Starfish or sea stars are
star-shaped echinoderm
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea ...
s belonging to the
class
Class or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used differentl ...
Asteroidea (). Common usage frequently finds these names being also applied to
ophiuroids, which are correctly referred to as
brittle star
Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomo ...
s or basket stars. Starfish are also known as asteroids due to being in the class Asteroidea. About 1,900 species of starfish live on the
seabed
The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'.
The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
in all the world's
ocean
The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...
s, from warm,
tropical zones to frigid,
polar regions
The polar regions, also called the frigid zones or polar zones, of Earth are the regions of the planet that surround its geographical poles (the North and South Poles), lying within the polar circles. These high latitudes are dominated by float ...
. They are found from the
intertidal zone
The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species o ...
down to
abyssal
The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. "Abyss" derives from the Greek word , meaning bottomless. At depths of , this zone remains in perpetual darkness. It covers 83% of the total area of the ocean a ...
depths, at below the surface.
Starfish are
marine invertebrates
Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a vertebral column, and some have evo ...
. They typically have a central disc and usually five arms, though some species have a larger number of arms. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey or brown. Starfish have
tube feet
Tube feet (technically podia) are small active tubular projections on the oral face of an echinoderm, whether the arms of a starfish, or the undersides of sea urchins, sand dollars and sea cucumbers; they are more discreet though present on britt ...
operated by a
hydraulic system
Hydraulics (from Greek language, Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is th ...
and a mouth at the centre of the oral or lower surface. They are
opportunistic
Opportunism is the practice of taking advantage of circumstances – with little regard for principles or with what the consequences are for others. Opportunist actions are expedient actions guided primarily by self-interested motives. The term ...
feeders and are mostly
predators
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
on
benthic
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...
invertebrates. Several species have specialized feeding behaviours including eversion of their stomachs and
suspension feeding
Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...
. They have complex
life cycles and can reproduce both sexually and
asexually. Most can
regenerate damaged parts or lost arms and they can shed arms as a means of defense. The Asteroidea occupy several significant
ecological roles. Starfish, such as the
ochre sea star
''Pisaster ochraceus'', generally known as the purple sea star, ochre sea star, or ochre starfish, is a common seastar found among the waters of the Pacific Ocean. Identified as a keystone species, ''P. ochraceus'' is considered an important indi ...
(''Pisaster ochraceus'') and the
reef sea star (''Stichaster australis''), have become widely known as examples of the
keystone species
A keystone species is a species which has a disproportionately large effect on its natural environment relative to its abundance, a concept introduced in 1969 by the zoologist Robert T. Paine. Keystone species play a critical role in maintaini ...
concept in ecology. The tropical
crown-of-thorns starfish
The crown-of-thorns starfish (frequently abbreviated to COTS), ''Acanthaster planci'', is a large starfish that preys upon hard, or stony, coral polyps (Scleractinia). The crown-of-thorns starfish receives its name from venomous thorn-like spine ...
(''Acanthaster planci'') is a voracious predator of
coral
Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...
throughout the Indo-Pacific region, and the
northern Pacific sea star is considered to be one of the
world's 100 worst invasive species.
The
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
record for starfish is ancient, dating back to the
Ordovician
The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...
around 450 million years ago, but it is rather sparse, as starfish tend to disintegrate after death. Only the
ossicles
The ossicles (also called auditory ossicles) are three bones in either middle ear that are among the smallest bones in the human body. They serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea). The absence of the auditory ...
and spines of the animal are likely to be preserved, making remains hard to locate. With their appealing symmetrical shape, starfish have played a part in literature, legend, design and popular culture. They are sometimes collected as curios, used in design or as logos, and in some cultures, despite possible toxicity, they are eaten.
Anatomy
![Luidia maculata, Ras Sedr, Egypt](https://upload.wikimedia.org/wikipedia/commons/5/56/Luidia_maculata%2C_Ras_Sedr%2C_Egypt.jpg)
Most starfish have five arms that radiate from a central disc, but the number varies with the group. Some species have six or seven arms and others have 10–15 arms.
[ The Antarctic '' Labidiaster annulatus'' can have over fifty.
]
Body wall
The body wall consists of a thin cuticle, an epidermis
The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...
consisting of a single layer of cells, a thick dermis
The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided i ...
formed of connective tissue
Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
and a thin coelom
The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it r ...
ic myoepithelial
Myoepithelial cells (sometimes referred to as myoepithelium) are cells usually found in glandular epithelium as a thin layer above the basement membrane but generally beneath the luminal cells. These may be positive for alpha smooth muscle actin a ...
layer, which provides the longitudinal and circular musculature. The dermis contains an endoskeleton
An endoskeleton (From Greek ἔνδον, éndon = "within", "inner" + σκελετός, skeletos = "skeleton") is an internal support structure of an animal, composed of mineralized tissue.
Overview
An endoskeleton is a skeleton that is on the i ...
of calcium carbonate components known as ossicles. These are honeycombed structures composed of calcite
Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
microcrystals arranged in a lattice.[Ruppert et al., 2004. p. 876] They vary in form, with some bearing external granules, tubercles and spines, but most are tabular plates that fit neatly together in a tessellated
A tessellation or tiling is the covering of a surface, often a plane, using one or more geometric shapes, called ''tiles'', with no overlaps and no gaps. In mathematics, tessellation can be generalized to higher dimensions and a variety of ...
manner and form the main covering of the aboral surface. Some are specialised structures such as the madreporite
The madreporite is a light colored calcareous opening used to filter water into the water vascular system of echinoderms. It acts like a pressure-equalizing valve. It is visible as a small red or yellow button-like structure, looking like a smal ...
(the entrance to the water vascular system), pedicellaria
A pedicellaria (plural: pedicellariae) is a small wrench- or claw-shaped appendage with movable jaws, called valves, commonly found on echinoderms (phylum Echinodermata), particularly in sea stars (class Asteroidea) and sea urchins (class Echi ...
e and paxillae.[ Pedicellariae are compound ossicles with forceps-like jaws. They remove debris from the body surface and wave around on flexible stalks in response to physical or chemical stimuli while continually making biting movements. They often form clusters surrounding spines.] Paxillae are umbrella-like structures found on starfish that live buried in sediment. The edges of adjacent paxillae meet to form a false cuticle with a water cavity beneath in which the madreporite and delicate gill structures are protected. All the ossicles, including those projecting externally, are covered by the epidermal layer.[
Several groups of starfish, including ]Valvatida
The Valvatida are an order of starfish in the class Asteroidea, which contains 695 species in 172 genera in 17 families.
Description
The order encompasses both tiny species, which are only a few millimetres in diameter, like those in the genu ...
and Forcipulatida
The Forcipulatida are an order of sea stars, containing three families and 49 genera.
Description
Forcipulatids share with the brisingid sea stars distinctive pedicellariae, consisting of a short stalk with three skeletal ossicles. Unlike that ...
, possess pedicellariae
A pedicellaria (plural: pedicellariae) is a small wrench- or claw-shaped appendage with movable jaws, called valves, commonly found on echinoderms (phylum Echinodermata), particularly in sea stars (class Asteroidea) and sea urchins (class Echinoi ...
. In Forcipulatida, such as ''Asterias
''Asterias'' is a genus of the Asteriidae family of sea stars. It includes several of the best-known species of sea stars, including the (Atlantic) common starfish, ''Asterias rubens'', and the northern Pacific seastar, ''Asterias amurensis''. Th ...
'' and '' Pisaster'', they occur in pompom
A pom-pom – also spelled pom-pon, pompom or pompon – is a decorative ball or tuft of fibrous material.
The term may refer to large tufts used by cheerleaders, or a small, tighter ball attached to the top of a hat, also known as a ...
-like tufts at the base of each spine, whereas in the Goniasteridae
Goniasteridae (the biscuit stars) constitute the largest family of sea stars, included in the order Valvatida. They are mostly deep-dwelling species, but the family also include several colorful shallow tropical species.
Description
Goniast ...
, such as '' Hippasteria phrygiana'', the pedicellariae are scattered over the body surface. Some are thought to assist in defence, while others aid in feeding or in the removal of organisms attempting to settle on the starfish's surface. Some species like ''Labidiaster annulatus'', '' Rathbunaster californicus'' and '' Novodinia antillensis'' use their large pedicellariae to capture small fish and crustaceans.
There may also be papula
Papulae (sing. papula; also occasionally papulla, papullae), also known as dermal branchiae or skin gills, are projections of the coelom of Asteroidea that serve in respiration and waste removal. Papulae are soft, covered externally with the epi ...
e, thin-walled protrusions of the body cavity that reach through the body wall and extend into the surrounding water. These serve a respiratory
The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies grea ...
function. The structures are supported by collagen fibres set at right angles to each other and arranged in a three-dimensional web with the ossicles and papulae in the interstices
An interstitial space or interstice is a space between structures or objects.
In particular, interstitial may refer to:
Biology
* Interstitial cell tumor
* Interstitial cell, any cell that lies between other cells
* Interstitial collagenase, ...
. This arrangement enables both easy flexion of the arms by the starfish and the rapid onset of stiffness and rigidity required for actions performed under stress.
Water vascular system
The water vascular
The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away f ...
system of the starfish is a hydraulic system
Hydraulics (from Greek language, Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is th ...
made up of a network of fluid-filled canals and is concerned with locomotion, adhesion, food manipulation and gas exchange
Gas exchange is the physical process by which gases move passively by Diffusion#Diffusion vs. bulk flow, diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liqui ...
. Water enters the system through the madreporite
The madreporite is a light colored calcareous opening used to filter water into the water vascular system of echinoderms. It acts like a pressure-equalizing valve. It is visible as a small red or yellow button-like structure, looking like a smal ...
, a porous, often conspicuous, sieve-like ossicle on the aboral surface. It is linked through a stone canal, often lined with calcareous material, to a ring canal around the mouth opening. A set of radial canals leads off this; one radial canal runs along the ambulacral
Ambulacral is a term typically used in the context of anatomical parts of the phylum Echinodermata or class Sea star, Asteroidea and Edrioasteroidea. Echinoderms can have ambulacral parts that include Ossicle (echinoderm), ossicles, plates, spines ...
groove in each arm. There are short lateral canals branching off alternately to either side of the radial canal, each ending in an ampulla. These bulb-shaped organs are joined to tube feet (podia) on the exterior of the animal by short linking canals that pass through ossicles in the ambulacral groove. There are usually two rows of tube feet but in some species, the lateral canals are alternately long and short and there appear to be four rows. The interior of the whole canal system is lined with cilia
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
.[Ruppert et al., 2004. pp. 879–883]
When longitudinal muscles in the ampullae contract, valves in the lateral canals close and water is forced into the tube feet. These extend to contact the substrate. Although the tube feet resemble suction cups in appearance, the gripping action is a function of adhesive chemicals rather than suction. Other chemicals and relaxation of the ampullae allow for release from the substrate. The tube feet latch on to surfaces and move in a wave, with one arm section attaching to the surface as another releases.[ Some starfish turn up the tips of their arms while moving which gives maximum exposure of the sensory tube feet and the eyespot to external stimuli.
Having descended from ]bilateral
Bilateral may refer to any concept including two sides, in particular:
* Bilateria, bilateral animals
*Bilateralism, the political and cultural relations between two states
*Bilateral, occurring on both sides of an organism ( Anatomical terms of ...
organisms, starfish may move in a bilateral fashion, particularly when hunting or in danger. When crawling, certain arms act as the leading arms, while others trail behind. Most starfish cannot move quickly, a typical speed being that of the leather star
The leather star (''Dermasterias imbricata'') is a sea star in the family Asteropseidae found at depths to off the western seaboard of North America. It was first described to science by Adolph Eduard Grube in 1857.
Description
The leather st ...
(''Dermasterias imbricata''), which can manage just in a minute. Some burrowing species from the genera ''Astropecten
''Astropecten'' is a genus of sea stars of the family Astropectinidae.
Identification
These sea stars are similar one to each other and it can be difficult to determine with certainty the species only from a photograph. To have a certain d ...
'' and ''Luidia
''Luidia'' is a genus of starfish in the Family (biology), family Luidiidae in which it is the only genus. Species of the family have a cosmopolitan distribution.
Characteristics
Members of the genus are characterised by having long arms with ...
'' have points rather than suckers on their long tube feet
Tube feet (technically podia) are small active tubular projections on the oral face of an echinoderm, whether the arms of a starfish, or the undersides of sea urchins, sand dollars and sea cucumbers; they are more discreet though present on britt ...
and are capable of much more rapid motion, "gliding" across the ocean floor. The sand star ('' Luidia foliolata'') can travel at a speed of per minute. When a starfish finds itself upside down, two adjacent arms are bent backwards to provide support, the opposite arm is used to stamp the ground while the two remaining arms are raised on either side; finally the stamping arm is released as the starfish turns itself over and recovers its normal stance.
Apart from their function in locomotion, the tube feet act as accessory gills. The water vascular system serves to transport oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
from, and carbon dioxide to, the tube feet and also nutrients from the gut to the muscles involved in locomotion. Fluid movement is bidirectional and initiated by cilia
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
. Gas exchange also takes place through other gills
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...
known as papulae, which are thin-walled bulges on the aboral surface of the disc and arms. Oxygen is transferred from these to the coelomic fluid, which acts as the transport medium for gasses. Oxygen dissolved in the water is distributed through the body mainly by the fluid in the main body cavity; the circulatory system may also play a minor role.
Digestive system and excretion
The gut of a starfish occupies most of the disc and extends into the arms. The mouth is located in the centre of the oral surface, where it is surrounded by a tough peristomial membrane and closed with a sphincter
A sphincter is a circular muscle that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. Sphincters are found in many animals. There are over 60 types in the hum ...
. The mouth opens through a short oesophagus
The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the ...
into a stomach
The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...
divided by a constriction into a larger, eversible cardiac portion and a smaller pyloric portion. The cardiac stomach is glandular and pouched, and is supported by ligament
A ligament is the fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body include the:
* Peritoneal li ...
s attached to ossicles in the arms so it can be pulled back into position after it has been everted. The pyloric stomach has two extensions into each arm: the pyloric caeca. These are elongated, branched hollow tubes that are lined by a series of glands, which secrete digestive enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
s and absorb nutrients from the food. A short intestine
The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans ...
and rectum
The rectum is the final straight portion of the large intestine in humans and some other mammals, and the Gastrointestinal tract, gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the s ...
run from the pyloric stomach to open at a small anus
The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...
at the apex of the aboral surface of the disc.[Ruppert et al., 2004. p. 885]
Primitive starfish, such as ''Astropecten'' and ''Luidia'', swallow their prey
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...
whole, and start to digest it in their cardiac stomachs. Shell valves and other inedible materials are ejected through their mouths. The semi-digested fluid is passed into their pyloric stomachs and caeca where digestion continues and absorption ensues. In more advanced species of starfish, the cardiac stomach can be everted from the organism's body to engulf and digest food. When the prey is a clam or other bivalve
Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...
, the starfish pulls with its tube feet to separate the two valves slightly, and inserts a small section of its stomach, which releases enzymes to digest the prey. The stomach and the partially digested prey are later retracted into the disc. Here the food is passed on to the pyloric stomach, which always remains inside the disc. The retraction and contraction of the cardiac stomach is activated by a neuropeptide
Neuropeptides are chemical messengers made up of small chains of amino acids that are synthesized and released by neurons. Neuropeptides typically bind to G protein-coupled receptors (GPCRs) to modulate neural activity and other tissues like the ...
known as NGFFYamide.
Because of this ability to digest food outside the body, starfish can hunt prey much larger than their mouths. Their diets include clams and oyster
Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not al ...
s, arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
s, small fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...
and gastropod molluscs. Some starfish are not pure carnivore
A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other sof ...
s, supplementing their diets with algae
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
or organic detritus. Some of these species are grazers, but others trap food particles from the water in sticky mucus
Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is ...
strands that are swept towards the mouth along cilia
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
ted grooves.
The main nitrogenous waste product is ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
. Starfish have no distinct excretory organs; waste ammonia is removed by diffusion through the tube feet and papulae.[ The body fluid contains ]phagocytic
Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...
cells called coelomocytes, which are also found within the hemal and water vascular systems. These cells engulf waste material, and eventually migrate to the tips of the papulae, where a portion of body wall is nipped off and ejected into the surrounding water. Some waste may also be excreted by the pyloric glands and voided with the faeces
Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a relati ...
.[Ruppert et al., 2004. pp. 886–887]
Starfish do not appear to have any mechanisms for osmoregulation
Osmoregulation is the active regulation of the osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content; that is, it maintains the fluid balance and the concentration o ...
, and keep their body fluids at the same salt concentration as the surrounding water. Although some species can tolerate relatively low salinity
Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...
, the lack of an osmoregulation system probably explains why starfish are not found in fresh water or even in many estuarine
An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environment ...
environments.[
]
Sensory and nervous systems
Although starfish do not have many well-defined sense organs, they are sensitive to touch, light, temperature, orientation and the status of the water around them. The tube feet, spines and pedicellariae are sensitive to touch. The tube feet, especially those at the tips of the rays, are also sensitive to chemicals, enabling the starfish to detect odour sources such as food.[ There are eyespots at the ends of the arms, each one made of 80–200 simple ]ocelli
A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-l ...
. These are composed of pigment
A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compo ...
ed epithelial cells that respond to light and are covered by a thick, transparent cuticle that both protects the ocelli and acts to focus light. Many starfish also possess individual photoreceptor cells
A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiatio ...
in other parts of their bodies and respond to light even when their eyespots are covered. Whether they advance or retreat depends on the species.[Ruppert et al., 2004. pp. 883–884]
While a starfish lacks a centralized brain, it has a complex nervous system
In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes th ...
with a nerve ring around the mouth and a radial nerve running along the ambulacral region of each arm parallel to the radial canal. The peripheral nerve system consists of two nerve nets: a sensory system in the epidermis and a motor system in the lining of the coelomic cavity. Neurons passing through the dermis connect the two. The ring nerves and radial nerves have sensory and motor components and coordinate the starfish's balance and directional systems. The sensory component receives input from the sensory organs while the motor nerves control the tube feet and musculature. The starfish does not have the capacity to plan its actions. If one arm detects an attractive odour, it becomes dominant and temporarily over-rides the other arms to initiate movement towards the prey. The mechanism for this is not fully understood.
Circulatory system
The body cavity contains the circulatory
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
or haemal system. The vessels form three rings: one around the mouth (the hyponeural haemal ring), another around the digestive system (the gastric ring) and the third near the aboral surface (the genital ring). The heart beats about six times a minute and is at the apex of a vertical channel (the axial vessel) that connects the three rings. At the base of each arm are paired gonad
A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sper ...
s; a lateral vessel extends from the genital ring past the gonads to the tip of the arm. This vessel has a blind end and there is no continuous circulation of the fluid within it. This liquid does not contain a pigment and has little or no respiratory function but is probably used to transport nutrients around the body.[Ruppert et al., 2004. p. 886]
Secondary metabolites
Starfish produce a large number of secondary metabolite
Secondary metabolites, also called specialised metabolites, toxins, secondary products, or natural products, are organic compounds produced by any lifeform, e.g. bacteria, fungi, animals, or plants, which are not directly involved in the norm ...
s in the form of lipid
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include ...
s, including steroid
A steroid is a biologically active organic compound with four rings arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes that alter membrane fluidity; and a ...
al derivatives of cholesterol
Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell mem ...
, and fatty acid
In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, fr ...
amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...
s of sphingosine
Sphingosine (2-amino-4-trans-octadecene-1,3-diol) is an 18-carbon amino alcohol with an unsaturated hydrocarbon chain, which forms a primary part of sphingolipids, a class of cell membrane lipids that include sphingomyelin, an important phospholip ...
. The steroids are mostly saponin
Saponins (Latin "sapon", soap + "-in", one of), also selectively referred to as triterpene glycosides, are bitter-tasting usually toxic plant-derived organic chemicals that have a foamy quality when agitated in water. They are widely distributed ...
s, known as asterosaponins, and their sulphated derivatives. They vary between species and are typically formed from up to six sugar molecules (usually glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...
and galactose
Galactose (, '' galacto-'' + '' -ose'', "milk sugar"), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epimer of glucose. A galactose molec ...
) connected by up to three glycosidic
A glycosidic bond or glycosidic linkage is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate.
A glycosidic bond is formed between the hemiacetal or hemiketal group ...
chains. Long-chain fatty acid amides of sphingosine occur frequently and some of them have known pharmacological activity
In pharmacology, biological activity or pharmacological activity describes the beneficial or adverse effects of a drug on living matter. When a drug is a complex chemical mixture, this activity is exerted by the substance's active ingredient or ...
. Various ceramide
Ceramides are a family of waxy lipid molecules. A ceramide is composed of N-acetylsphingosine and a fatty acid. Ceramides are found in high concentrations within the cell membrane of eukaryotic cells, since they are component lipids that make up ...
s are also known from starfish and a small number of alkaloid
Alkaloids are a class of basic, naturally occurring organic compounds that contain at least one nitrogen atom. This group also includes some related compounds with neutral and even weakly acidic properties. Some synthetic compounds of similar ...
s have also been identified. The functions of these chemicals in the starfish have not been fully investigated but most have roles in defence and communication. Some are feeding deterrents used by the starfish to discourage predation. Others are antifoulants and supplement the pedicellariae to prevent other organisms from settling on the starfish's aboral surface. Some are alarm pheromone
A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
s and escape-eliciting chemicals, the release of which trigger responses in conspecific
Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species.
Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organ ...
starfish but often produce escape responses in potential prey. Research into the efficacy of these compounds for possible pharmacological or industrial use occurs worldwide.
Life cycle
Sexual reproduction
Most species of starfish are gonochorous, there being separate male and female individuals. These are usually not distinguishable externally as the gonads cannot be seen, but their sex is apparent when they spawn
Spawn or spawning may refer to:
* Spawn (biology), the eggs and sperm of aquatic animals
Arts, entertainment, and media
* Spawn (character), a fictional character in the comic series of the same name and in the associated franchise
** '' Spawn: ...
. Some species are simultaneous hermaphrodites, producing eggs and sperm at the same time and in a few of these, the same gonad, called an ovotestis
An ovotestis is a gonad with both testicular and ovarian aspects. In humans, ovotestes are an infrequent anatomical variation associated with gonadal dysgenesis. The only mammals where ovotestes are not symptomatic of an intersex variation are mole ...
, produces both eggs and sperm. Other starfish are sequential hermaphrodites. Protandrous
Sequential hermaphroditism (called dichogamy in botany) is a type of hermaphroditism that occurs in many fish, gastropods, and plants. Sequential hermaphroditism occurs when the individual changes its sex at some point in its life. In particular, ...
individuals of species like ''Asterina gibbosa
''Asterina gibbosa'', commonly known as the starlet cushion star, is a species of starfish in the family Asterinidae. It is native to the northeastern Atlantic Ocean and the Mediterranean Sea.
Description
''Asterina gibbosa'' is a pentagonal sta ...
'' start life as males before changing sex into females as they grow older. In some species such as ''Nepanthia belcheri
''Nepanthia belcheri'' is a species of starfish in the family Asterinidae. It is found in shallow water in Southeast Asia and northeastern Australia. It is an unusual species in that it can reproduce sexually or can split in two by fission to fo ...
'', a large female can split in half and the resulting offspring are males. When these grow large enough they change back into females.
Each starfish arm contains two gonads that release gametes
A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...
through openings called gonoducts, located on the central disc between the arms. Fertilization
Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...
is generally external but in a few species, internal fertilization takes place. In most species, the buoyant eggs and sperm are simply released into the water (free spawning) and the resulting embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
s and larva
A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.
The ...
e live as part of the plankton
Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...
. In others, the eggs may be stuck to the undersides of rocks. In certain species of starfish, the females brood
Brood may refer to:
Nature
* Brood, a collective term for offspring
* Brooding, the incubation of bird eggs by their parents
* Bee brood, the young of a beehive
* Individual broods of North American Periodical Cicadas:
** Brood X, the largest b ...
their eggs – either by simply enveloping them[ or by holding them in specialised structures. Brooding may be done in pockets on the starfish's aboral surface,][ inside the pyloric stomach ('']Leptasterias tenera
''Leptasterias tenera'' is a species of starfish in the Family (biology), family Asteriidae. It is found on the eastern coast of North America.
Description
''Leptasterias tenera'' is a small starfish with five arms and a slow growth rate. It c ...
'') or even in the interior of the gonads themselves.[ Those starfish that brood their eggs by "sitting" on them usually assume a humped posture with their discs raised off the substrate.] '' Pteraster militaris'' broods a few of its young and disperses the remaining eggs, that are too numerous to fit into its pouch.[ In these brooding species, the eggs are relatively large, and supplied with ]yolk
Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example bec ...
, and they generally develop directly into miniature starfish without an intervening larval stage.[ The developing young are called lecithotrophic because they obtain their nutrition from the yolk as opposed to "planktotrophic" larvae that feed in the ]water column
A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...
. In '' Parvulastra parvivipara'', an intragonadal brooder, the young starfish obtain nutrients by eating other eggs and embryos in the brood pouch. Brooding is especially common in polar and deep-sea species that live in environments unfavourable for larval development[Ruppert et al., 2004. pp. 887–888] and in smaller species that produce just a few eggs.
In the tropics, a plentiful supply of phytoplankton is continuously available for starfish larvae to feed on. Spawning takes place at any time of year, each species having its own characteristic breeding season. In temperate regions, the spring and summer brings an increase in food supplies. The first individual of a species to spawn may release a pheromone
A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
that serves to attract other starfish to aggregate and to release their gametes synchronously. In other species, a male and female may come together and form a pair. This behaviour is called pseudocopulation
Pseudocopulation describes behaviors similar to copulation that serve a reproductive function for one or both participants but do not involve actual sexual union between the individuals. It is most generally applied to a pollinator attempting to co ...
and the male climbs on top, placing his arms between those of the female. When she releases eggs into the water, he is induced to spawn.[ Starfish may use environmental signals to coordinate the time of spawning (day length to indicate the correct time of the year,][ dawn or dusk to indicate the correct time of day), and chemical signals to indicate their readiness to breed. In some species, mature females produce chemicals to attract sperm in the sea water.]
Larval development
Most starfish embryos hatch at the blastula
Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. The blastula (from ...
stage. The original ball of cells develops a lateral pouch, the archenteron
The primary gut that forms during gastrulation in a developing embryo is known as the archenteron, the gastrocoel or the primitive digestive tube. It develops into the endoderm and mesoderm of an animal.
Formation in sea urchins
As primary mesen ...
. The entrance to this is known as the blastopore and it will later develop into the anus—together with chordate
A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fiv ...
s, echinoderm
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea ...
s are deuterostomes, meaning the second (''deutero'') invagination becomes the mouth (''stome''); members of all other phyla are protostome
Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryogenesis, embryonic development. This nature has since been discovered to be extremely variable among Pro ...
s, and their first invagination becomes the mouth. Another invagination of the surface will fuse with the tip of the archenteron as the mouth while the interior section will become the gut. At the same time, a band of cilia
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
develops on the exterior. This enlarges and extends around the surface and eventually onto two developing arm-like outgrowths. At this stage the larva is known as a bipinnaria A bipinnaria is the first stage in the larval development of most starfish, and is usually followed by a brachiolaria stage. Movement and feeding is accomplished by the bands of cilia. Starfish that brood their young generally lack a bipinnaria st ...
. The cilia are used for locomotion and feeding, their rhythmic beat wafting phytoplankton
Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'.
Ph ...
towards the mouth.[
The next stage in development is a brachiolaria larva and involves the growth of three short, additional arms. These are at the anterior end, surround a sucker and have adhesive cells at their tips. Both bipinnaria and brachiolaria larvae are bilaterally symmetrical. When fully developed, the brachiolaria settles on the seabed and attaches itself with a short stalk formed from the ventral arms and sucker. Metamorphosis now takes place with a radical rearrangement of tissues. The left side of the larval body becomes the oral surface of the juvenile and the right side the aboral surface. Part of the gut is retained, but the mouth and anus move to new positions. Some of the body cavities degenerate but others become the water vascular system and the visceral coelom. The starfish is now pentaradially symmetrical. It casts off its stalk and becomes a free-living juvenile starfish about in diameter. Starfish of the order Paxillosida have no brachiolaria stage, with the bipinnaria larvae settling on the seabed and developing directly into juveniles.]
Asexual reproduction
Some species of starfish are able to reproduce asexually as adults either by fission of their central discs or by autotomy
Autotomy (from the Greek language, Greek ''auto-'', "self-" and ''tome'', "severing", wikt:αὐτοτομία, αὐτοτομία) or self-amputation, is the behaviour whereby an animal sheds or discards one or more of its own appendages, usual ...
of one or more of their arms. Which of these processes occurs depends on the genus. Among starfish that are able to regenerate their whole body from a single arm, some can do so even from fragments just long. Single arms that regenerate a whole individual are called comet forms. The division of the starfish, either across its disc or at the base of the arm, is usually accompanied by a weakness in the structure that provides a fracture zone.
The larvae of several species of starfish can reproduce asexually before they reach maturity. They do this by autotomising some parts of their bodies or by budding
Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is know ...
. When such a larva senses that food is plentiful, it takes the path of asexual reproduction rather than normal development. Though this costs it time and energy and delays maturity, it allows a single larva to give rise to multiple adults when the conditions are appropriate.[
]
Regeneration
Some species of starfish have the ability to regenerate lost arms and can regrow an entire new limb given time.[ A few can regrow a complete new disc from a single arm, while others need at least part of the central disc to be attached to the detached part.][ Regrowth can take several months or years,][ and starfish are vulnerable to infections during the early stages after the loss of an arm. A separated limb lives off stored nutrients until it regrows a disc and mouth and is able to feed again.][ Other than fragmentation carried out for the purpose of reproduction, the division of the body may happen inadvertently due to part being detached by a predator, or part may be actively shed by the starfish in an escape response.][ The loss of parts of the body is achieved by the rapid softening of a special type of connective tissue in response to nervous signals. This type of tissue is called catch connective tissue and is found in most echinoderms.] An autotomy-promoting factor has been identified which, when injected into another starfish, causes rapid shedding of arms.
Lifespan
The lifespan of a starfish varies considerably between species, generally being longer in larger forms and in those with planktonic larvae. For example, '' Leptasterias hexactis'' broods a small number of large-yolked eggs. It has an adult weight of , reaches sexual maturity in two years and lives for about ten years. ''Pisaster ochraceus
''Pisaster ochraceus'', generally known as the purple sea star, ochre sea star, or ochre starfish, is a common seastar found among the waters of the Pacific Ocean. Identified as a keystone species, ''P. ochraceus'' is considered an important indi ...
'' releases a large number of eggs into the sea each year and has an adult weight of up to . It reaches maturity in five years and has a maximum recorded lifespan of 34 years.
Ecology
Distribution and habitat
Echinoderms, including starfish, maintain a delicate internal electrolyte
An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
balance that is in equilibrium with sea water, making it impossible for them to live in a freshwater
Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...
habitat.[ Starfish species inhabit all of the world's oceans. Habitats range from tropical ]coral reef
A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.
Co ...
s, rocky shores, tidal pool
A tide pool or rock pool is a shallow pool of seawater that forms on the rocky intertidal shore. Many of these pools exist as separate bodies of water only at low tide.
Many tide pool habitats are home to especially adaptable animals that ...
s, mud, and sand to kelp forest
Kelp forests are underwater areas with a high density of kelp, which covers a large part of the world's coastlines. Smaller areas of anchored kelp are called kelp beds. They are recognized as one of the most productive and dynamic ecosystems on Ea ...
s, seagrass meadow
A seagrass meadow or seagrass bed is an underwater ecosystem formed by seagrasses. Seagrasses are marine (saltwater) plants found in shallow coastal waters and in the brackish waters of estuaries. Seagrasses are flowering plants with stems and ...
s and the deep-sea floor down to at least . The greatest diversity of species occurs in coastal areas.[
]
Diet
Most species are generalist predators, eating microalgae
Microalgae or microphytes are microscopic algae invisible to the naked eye. They are phytoplankton typically found in freshwater and marine systems, living in both the water column and sediment. They are unicellular species which exist indiv ...
, sponge
Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...
s, bivalve
Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...
s, snail
A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gastro ...
s and other small animals.[ The ]crown-of-thorns starfish
The crown-of-thorns starfish (frequently abbreviated to COTS), ''Acanthaster planci'', is a large starfish that preys upon hard, or stony, coral polyps (Scleractinia). The crown-of-thorns starfish receives its name from venomous thorn-like spine ...
consumes coral
Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...
polyps,[ while other species are ]detritivore
Detritivores (also known as detrivores, detritophages, detritus feeders, or detritus eaters) are heterotrophs that obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces). There are many kinds of invertebrates, ...
s, feeding on decomposing organic material and faecal matter.[ in Lawrence (2013)] A few are suspension feeders, gathering in phytoplankton
Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'.
Ph ...
; ''Henricia
''Henricia'' is a large genus of slender-armed sea stars belonging to the family Echinasteridae. It contains about fifty species.
The sea stars from this genus are ciliary suspension-feeders, filtering phytoplankton.
Species
According to the Wo ...
'' and '' Echinaster'' often occur in association with sponges, benefiting from the water current they produce. Various species have been shown to be able to absorb organic nutrients from the surrounding water, and this may form a significant portion of their diet.[
The processes of feeding and capture may be aided by special parts; '']Pisaster brevispinus
''Pisaster brevispinus'', commonly called the pink sea star, giant pink sea star, or short-spined sea star, is a species of sea star in the northeast Pacific Ocean. It was first described to science by William Stimson in 1857. The type specime ...
'', the short-spined pisaster from the West Coast West Coast or west coast may refer to:
Geography Australia
* Western Australia
*Regions of South Australia#Weather forecasting, West Coast of South Australia
* West Coast, Tasmania
**West Coast Range, mountain range in the region
Canada
* Britis ...
of America, can use a set of specialized tube feet to dig itself deep into the soft substrate to extract prey (usually clam
Clam is a common name for several kinds of bivalve molluscs. The word is often applied only to those that are edible and live as infauna, spending most of their lives halfway buried in the sand of the seafloor or riverbeds. Clams have two she ...
s). Grasping the shellfish, the starfish slowly pries open the prey's shell by wearing out its adductor muscle, and then inserts its everted stomach into the crack to digest the soft tissues. The gap between the valves need only be a fraction of a millimetre wide for the stomach to gain entry. Cannibalism has been observed in juvenile sea stars as early as four days after metamorphosis.
Ecological impact
Starfish are keystone species
A keystone species is a species which has a disproportionately large effect on its natural environment relative to its abundance, a concept introduced in 1969 by the zoologist Robert T. Paine. Keystone species play a critical role in maintaini ...
in their respective marine communities
A community is a Level of analysis, social unit (a group of living things) with commonality such as place (geography), place, Norm (social), norms, religion, values, Convention (norm), customs, or Identity (social science), identity. Communiti ...
. Their relatively large sizes, diverse diets and ability to adapt to different environments makes them ecologically important. The term "keystone species" was in fact first used by Robert Paine in 1966 to describe a starfish, ''Pisaster ochraceus''. When studying the low intertidal coasts of Washington state
Washington (), officially the State of Washington, is a state in the Pacific Northwest region of the Western United States. Named for George Washington—the first U.S. president—the state was formed from the western part of the Washington ...
, Paine found that predation by ''P. ochraceus'' was a major factor in the diversity of species. Experimental removals of this top predator from a stretch of shoreline resulted in lower species diversity and the eventual domination of '' Mytilus'' mussels, which were able to outcompete other organisms for space and resources. Similar results were found in a 1971 study of '' Stichaster australis'' on the intertidal coast of the South Island
The South Island, also officially named , is the larger of the two major islands of New Zealand in surface area, the other being the smaller but more populous North Island. It is bordered to the north by Cook Strait, to the west by the Tasman ...
of New Zealand
New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...
. ''S. australis'' was found to have removed most of a batch of transplanted mussels within two or three months of their placement, while in an area from which ''S. australis'' had been removed, the mussels increased in number dramatically, overwhelming the area and threatening biodiversity
Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') l ...
.
The feeding activity of the omnivorous
An omnivore () is an animal that has the ability to eat and survive on both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize the nutri ...
starfish ''Oreaster reticulatus
''Oreaster reticulatus'', commonly known as the red cushion sea star or the West Indian sea star, is a species of marine invertebrate, a starfish in the family Oreasteridae. It is found in shallow water in the western Atlantic Ocean and the Ca ...
'' on sandy and seagrass bottoms in the Virgin Islands
The Virgin Islands ( es, Islas Vírgenes) are an archipelago in the Caribbean Sea. They are geologically and biogeographically the easternmost part of the Greater Antilles, the northern islands belonging to the Puerto Rico Trench and St. Croix ...
appears to regulate the diversity, distribution and abundance of microorganisms. These starfish engulf piles of sediment removing the surface films and algae adhering to the particles. Organisms that dislike this disturbance are replaced by others better able to rapidly recolonise "clean" sediment. In addition, foraging by these migratory starfish creates diverse patches of organic matter, which may play a role in the distribution and abundance of organisms such as fish, crabs and sea urchins that feed on the sediment.
Starfish sometimes have negative effects on ecosystems. Outbreaks of crown-of-thorns starfish have caused damage to coral reefs in Northeast Australia and French Polynesia
)Territorial motto: ( en, "Great Tahiti of the Golden Haze")
, anthem =
, song_type = Regional anthem
, song = " Ia Ora 'O Tahiti Nui"
, image_map = French Polynesia on the globe (French Polynesia centered).svg
, map_alt = Location of Frenc ...
. A study in Polynesia found that coral cover declined drastically with the arrival of migratory starfish in 2006, dropping from 50% to under 5% in three years. This had a cascading effect on the whole benthic
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...
community and reef-feeding fish.[ '']Asterias amurensis
''Asterias amurensis'', also known as the Northern Pacific seastar and Japanese common starfish, is a seastar found in shallow seas and estuary, estuaries, native to the coasts of northern China, Korea, far eastern Russia, Japan, Alaska, the Aleu ...
'' is one of a few echinoderm invasive species
An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species ad ...
. Its larvae likely arrived in Tasmania
)
, nickname =
, image_map = Tasmania in Australia.svg
, map_caption = Location of Tasmania in AustraliaCoordinates:
, subdivision_type = Country
, subdi ...
from central Japan via water discharged from ships in the 1980s. The species has since grown in numbers to the point where they threaten commercially important bivalve
Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...
populations. As such, they are considered pests,[ in Lawrence (2013)] and are on the Invasive Species Specialist Group's list of the world's 100 worst invasive species
100 of the World's Worst Invasive Alien Species is a list of invasive species compiled in 2014 by the Global Invasive Species Database, a database of invasive species around the world. The database is run by the Invasive Species Specialist Group ...
.
Sea Stars (starfish) are the main predators of kelp-eating sea urchins. Satellite imagery shows that sea urchin populations have exploded due to starfish mass deaths, and that by 2021, sea urchins have destroyed 95% of California's kelp forests.
Threats
Starfish may be preyed on by conspecifics, sea anemones, other starfish species, tritons, crabs, fish, gull
Gulls, or colloquially seagulls, are seabirds of the family Laridae in the suborder Lari. They are most closely related to the terns and skimmers and only distantly related to auks, and even more distantly to waders. Until the 21st century, m ...
s and sea otter
The sea otter (''Enhydra lutris'') is a marine mammal native to the coasts of the northern and eastern North Pacific Ocean. Adult sea otters typically weigh between , making them the heaviest members of the weasel family, but among the small ...
s.[ in Lawrence (2013)][ in Lawrence (2013)][ in Lawrence (2013)] Their first lines of defence are the saponin
Saponins (Latin "sapon", soap + "-in", one of), also selectively referred to as triterpene glycosides, are bitter-tasting usually toxic plant-derived organic chemicals that have a foamy quality when agitated in water. They are widely distributed ...
s present in their body walls, which have unpleasant flavours.[ Some starfish such as '' Astropecten polyacanthus'' also include powerful toxins such as ]tetrodotoxin
Tetrodotoxin (TTX) is a potent neurotoxin. Its name derives from Tetraodontiformes, an order that includes pufferfish, porcupinefish, ocean sunfish, and triggerfish; several of these species carry the toxin. Although tetrodotoxin was discovered ...
among their chemical armoury, and the slime star can ooze out large quantities of repellent mucus. They also have body armour in the form of hard plates and spines. The crown-of-thorns starfish
The crown-of-thorns starfish (frequently abbreviated to COTS), ''Acanthaster planci'', is a large starfish that preys upon hard, or stony, coral polyps (Scleractinia). The crown-of-thorns starfish receives its name from venomous thorn-like spine ...
is particularly unattractive to potential predators, being heavily defended by sharp spines, laced with toxins and sometimes with bright warning colours. Other species protect their vulnerable tube feet and arm tips by lining their ambulacral grooves with spines and heavily plating their extremities.[
]
Several species sometimes suffer from a wasting
In medicine, wasting, also known as wasting syndrome, refers to the process by which a debilitating disease causes muscle and fat tissue to "waste" away. Wasting is sometimes referred to as "acute malnutrition" because it is believed that episo ...
condition caused by bacteria in the genus ''Vibrio
''Vibrio'' is a genus of Gram-negative bacteria, possessing a curved-rod (comma) shape, several species of which can cause foodborne infection, usually associated with eating undercooked seafood. Being highly salt tolerant and unable to survive ...
'';[ however, a more widespread ]wasting disease
In medicine, wasting, also known as wasting syndrome, refers to the process by which a debilitating disease causes muscle and fat tissue to "waste" away. Wasting is sometimes referred to as "acute malnutrition" because it is believed that epis ...
, causing mass mortalities among starfish, appears sporadically. A paper published in November 2014 revealed the most likely cause of this disease to be a densovirus the authors named sea star-associated densovirus (SSaDV).
The protozoan ''Orchitophrya stellarum
''Orchitophrya stellarum'' is a species of single-celled marine ciliates, a member of the class Oligohymenophorea. It is found living freely in the north Atlantic and Pacific Oceans but is also parasitic, being found inside the gonads of starfish ...
'' is known to infect the gonads of starfish and damage tissue.[ Starfish are vulnerable to high temperatures. Experiments have shown that the feeding and growth rates of ''P. ochraceus'' reduce greatly when their body temperatures rise above and that they die when their temperature rises to . This species has a unique ability to absorb seawater to keep itself cool when it is exposed to sunlight by a receding tide. It also appears to rely on its arms to absorb heat, so as to protect the central disc and vital organs like the stomach.
Starfish and other echinoderms are sensitive to ]marine pollution
Marine pollution occurs when substances used or spread by humans, such as industrial waste, industrial, agricultural pollution, agricultural and municipal solid waste, residential waste, particle (ecology), particles, noise, excess carbon dioxid ...
. The common starfish
The common starfish, common sea star or sugar starfish (''Asterias rubens'') is the most common and familiar starfish in the north-east Atlantic. Belonging to the family Asteriidae, it has five arms and usually grows to between 10–30 c ...
is considered to be a bioindicator
A bioindicator is any species (an indicator species) or group of species whose function, population, or status can reveal the qualitative status of the environment. The most common indicator species are animals. For example, copepods and other sma ...
for marine ecosystems. A 2009 study found that ''P. ochraceus'' is unlikely to be affected by ocean acidification
Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14. The root cause of ocean acidification is carbon dioxid ...
as severely as other marine animals with calcareous
Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.
In zoology
''Calcareous'' is used as an adje ...
skeletons. In other groups, structures made of calcium carbonate are vulnerable to dissolution when the pH is lowered. Researchers found that when ''P. ochraceus'' were exposed to and 770 ppm carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
(beyond rises expected in the next century), they were relatively unaffected. Their survival is likely due to the nodular nature of their skeletons, which are able to compensate for a shortage of carbonate by growing more fleshy tissue.
Evolution
Fossil record
Echinoderms first appeared in the fossil record
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...
in the Cambrian
The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...
. The first known asterozoans were the Somasteroidea
The Somasteroidea, or Stomasteroidea, is an extinct order of asterozoan echinoderms first defined in 1951 by W. K. Spencer.W. K. Spencer. (1951). Early Palaeozoic starfish. ''Philosophical Transactions of the Royal Society of London, series B'' 23 ...
, which exhibit characteristics of both groups. Starfish are infrequently found as fossils, possibly because their hard skeletal components separate as the animal decays. Despite this, there are a few places where accumulations of complete skeletal structures occur, fossilized in place in Lagerstätte
A Lagerstätte (, from ''Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These for ...
n – so-called "starfish beds".
By the late Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ' ...
, the crinoids
Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which are ...
and blastoid
Blastoids (class Blastoidea) are an extinct type of stemmed echinoderm, often referred to as sea buds. They first appear, along with many other echinoderm classes, in the Ordovician period, and reached their greatest diversity in the Mississi ...
s were the predominant echinoderms, and some limestones from this period are made almost entirely from fragments from these groups. In the two major extinction event
An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. I ...
s that occurred during the late Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
and late Permian
The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...
, the blastoids were wiped out and only a few species of crinoids survived.[ Many starfish species also became extinct in these events, but afterwards the surviving few species diversified rapidly within about sixty million years during the ]Early Jurassic
The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-J ...
and the beginning of the Middle Jurassic
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare, but geological formations co ...
. A 2012 study found that speciation
Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...
in starfish can occur rapidly. During the last 6,000 years, divergence in the larval development of '' Cryptasterina hystera'' and '' Cryptasterina pentagona'' has taken place, the former adopting internal fertilization and brooding and the latter remaining a broadcast spawner.
Diversity
The scientific name Asteroidea was given to starfish by the French zoologist de Blainville
Henri Marie Ducrotay de Blainville (; 12 September 1777 – 1 May 1850) was a French zoologist and anatomist.
Life
Blainville was born at Arques, near Dieppe. As a young man he went to Paris to study art, but ultimately devoted himself to natur ...
in 1830. It is derived from the Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
''aster'', ἀστήρ (a star) and the Greek ''eidos'', εἶδος (form, likeness, appearance). The class Asteroidea belongs to the phylum Echinodermata
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...
. As well as the starfish, the echinoderms include sea urchin
Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) of ...
s, sand dollar
Sand dollars (also known as a sea cookie or snapper biscuit in New Zealand, or pansy shell in South Africa) are species of flat, burrowing sea urchins belonging to the order Clypeasteroida. Some species within the order, not quite as flat, are k ...
s, brittle
A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Bre ...
and basket star
The Euryalida are an order of brittle stars, which includes large species with either branching arms (called "basket stars") or long and curling arms (called "snake stars").
Characteristics
Many of the species in this order have characteristi ...
s, sea cucumber
Sea cucumbers are echinoderms from the class Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothuria ...
s and crinoid
Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which are ...
s. The larvae of echinoderms have bilateral symmetry, but during metamorphosis
Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some inse ...
this is replaced with radial symmetry
Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a pla ...
, typically pentameric
A pentamer is an entity composed of five sub-units.
In chemistry, it applies to molecules made of five monomers.
In biochemistry, it applies to macromolecules, in particular to pentameric proteins, made of five proteic sub-units.
In microbiolo ...
. Adult echinoderms are characterized by having a water vascular system
The water vascular system is a hydraulic system used by echinoderms, such as sea stars and sea urchins, for locomotion, food and waste transportation, and respiration. The system is composed of canals connecting numerous tube feet. Echinoderms mov ...
with external tube feet
Tube feet (technically podia) are small active tubular projections on the oral face of an echinoderm, whether the arms of a starfish, or the undersides of sea urchins, sand dollars and sea cucumbers; they are more discreet though present on britt ...
and a calcareous
Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.
In zoology
''Calcareous'' is used as an adje ...
endoskeleton consisting of ossicles
The ossicles (also called auditory ossicles) are three bones in either middle ear that are among the smallest bones in the human body. They serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea). The absence of the auditory ...
connected by a mesh of collagen
Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...
fibres. Starfish are included in the subphylum Asterozoa
The Asterozoa are a subphylum in the phylum Echinodermata. Characteristics include a star-shaped body and radially divergent axes of symmetry. The subphylum includes the class Asteroidea (the starfish), the class Ophiuroidea (the brittle stars ...
, the characteristics of which include a flattened, star-shaped body as adults consisting of a central disc and multiple radiating arms. The subphylum includes the two classes of Asteroidea, the starfish, and Ophiuroidea
Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomot ...
, the brittle stars and basket stars. Asteroids have broad-based arms with skeletal support provided by calcareous plates in the body wall while ophiuroids have clearly demarcated slender arms strengthened by paired fused ossicles forming jointed "vertebrae".
The starfish are a large and diverse class with over 1,900 living species. There are seven extant
Extant is the opposite of the word extinct. It may refer to:
* Extant hereditary titles
* Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English
* Extant taxon, a taxon which is not extinct, ...
orders, Brisingida
The Brisingids are deep-sea-dwelling starfish in the order Brisingida.
Description
These starfish have between 6 to 18 long, attenuated arms which they use for suspension feeding. Other characteristics include a single series of marginals, a fu ...
, Forcipulatida
The Forcipulatida are an order of sea stars, containing three families and 49 genera.
Description
Forcipulatids share with the brisingid sea stars distinctive pedicellariae, consisting of a short stalk with three skeletal ossicles. Unlike that ...
, Notomyotida
Benthopectinidae is a family of sea stars containing at least 75 species in eight genera. It is the only family in the monotypic
In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. ...
, Paxillosida
The Paxillosida are a large order (biology), order of sea stars.
Characteristics
Paxillosida adults lack an anus and have no suckers on their tube feet. They do not develop the brachiolaria stage in their early development.Matsubara, M., Komatsu ...
, Spinulosida
The Spinulosida are an order of sea stars containing at least 120 species in seven genera and one family.
Spinulosids completely lack pedicellariae and have a delicate skeletal arrangement. Their name comes from the presence of numerous low spi ...
, Valvatida
The Valvatida are an order of starfish in the class Asteroidea, which contains 695 species in 172 genera in 17 families.
Description
The order encompasses both tiny species, which are only a few millimetres in diameter, like those in the genu ...
and Velatida
The Velatida are an order of sea stars containing about 200 species in five families. These sea stars normally have thick bodies with large discs.
Description and characteristics
This order contains mostly deep or cold seas sea stars, often w ...
and two extinct ones, Calliasterellidae and Trichasteropsida.[ Living asteroids, the Neoasteroidea, are morphologically distinct from their forerunners in the Paleozoic. The taxonomy of the group is relatively stable but there is ongoing debate about the status of the ]Paxillosida
The Paxillosida are a large order (biology), order of sea stars.
Characteristics
Paxillosida adults lack an anus and have no suckers on their tube feet. They do not develop the brachiolaria stage in their early development.Matsubara, M., Komatsu ...
, and the deep-water sea daisies, though clearly Asteroidea and currently included in Velatida
The Velatida are an order of sea stars containing about 200 species in five families. These sea stars normally have thick bodies with large discs.
Description and characteristics
This order contains mostly deep or cold seas sea stars, often w ...
, do not fit easily in any accepted lineage. Phylogenetic data suggests that they may be a sister group
In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree.
Definition
The expression is most easily illustrated by a cladogram:
Taxon A and t ...
, the Concentricycloidea, to the Neoasteroidea, or that the Velatida themselves may be a sister group.[
]
Living groups
; Brisingida
The Brisingids are deep-sea-dwelling starfish in the order Brisingida.
Description
These starfish have between 6 to 18 long, attenuated arms which they use for suspension feeding. Other characteristics include a single series of marginals, a fu ...
(2 families, 17 genera, 111 species)
:Species in this order have a small, inflexible disc and 6–20 long, thin arms, which they use for suspension feeding. They have a single series of marginal plates, a fused ring of disc plates, a reduced number of aboral plates, crossed pedicellariae, and several series of long spines on the arms. They live almost exclusively in deep-sea habitats, although a few live in shallow waters in the Antarctic. In some species, the tube feet have rounded tips and lack suckers.
; Forcipulatida
The Forcipulatida are an order of sea stars, containing three families and 49 genera.
Description
Forcipulatids share with the brisingid sea stars distinctive pedicellariae, consisting of a short stalk with three skeletal ossicles. Unlike that ...
(6 families, 63 genera, 269 species)
:Species in this order have distinctive pedicellariae, consisting of a short stalk with three skeletal ossicles. They tend to have robust bodies and have tube feet with flat-tipped suckers usually arranged in four rows.[ The order includes well-known species from temperate regions, including the ]common starfish
The common starfish, common sea star or sugar starfish (''Asterias rubens'') is the most common and familiar starfish in the north-east Atlantic. Belonging to the family Asteriidae, it has five arms and usually grows to between 10–30 c ...
of North Atlantic coasts and rock pools, as well as cold-water and abyssal species.
; Notomyotida
Benthopectinidae is a family of sea stars containing at least 75 species in eight genera. It is the only family in the monotypic
In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. ...
(1 family, 8 genera, 75 species)
:These starfish are deep-sea dwelling and have particularly flexible arms. The inner dorso-lateral surfaces of the arms contain characteristic longitudinal muscle bands. In some species, the tube feet lack suckers.[
]
; Paxillosida
The Paxillosida are a large order (biology), order of sea stars.
Characteristics
Paxillosida adults lack an anus and have no suckers on their tube feet. They do not develop the brachiolaria stage in their early development.Matsubara, M., Komatsu ...
(7 families, 48 genera, 372 species)
:This is a primitive order and members do not extrude their stomach when feeding, lack an anus and have no suckers on their tube feet. Papulae are plentiful on their aboral surface and they possess marginal plates and paxillae. They mostly inhabit soft-bottomed areas of sand or mud.[Ruppert et al, 2004. pp. 888–889] There is no brachiolaria stage in their larval development. The comb starfish ('' Astropecten polyacanthus'') is a member of this order.
; Spinulosida
The Spinulosida are an order of sea stars containing at least 120 species in seven genera and one family.
Spinulosids completely lack pedicellariae and have a delicate skeletal arrangement. Their name comes from the presence of numerous low spi ...
(1 family, 8 genera, 121 species)
:Most species in this order lack pedicellariae and all have a delicate skeletal arrangement with small or no marginal plates on the disc and arms. They have numerous groups of short spines on the aboral surface.[ This group includes the red starfish '']Echinaster sepositus
''Echinaster sepositus'', the Mediterranean red sea star (sometimes only red sea star, but this name is also used for other species), is a species of starfish from the East Atlantic, including the Mediterranean Sea.Villamor, A.; and M. A. Becerr ...
''.
; Valvatida
The Valvatida are an order of starfish in the class Asteroidea, which contains 695 species in 172 genera in 17 families.
Description
The order encompasses both tiny species, which are only a few millimetres in diameter, like those in the genu ...
(16 families, 172 genera, 695 species)
:Most species in this order have five arms and two rows of tube feet with suckers. There are conspicuous marginal plates on the arms and disc. Some species have paxillae and in some, the main pedicellariae are clamp-like and recessed into the skeletal plates. This group includes the cushion stars, the leather star and the sea daisies.
; Velatida
The Velatida are an order of sea stars containing about 200 species in five families. These sea stars normally have thick bodies with large discs.
Description and characteristics
This order contains mostly deep or cold seas sea stars, often w ...
(4 families, 16 genera, 138 species)
:This order of starfish consists mostly of deep-sea and other cold-water starfish often with a global distribution. The shape is pentagonal or star-shaped with five to fifteen arms. They mostly have poorly developed skeletons with papulae widely distributed on the aboral surface and often spiny pedicellariae. This group includes the slime star.
Extinct groups
Extinct groups within the Asteroidea include:
* † Calliasterellidae, with the type genus '' Calliasterella'' from the Devonian and Carboniferous
* † Palastericus, a Devonian genus
* † Trichasteropsida, with the Triassic
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
genus '' Trichasteropsis'' (at least 2 species)[
]
Phylogeny
External
Starfish are deuterostome animals, like the chordate
A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fiv ...
s. A 2014 analysis of 219 genes from all classes of echinoderms gives the following phylogenetic tree
A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...
. The times at which the clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
s diverged is shown under the labels in millions of years ago (mya).
Internal
The phylogeny of the Asteroidea has been difficult to resolve, with visible (morphological) features proving inadequate, and the question of whether traditional taxa
In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...
are clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
s in doubt.[ The phylogeny proposed by Gale in 1987 is:][
The phylogeny proposed by Blake in 1987 is:][
Later work making use of molecular evidence, with or without the use of morphological evidence, had by 2000 failed to resolve the argument.][ In 2011, on further molecular evidence, Janies and colleagues noted that the phylogeny of the echinoderms "has proven difficult", and that "the overall phylogeny of extant echinoderms remains sensitive to the choice of analytical methods". They presented a phylogenetic tree for the living Asteroidea only; using the traditional names of starfish orders where possible, and indicating "part of" otherwise, the phylogeny is shown below. The Solasteridae are split from the Velatida, and the old Spinulosida is broken up.]
Human relations
In research
Starfish are deuterostomes, closely related, together with all other echinoderms, to chordate
A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fiv ...
s, and are used in reproductive and developmental studies. Female starfish produce large numbers of oocyte
An oocyte (, ), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ...
s that are easily isolated; these can be stored in a pre-meiosis phase and stimulated to complete division by the use of 1-methyladenine. Starfish oocytes are well suited for this research as they are large and easy to handle, transparent, simple to maintain in sea water at room temperature, and they develop rapidly. ''Asterina pectinifera
''Patiria pectinifera'', the blue bat star, is a species of starfish in the family Asterinidae. It is found in the northern Pacific Ocean along the coasts of Japan, China and Russia. It is used as a model organism in developmental biology.
T ...
'', used as a model organism
A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workin ...
for this purpose, is resilient and easy to breed and maintain in the laboratory.
Another area of research is the ability of starfish to regenerate lost body parts. The stem cell
In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...
s of adult humans are incapable of much differentiation and understanding the regrowth, repair and cloning processes in starfish may have implications for human medicine.
Starfish also have an unusual ability to expel foreign objects from their bodies, which makes them difficult to tag for research tracking purposes.
In legend and culture
An aboriginal Australian
Aboriginal Australians are the various Indigenous peoples of the Australian mainland and many of its islands, such as Tasmania, Fraser Island, Hinchinbrook Island, the Tiwi Islands, and Groote Eylandt, but excluding the Torres Strait Islands ...
fable retold by the Welsh school headmaster William Jenkyn Thomas (1870–1959) tells how some animals needed a canoe to cross the ocean. Whale had one but refused to lend it, so Starfish kept him busy, telling him stories and grooming him to remove parasites, while the others stole the canoe. When Whale realized the trick he beat Starfish ragged, which is how Starfish still is today.
In 1900, the scholar Edward Tregear
Edward Robert Tregear , Ordre des Palmes académiques (1846–1931) was a New Zealand public servant and scholar. He was an architect of New Zealand's advanced social reforms and progressive labour legislation during the 1890s.
Biography
He was ...
documented ''The Creation Song'', which he describes as "an ancient prayer for the dedication of a high chief" of Hawaii
Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...
. Among the "uncreated gods" described early in the song are the male Kumulipo ("Creation") and the female Poele, both born in the night, a coral insect, the earthworm, and the starfish.
Georg Eberhard Rumpf
Georg Eberhard Rumphius (originally: Rumpf; baptized c. 1 November 1627 – 15 June 1702) was a German-born botanist employed by the Dutch East India Company in what is now eastern Indonesia, and is best known for his work ''Herbarium Amboinense' ...
's 1705 ''The Ambonese Curiosity Cabinet'' describes the tropical varieties of ''Stella Marina'' or ''Bintang Laut'', "Sea Star", in Latin and Malay
Malay may refer to:
Languages
* Malay language or Bahasa Melayu, a major Austronesian language spoken in Indonesia, Malaysia, Brunei and Singapore
** History of the Malay language, the Malay language from the 4th to the 14th century
** Indonesi ...
respectively, known in the waters around Ambon
Ambon may refer to:
Places
* Ambon Island, an island in Indonesia
** Ambon, Maluku, a city on Ambon Island, the capital of Maluku province
** Governorate of Ambon, a colony of the Dutch East India Company from 1605 to 1796
* Ambon, Morbihan, a c ...
. He writes that the ''Histoire des Antilles'' reports that when the sea stars "see thunder storms approaching, hey
Hey or Hey! may refer to:
Music
* Hey (band), a Polish rock band
Albums
* ''Hey'' (Andreas Bourani album) or the title song (see below), 2014
* ''Hey!'' (Julio Iglesias album) or the title song, 1980
* ''Hey!'' (Jullie album) or the title s ...
grab hold of many small stones with their little legs, looking to ... hold themselves down as if with anchors".
''Starfish'' is the title of novels by Peter Watts and Jennie Orbell, and in 2012, Alice Addison wrote a non-fiction book titled "Starfish - A year in the life of bereavement and depression". ''The Starfish and the Spider
''The Starfish and the Spider: The Unstoppable Power of Leaderless Organizations'' is a 2006 book by Ori Brafman (author of the 2010 book ''Click: The Magic of Instant Connections'') and Rod Beckstrom. It is an exploration of the implications of ...
'' is a 2006 business management
Business administration, also known as business management, is the administration of a commercial enterprise. It includes all aspects of overseeing and supervising the business operations of an organization. From the point of view of management ...
book by Ori Brafman and Rod Beckstrom
Rod Beckstrom (born February 1961) is an American author, high-tech entrepreneur, and former CEO and President of ICANN. He previously served as Director of the National Cybersecurity Center.
Education and early work
Beckstrom received his BA ...
; its title alludes to the ability of the starfish to regenerate itself because of its decentralized nervous system, and the book suggests ways that a decentralized organisation may flourish.
In the Nickelodeon
Nickelodeon (often shortened to Nick) is an American pay television television channel, channel which launched on April 1, 1979, as the first cable channel for children. It is run by Paramount Global through its List of assets owned by Param ...
animated television series
A television show – or simply TV show – is any content produced for viewing on a television set which can be broadcast via over-the-air, satellite television, satellite, or cable television, cable, excluding breaking news, television adverti ...
''SpongeBob SquarePants
''SpongeBob SquarePants'' (or simply ''SpongeBob'') is an American animated comedy television series created by marine science educator and animator Stephen Hillenburg for Nickelodeon. It chronicles the adventures of the title character a ...
'', the eponymous
An eponym is a person, a place, or a thing after whom or which someone or something is, or is believed to be, named. The adjectives which are derived from the word eponym include ''eponymous'' and ''eponymic''.
Usage of the word
The term ''epon ...
character's best friend is a dim-witted starfish, Patrick Star.
As food
Starfish are widespread in the oceans, but are only occasionally used as food. There may be good reason for this: the bodies of numerous species are dominated by bony ossicles, and the body wall of many species contains saponins, which have an unpleasant taste, and others contain tetrodotoxin
Tetrodotoxin (TTX) is a potent neurotoxin. Its name derives from Tetraodontiformes, an order that includes pufferfish, porcupinefish, ocean sunfish, and triggerfish; several of these species carry the toxin. Although tetrodotoxin was discovered ...
s which are poisonous. Some species that prey on bivalve molluscs can transmit paralytic shellfish poisoning. Georg Eberhard Rumpf found few starfish being used for food in the Indonesian archipelago, other than as bait in fish traps, but on the island of "Huamobel" the people cut them up, squeeze out the "black blood" and cook them with sour tamarind leaves; after resting the pieces for a day or two, they remove the outer skin and cook them in coconut milk.[ Starfish are sometimes eaten in China, Japan and in Micronesia.
]
As collectables
Starfish are in some cases taken from their habitat and sold to tourists as souvenirs, Ornament (art), ornaments, curios or for display in aquariums. In particular, ''Oreaster reticulatus'', with its easily accessed habitat and conspicuous coloration, is widely collected in the Caribbean. In the early to mid 20th century, this species was common along the coasts of the West Indies, but collection and trade have severely reduced its numbers. In the Florida, State of Florida, ''O. reticulatus'' is listed as endangered and its collection is illegal. Nevertheless, it is still sold throughout its range and beyond.[ A similar phenomenon exists in the Indo-Pacific for species such as ''Protoreaster nodosus''.
]
In industry and military history
With its multiple arms, the starfish provides a popular metaphor for computer networks, companies and software tools. It is also the name of a Sonar, seabed imaging system and company.
Starfish has repeatedly been chosen as a name in military history. Three ships of the Royal Navy have borne the name HMS ''Starfish'': an HMS Starfish (1894), A-class destroyer launched in 1894; an HMS Starfish (1916), R-class destroyer launched in 1916; and HMS Starfish (19S), an S-class submarine launched in 1933 and lost in 1940. In the World War II, Starfish sites were large-scale night-time Military deception, decoys created during The Blitz to simulate burning British cities. Starfish Prime was a high-altitude nuclear explosion, high-altitude nuclear test conducted by the United States on 9 July 1962.
References
Bibliography
*
*
External links
* , a blog about sea stars by a passionate and professional specialist.
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Asteroidea,
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Late Ordovician first appearances
Extant Ordovician first appearances
Taxa named by Henri Marie Ducrotay de Blainville