Sea anemones are a group of
predatory
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 ...
marine invertebrates of the
order Actiniaria. Because of their colourful appearance, they are named after the ''
Anemone
''Anemone'' () is a genus of flowering plants in the buttercup family Ranunculaceae. Plants of the genus are commonly called windflowers. They are native to the temperate and subtropical regions of all continents except Australia, New Zealand an ...
'', a terrestrial flowering plant. Sea anemones are classified in the phylum
Cnidaria
Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter.
Their distinguishing feature is cnidocytes, specialized cells that th ...
, class
Anthozoa
Anthozoa is a subphylum of marine invertebrates which includes the sea anemones, Scleractinia, stony corals and Alcyonacea, soft corals. Adult anthozoans are almost all attached to the seabed, while their larvae can disperse as part of the plank ...
, subclass
Hexacorallia
Hexacorallia is a Class (biology), class of Anthozoa comprising approximately 4,300 species of aquatic organisms formed of polyp (zoology), polyps, generally with 6-fold symmetry. It includes all of the stony corals, most of which are Colony (b ...
. As
cnidaria
Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter.
Their distinguishing feature is cnidocytes, specialized cells that th ...
ns, sea anemones are related to
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 ...
s,
jellyfish
Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella- ...
,
tube-dwelling anemone
Tube-dwelling anemones or ceriantharians look very similar to sea anemones but belong to an entirely different Class (biology), class of anthozoans. They are solitary, living buried in soft sediments. Tube anemones live inside and can withdraw i ...
s, and ''
Hydra''. Unlike jellyfish, sea anemones do not have a
medusa
In Greek mythology, Medusa (; Ancient Greek: Μέδουσα "guardian, protectress"), also called Gorgo, was one of the three monstrous Gorgons, generally described as winged human females with living venomous snakes in place of hair. Those ...
stage in their life cycle.
A typical sea anemone is a single
polyp attached to a hard surface by its base, but some species live in soft sediment, and a few float near the surface of the water. The polyp has a columnar trunk topped by an oral disc with a ring of
tentacle
In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...
s and a central mouth. The tentacles can be retracted inside the body cavity or expanded to catch passing prey. They are armed with
cnidocyte
A cnidocyte (also known as a cnidoblast or nematocyte) is an explosive cell containing one large secretory organelle called a cnidocyst (also known as a cnida () or nematocyst) that can deliver a sting to other organisms. The presence of this ce ...
s (stinging cells). In many species, additional nourishment comes from a
symbiotic relationship
Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
with
single-celled dinoflagellate
The dinoflagellates (Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates are ...
s, with
zooxanthellae
Zooxanthellae is a colloquial term for single-celled dinoflagellates that are able to live in symbiosis with diverse marine invertebrates including demosponges, corals, jellyfish, and nudibranchs. Most known zooxanthellae are in the genus ''Sy ...
, or with green algae,
zoochlorella
''Zoochlorella'' is a ''nomen rejiciendum'' for a genus of green algae assigned to ''Chlorella''. The term zoochlorella (plural zoochlorellae) is sometimes used to refer to any green algae that lives symbiotically within the body of a freshwater ...
e, that live within the cells. Some species of sea anemone live in association with
clownfish
Clownfish or anemonefish are fishes from the subfamily Amphiprioninae in the family Pomacentridae. Thirty species of clownfish are recognized: one in the genus ''Premnas'', while the remaining are in the genus ''Amphiprion''. In the wild, th ...
,
hermit crab
Hermit crabs are anomuran decapod crustaceans of the superfamily Paguroidea that have adapted to occupy empty scavenged mollusc shells to protect their fragile exoskeletons. There are over 800 species of hermit crab, most of which possess an a ...
s, small fish, or other animals to their
mutual benefit.
Sea anemones breed by liberating sperm and eggs through the mouth into the sea. The resulting fertilized eggs develop into
planula
A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species and also in some species of Ctenophores. Some groups of Nemerteans also produce larvae that are very similar to the planula, which ...
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 which, after being
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 ...
ic for a while, settle on the seabed and develop directly into juvenile polyps. Sea anemones also breed
asexually, by breaking in half or into smaller pieces which regenerate into polyps. Sea anemones are sometimes kept in
reef aquarium
A reef aquarium or reef tank is a marine aquarium that prominently displays live corals and other marine invertebrates as well as fish that play a role in maintaining the tropical coral reef environment. A reef aquarium requires appropriately ...
s; the global trade in marine ornamentals for this purpose is expanding and threatens sea anemone populations in some localities, as the trade depends on collection from the wild.
Anatomy
file:Sea Anemone Structure.svg, Sea anemone anatomy.
1. Tentacles 2. Mouth 3. Retracting muscles 4. Gonads 5. Acontial filaments 6. Pedal disk 7. Ostium 8. Coelenteron 9. Sphincter muscle 10. Mesentery 11. Column 12. Pharynx
A typical sea anemone is a sessile
polyp attached at the base to the surface beneath it by an adhesive foot, called a basal or
pedal disc
The pedal disc (also known as a basal plate) is the surface opposite to the mouth of the sea anemone. IIt serves to attach the anemone to the substrate, or hard surface, upon which it lives.
The pedal disc is composed of a thin tissue plate an ...
, with a column-shaped body topped by an oral disc. Most are from in diameter and in length, but they are inflatable and vary greatly in dimensions. Some are very large; ''
Urticina columbiana
''Urticina columbiana'', common names crusty red anemone, Columbia sand anemone, sand anemone, and the sand-rose anemone, is a species of sea anemone in the family Actiniidae.
Description
This species can grow to 25 cm high and can reach a ...
'' and ''
Stichodactyla mertensii'' can both exceed in diameter and ''
Metridium farcimen
''Metridium farcimen'' is a species of sea anemone in the Family (biology), family Metridiidae. It is commonly known as the giant plumose anemone or white-plumed anemone. It is found in the eastern Pacific Ocean from Alaska down to Santa Catalin ...
'' a metre in length.
[ Some species burrow in soft sediment and lack a basal disc, having instead a bulbous lower end, the physa, which anchors them in place.][
The column or trunk is generally more or less cylindrical and may be plain and smooth or may bear specialised structures; these include solid papillae (fleshy protuberances), adhesive papillae, cinclides (slits), and small protruding ]vesicles
Vesicle may refer to:
; In cellular biology or chemistry
* Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane
* Synaptic vesicle
; In human embryology
* Vesicle (embryology), bulge-like features o ...
. In some species the part immediately below the oral disc is constricted and is known as the capitulum. When the animal contracts, the oral disc, tentacles and capitulum fold inside the pharynx and are held in place by a strong 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 ...
muscle part way up the column. There may be a fold in the body wall, known as a parapet, at this point, and this parapet covers and protects the anemone when it is retracted.[
The oral disc has a central mouth, usually slit-shaped, surrounded by one or more whorls of tentacles. The ends of the slit lead to grooves in the wall of the pharynx known as ]siphonoglyph The siphonoglyph is a ciliated groove at one or both ends of the mouth of sea anemones and some corals. The siphonoglyph extends into a pharynx and is used to create currents of water into the pharynx. These water currents are important for res ...
s; there are usually two of these grooves, but some groups have a single one. The tentacles are generally tapered and often tipped by a pore, but in some species they are branched, club-tipped, or reduced to low knobs.[ The tentacles are armed with many ]cnidocyte
A cnidocyte (also known as a cnidoblast or nematocyte) is an explosive cell containing one large secretory organelle called a cnidocyst (also known as a cnida () or nematocyst) that can deliver a sting to other organisms. The presence of this ce ...
s, cells that are both defensive and used to capture prey. Cnidocytes contain stinging nematocysts, capsule-like organelle
In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...
s capable of everting suddenly, giving the phylum Cnidaria its name. Each nematocyst contains a small venom vesicle filled with actinotoxins, an inner filament, and an external sensory hair. A touch to the hair mechanically triggers a cell explosion, which launches a harpoon-like structure that attaches to the organism that triggered it, and injects a dose of venom in the flesh of the aggressor or prey.
At the base of the tentacles in some species lie acrorhagi, elongated inflatable tentacle-like organs armed with cnidocytes, that can flail around and fend off other encroaching anemones; one or both anemones can be driven off or suffer injury in such battles.[
]
The venom is a mix of toxin
A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849– ...
s, including neurotoxin
Neurotoxins are toxins that are destructive to nerve tissue (causing neurotoxicity). Neurotoxins are an extensive class of exogenous chemical neurological insultsSpencer 2000 that can adversely affect function in both developing and mature ner ...
s, that paralyzes the prey so the anemone can move it to the mouth for digestion inside the gastrovascular cavity
The gastrovascular cavity is the primary organ of digestion and circulation in two major animal phyla: the Coelenterates or cnidarians (including jellyfish and corals) and Platyhelminthes (flatworms). The cavity may be extensively branched into ...
. Actinotoxins are highly toxic to prey species of fish and crustacean
Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...
s. However, Amphiprioninae
Clownfish or anemonefish are fishes from the subfamily Amphiprioninae in the family Pomacentridae. Thirty species of clownfish are recognized: one in the genus '' Premnas'', while the remaining are in the genus '' Amphiprion''. In the wild, t ...
(clownfish), small banded fish in various colours, are not affected by their host anemone's sting and shelter themselves from predators among its tentacles. Several other species have similar adaptions and are also unaffected (see '' Mutualistic relationships''). Most sea anemones are harmless to humans, but a few highly toxic species (notably '' Actinodendron arboreum'', '' Phyllodiscus semoni'' and '' Stichodactyla'' spp.) have caused severe injuries and are potentially lethal.
Digestive system
Sea anemones have what can be described as an incomplete gut; the gastrovascular cavity functions as a stomach and possesses a single opening to the outside, which operates as both a mouth
In animal anatomy, the mouth, also known as the oral cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on ...
and 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 ...
. Waste and undigested matter is excreted through this opening. The mouth is typically slit-like in shape, and bears a groove at one or both ends. The groove, termed a ''siphonoglyph'', is ciliated
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 ...
, and helps to move food particles inwards and circulate water through the gastrovascular cavity.
The mouth opens into a flattened pharynx
The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...
. This consists of an in-folding of the body wall, and is therefore lined by the animal's 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 ...
. The pharynx typically runs for about one third the length of the body before opening into the gastrovascular cavity that occupies the remainder of the body.[
The gastrovascular cavity itself is divided into a number of chambers by ]mesenteries
In zoology, a mesentery is a membrane inside the body cavity of an animal. The term identifies different structures in different phyla: in vertebrates it is a double fold of the peritoneum enclosing the intestines; in other organisms it forms com ...
radiating inwards from the body wall. Some of the mesenteries form complete partitions with a free edge at the base of the pharynx, where they connect, but others reach only partway across. The mesenteries are usually found in multiples of twelve, and are symmetrically arranged around the central lumen. They have stomach lining on both sides, separated by a thin layer of mesoglea
Mesoglea refers to the extracellular matrix found in cnidarians like coral or jellyfish that functions as a hydrostatic skeleton. It is related to but distinct from mesohyl, which generally refers to extracellular material found in sponges.
Desc ...
, and include filaments of tissue specialised for secreting digestive enzyme
Digestive enzymes are a group of enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their absorption into the cells of the body. Digestive enzymes are found in the digestive tracts of anim ...
s. In some species, these filaments extend below the lower margin of the mesentery, hanging free in the gastrovascular cavity as thread-like acontial filaments. These acontia are armed with nematocysts and can be extruded through cinclides, blister-like holes in the wall of the column, for use in defence.
Musculature and nervous system
A primitive nervous system, without centralization, coordinates the processes involved in maintaining homeostasis
In biology, homeostasis (British English, British also homoeostasis) Help:IPA/English, (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physics, physical, and chemistry, chemical conditions maintained by organism, living systems. Thi ...
, as well as biochemical and physical responses to various stimuli. There are two nerve nets, one in the epidermis and one in the gastrodermis; these unite at the pharynx, the junctions of the septa with the oral disc and the pedal disc, and across the mesogloea. No specialized sense organs are present, but sensory cells include nematocytes and chemoreceptor
A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance (endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorecept ...
s.[
The muscles and nerves are much simpler than those of most other animals, although more specialised than in other cnidarians, such as corals. Cells in the outer layer (epidermis) and the inner layer (]gastrodermis The gastrodermis is the inner layer of cells that serves as a lining membrane of the gastrovascular cavity of Cnidarians. The term is also used for the analogous inner epithelial layer of Ctenophore
Ctenophora (; ctenophore ; ) comprise a ph ...
) have microfilaments that group into contractile fibers. These fibers are not true muscles because they are not freely suspended in the body cavity as they are in more developed animals. Longitudinal fibres are found in the tentacles and oral disc, and also within the mesenteries, where they can contract the whole length of the body. Circular fibers are found in the body wall and, in some species, around the oral disc, allowing the animal to retract its tentacles into a protective 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 ...
.[
Since the anemone lacks a rigid skeleton, the contractile cells pull against the fluid in the gastrovascular cavity, forming a hydrostatic skeleton. The anemone stabilizes itself by flattening its pharynx, which acts as a valve, keeping the gastrovascular cavity at a constant volume and making it rigid. When the longitudinal muscles relax, the pharynx opens and the cilia lining the siphonoglyphs beat, wafting water inwards and refilling the gastrovascular cavity. In general, the sea anemone inflates its body to extend its tentacles and feed, and deflates it when resting or disturbed. The inflated body is also used to anchor the animal inside a crevice, burrow or tube.][
]
Life cycle
Unlike other cnidarians, anemones (and other anthozoa
Anthozoa is a subphylum of marine invertebrates which includes the sea anemones, Scleractinia, stony corals and Alcyonacea, soft corals. Adult anthozoans are almost all attached to the seabed, while their larvae can disperse as part of the plank ...
ns) entirely lack the free-swimming medusal stage of their life cycle; the polyp produces eggs and sperm, and the fertilized egg develops into a planula larva, which develops directly into another polyp. Both sexual
Sex is the biological distinction of an organism between male and female.
Sex or SEX may also refer to:
Biology and behaviour
*Animal sexual behaviour
**Copulation (zoology)
**Human sexual activity
**Non-penetrative sex, or sexual outercourse
** ...
and asexual reproduction
Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the fu ...
can occur.[
The sexes in sea anemones are separate in some species, while other species are sequential hermaphrodites, changing sex at some stage in their life. The ]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 are strips of tissue within the mesenteries
In zoology, a mesentery is a membrane inside the body cavity of an animal. The term identifies different structures in different phyla: in vertebrates it is a double fold of the peritoneum enclosing the intestines; in other organisms it forms com ...
.[ In sexual reproduction, males may release sperm to stimulate females to release eggs, and fertilization occurs, either internally in the gastrovascular cavity or 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 ...
. The eggs and sperm, or the larvae, usually emerge through the mouth,[ but in some species, such as '']Metridium dianthus
''Metridium dianthus'' is a species of sea anemone in the family Metridiidae. It is found in the northern Atlantic Ocean (including the coasts of the British Isles) and in the northeast Pacific Ocean. There is also a record from South Africa, po ...
'', may be swept out from the body cavity through the cinclides. In many species the eggs and sperm rise to the surface where fertilisation occurs. The fertilized egg develops into a planula larva, which drifts for a while before sinking to the seabed and undergoing 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 ...
into a juvenile sea anemone. Some larvae preferentially settle onto certain suitable substrates; the mottled anemone (''Urticina crassicornis'') for example, settles onto green algae, perhaps attracted by a biofilm
A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular ...
on the surface.
The brooding anemone (''Epiactis prolifera'') is gynodioecious, starting life as a female and later becoming hermaphroditic, so that populations consist of females and hermaphrodites. As a female, the eggs can develop parthenogenetically into female offspring without fertilisation, and as a hermaphrodite, the eggs are routinely self-fertilised.[ The larvae emerge from the anemone's mouth and tumble down the column, lodging in a fold near the pedal disc. Here they develop and grow, remaining for about three months before crawling off to start independent lives.][
Sea anemones have great powers of regeneration and can reproduce asexually, 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 ...
, fragmentation, or longitudinal or transverse binary fission
Binary may refer to:
Science and technology Mathematics
* Binary number, a representation of numbers using only two digits (0 and 1)
* Binary function, a function that takes two arguments
* Binary operation, a mathematical operation that t ...
. Some species such as certain ''Anthopleura
''Anthopleura'' is a genus of sea anemones, of the family Actiniidae.
Etymology
Anthopleura is Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek lang ...
'' divide longitudinally, pulling themselves apart, resulting in groups of individuals with identical colouring and markings. Transverse fission is less common, but occurs in '' Anthopleura stellula'' and '' Gonactinia prolifera'', with a rudimentary band of tentacles appearing halfway up the column before it splits horizontally. Some species can also reproduce by pedal laceration. In this process, a ring of material may break off from the pedal disc
The pedal disc (also known as a basal plate) is the surface opposite to the mouth of the sea anemone. IIt serves to attach the anemone to the substrate, or hard surface, upon which it lives.
The pedal disc is composed of a thin tissue plate an ...
at the base of the column, which then fragments, the pieces regenerating into new clonal individuals. Alternatively, fragments detach separately as the animal creeps across a surface. In ''Metridium dianthus
''Metridium dianthus'' is a species of sea anemone in the family Metridiidae. It is found in the northern Atlantic Ocean (including the coasts of the British Isles) and in the northeast Pacific Ocean. There is also a record from South Africa, po ...
'', fragmentation rates were higher in individuals living among live mussel
Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and Freshwater bivalve, freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other ...
s than among dead shells, and all the new individuals had tentacles within three weeks.
The sea anemone '' Aiptasia diaphana'' displays sexual plasticity. Thus asexually produced clones derived from a single founder individual can contain both male and female individuals (ramets). When eggs and sperm (gametes) are formed, they can produce zygote
A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism.
In multicellula ...
s derived from "selfing" (within the founding clone) or out-crossing, which then develop into swimming planula larvae. Anemones tend to grow and reproduce relatively slowly. The magnificent sea anemone (''Heteractis magnifica''), for example, may live for decades, with one individual surviving in captivity for eighty years.
Behaviour and ecology
Movement
A sea anemone is capable of changing its shape dramatically. The column and tentacles have longitudinal, transverse and diagonal sheets of muscle and can lengthen and contract, as well as bend and twist. The gullet and mesenteries can evert (turn inside out), or the oral disc and tentacles can retract inside the gullet, with the sphincter closing the aperture; during this process, the gullet folds transversely and water is discharged through the mouth.
Locomotion
Although some species of sea anemone burrow in soft sediment, the majority are mainly sessile
Sessility, or sessile, may refer to:
* Sessility (motility), organisms which are not able to move about
* Sessility (botany), flowers or leaves that grow directly from the stem or peduncle of a plant
* Sessility (medicine), tumors and polyps that ...
, attaching to a hard surface with their pedal disc, and tend to stay in the same spot for weeks or months at a time. They can move, however, being able to creep around on their bases; this gliding can be seen with time-lapse photography but the motion is so slow as to be almost imperceptible to the naked eye.[ The process resembles the locomotion of a ]gastropod
The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda ().
This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...
mollusc, a wave of contraction moving from the functionally posterior portion of the foot towards the front edge, which detaches and moves forwards. Sea anemones can also cast themselves loose from the substrate and drift to a new location. '' Gonactinia prolifera'' is unusual in that it can both walk and swim; walking is by making a series of short, looping steps, rather like a caterpillar, attaching its tentacles to the substrate and drawing its base closer; swimming is done by rapid movements of the tentacles beating synchronously like oar strokes. '' Stomphia coccinea'' can swim by flexing its column, and the sea onion anemone inflates and casts itself loose, adopting a spherical shape and allowing itself to be rolled about by the waves and currents.[ There are no truly ]pelagic
The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or wa ...
sea anemones, but some stages in the life cycle post-metamorphosis are able, in response to certain environmental factors, to cast themselves off and have a free-living stage that aids in their dispersal.
The sea onion '' Paranthus rapiformis'' lives on subtidal mud flats and burrows into the sediment, holding itself in place by expanding its basal disc to form an anchor. If it gets washed out of its burrow by strong currents, it contracts into a pearly glistening ball which rolls about. Tube-dwelling anemone
Tube-dwelling anemones or ceriantharians look very similar to sea anemones but belong to an entirely different Class (biology), class of anthozoans. They are solitary, living buried in soft sediments. Tube anemones live inside and can withdraw i ...
s, which live in parchment-like tubes, are in the anthozoan subclass Ceriantharia
Tube-dwelling anemones or ceriantharians look very similar to sea anemones but belong to an entirely different class of anthozoans. They are solitary, living buried in soft sediments. Tube anemones live inside and can withdraw into tubes, which ...
, and are only distantly related to sea anemones.
Feeding and diet
Sea anemones are typically 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 ...
, ensnaring prey of suitable size that comes within reach of their tentacles and immobilizing it with the aid of their nematocysts
A cnidocyte (also known as a cnidoblast or nematocyte) is an explosive cell containing one large secretory organelle called a cnidocyst (also known as a cnida () or nematocyst) that can deliver a sting to other organisms. The presence of this c ...
. The prey is then transported to the mouth and thrust into the pharynx. The lips can stretch to aid in prey capture and can accommodate larger items such as crab
Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all the ...
s, dislodged mollusc
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...
s and even small fish. ''Stichodactyla helianthus
''Stichodactyla helianthus'', commonly known as sun anemone, is a sea anemone of the family Stichodactylidae. ''Helianthus'' stems from the Greek words ἡλιος (meaning sun), and ἀνθος, meaning flower. ''S. helianthus'' is a large, gr ...
'' is reported to trap 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 by enfolding them in its carpet-like oral disc.[ A few species are ]parasitic
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has c ...
on other marine organisms. One of these is '' Peachia quinquecapitata'', the larvae of which develop inside the medusa
In Greek mythology, Medusa (; Ancient Greek: Μέδουσα "guardian, protectress"), also called Gorgo, was one of the three monstrous Gorgons, generally described as winged human females with living venomous snakes in place of hair. Those ...
e of jellyfish, feeding on their gonads and other tissues, before being liberated into the sea as free-living juvenile anemones.
Mutualistic relationships
Although not plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...
s and therefore incapable of photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
themselves, many sea anemones form an important facultative mutualistic relationship with certain single-celled 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 ...
species that reside in the animals' gastrodermal cells, especially in the tentacles and oral disc. These algae may be either zooxanthellae
Zooxanthellae is a colloquial term for single-celled dinoflagellates that are able to live in symbiosis with diverse marine invertebrates including demosponges, corals, jellyfish, and nudibranchs. Most known zooxanthellae are in the genus ''Sy ...
, zoochlorella
''Zoochlorella'' is a ''nomen rejiciendum'' for a genus of green algae assigned to ''Chlorella''. The term zoochlorella (plural zoochlorellae) is sometimes used to refer to any green algae that lives symbiotically within the body of a freshwater ...
e or both. The sea anemone benefits from the products of the algae's photosynthesis, namely 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 ...
and food in the form of glycerol
Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
, 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 alanine
Alanine (symbol Ala or A), or α-alanine, is an α-amino acid that is used in the biosynthesis of proteins. It contains an amine group and a carboxylic acid group, both attached to the central carbon atom which also carries a methyl group side c ...
; the algae in turn are assured a reliable exposure to sunlight and protection from micro-feeders, which the sea anemones actively maintain. The algae also benefit by being protected by the sea anemone's stinging cells, reducing the likelihood of being eaten by herbivores. In the aggregating anemone
The aggregating anemone (''Anthopleura elegantissima''), or clonal anemone, is the most abundant species of sea anemone found on rocky, tide swept shores along the Pacific coast of North America. This cnidarian hosts endosymbiotic algae calle ...
(''Anthopleura elegantissima''), the colour of the anemone is largely dependent on the proportions and identities of the zooxanthellae and zoochlorellae present.[ The hidden anemone (''Lebrunia coralligens'') has a whorl of seaweed-like pseudotentacles, rich in zooxanthellae, and an inner whorl of tentacles. A daily rhythm sees the pseudotentacles spread widely in the daytime for photosynthesis, but they are retracted at night, at which time the tentacles expand to search for prey.]
Several species of fish and invertebrate
Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
s live in symbiotic or mutualistic relationships with sea anemones, most famously the clownfish
Clownfish or anemonefish are fishes from the subfamily Amphiprioninae in the family Pomacentridae. Thirty species of clownfish are recognized: one in the genus ''Premnas'', while the remaining are in the genus ''Amphiprion''. In the wild, th ...
. The symbiont receives the protection from predators provided by the anemone's stinging cells, and the anemone utilises the nutrients present in its faeces. Other animals that associate with sea anemones include cardinalfish
Cardinalfishes are a family, Apogonidae, of ray-finned fishes found in the Atlantic, Indian, and Pacific Oceans; they are chiefly marine, but some species are found in brackish water and a few (notably '' Glossamia'') are found in fresh water. A ...
(such as Banggai cardinalfish
The Banggai cardinalfish (''Pterapogon kauderni'') is a small tropical cardinalfish in the family Apogonidae. It is the only member of its genus. This attractive fish is popular in the aquarium trade. It is among the relatively few marine fish ...
), juvenile threespot dascyllus
The threespot dascyllus (''Dascyllus trimaculatus''), also known as the domino damsel or simply domino, is a species of damselfish from the family Pomacentridae. It is native to the Indo-Pacific from the Red Sea and East Africa, to the Pitcairn ...
, incognito (or anemone) goby, juvenile painted greenling, various crabs (such as '' Inachus phalangium'', '' Mithraculus cinctimanus'' and ''Neopetrolisthes
''Neopetrolisthes'' is a genus of porcelain crabs that live on sea anemones., and contains the following three species:
* ''Neopetrolisthes maculatus'' (H. Milne-Edwards, 1837) – ''N. ohshimai'' Miyake, 1937 is a subjective synonym
The Bota ...
''), shrimp (such as certain '' Alpheus'', '' Lebbeus'', ''Periclimenes
''Periclimenes'', commonly known as glass shrimp or cleaner shrimp, is a commensal and often symbiotic genus of semi-transparent shrimp within the family Palaemonidae. Species of this large genus feature a wide variety of coloration and patterns, ...
'' and ''Thor
Thor (; from non, Þórr ) is a prominent god in Germanic paganism. In Norse mythology, he is a hammer-wielding æsir, god associated with lightning, thunder, storms, sacred trees and groves in Germanic paganism and mythology, sacred groves ...
''),[Debelius, H. (2001). ''Crustacea Guide of the World.'' ] opossum shrimp
Mysida is an order of small, shrimp-like crustaceans in the malacostracan superorder Peracarida. Their common name opossum shrimps stems from the presence of a brood pouch or "marsupium" in females. The fact that the larvae are reared in ...
(such as '' Heteromysis'' and '' Leptomysis''), and various marine 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 G ...
s.[Mercier, A.; and J. Hamel (2008). ''Nature and role of newly described symbiotic associations between a sea anemone and gastropods at bathyal depths in the NW Atlantic.'' Journal of Experimental Marine Biology and Ecology 358(1): 57–69.][Goodwill, R.H.; D.G. Fautin; J. Furey; and M. Daly (2009). ''A sea anemone symbiotic with gastropods of eight species in the Mariana Islands.'' Micronesica 41(1): 117–130.][Ates, R.M.L. (1997). ''Gastropod carrying actinians.'' In: J. C. den Hartog, eds, Proceedings of the 6th International Conference on Coelenterate Biology, 1995, pp. 11–20. National Naturhistorisch Museum, Leiden, The Netherlands.]
Two of the more unusual relationships are those between certain anemones (such as '' Adamsia'', ''Calliactis
''Calliactis'' is a genus of sea anemones. Species in this genus are Symbiosis, mutually symbiotic with hermit crabs. The anemone gets a place to live and discarded scraps of the crab's food in exchange for its help in defending the crab.
Speci ...
'' and '' Neoaiptasia'') and hermit crab
Hermit crabs are anomuran decapod crustaceans of the superfamily Paguroidea that have adapted to occupy empty scavenged mollusc shells to protect their fragile exoskeletons. There are over 800 species of hermit crab, most of which possess an a ...
s or snails, and '' Bundeopsis'' or '' Triactis'' anemones and ''Lybia
''Lybia'' is a genus of small crabs in the family Xanthidae. Their common names include boxer crabs, boxing crabs and pom-pom crabs. They are notable for their mutualism with sea anemones, which they hold in their claws for defense. In retur ...
'' boxing crabs. In the former, the anemones live on the shell of the hermit crab or snail.[ In the latter, the small anemones are carried in the claws of the boxing crab.][
]
Habitats
Sea anemones are found in both deep oceans and shallow coastal waters worldwide. The greatest diversity is in the tropics, although there are many species adapted to relatively cold waters. The majority of species cling on to rocks, shells or submerged timber, often hiding in cracks or under seaweed, but some burrow into sand and mud, and a few are pelagic
The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or wa ...
.[
]
Relationship with humans
Sea anemones and their attendant anemone fish can make attractive aquarium exhibits, and both are often harvested from the wild as adults or juveniles. These fishing activities significantly impact the populations of anemones and anemone fish by drastically reducing the densities of each in exploited areas. Besides their collection from the wild for use in reef aquaria, sea anemones are also threatened by alterations to their environment. Those living in shallow-water coastal locations are affected directly by pollution and siltation, and indirectly by the effect these have on their photosynthetic symbionts and the prey on which they feed.
In southwestern Spain and Sardinia, the snakelocks anemone
The snakelocks anemone (''Anemonia viridis'') is a sea anemone found in the eastern Atlantic Ocean and the Mediterranean Sea. The latter population is however sometimes considered a separate species, the Mediterranean snakelocks anemone (''Anem ...
(''Anemonia viridis'') is consumed as a delicacy. The whole animal is marinated in vinegar, then coated in a batter similar to that used to make calamari
Squid is eaten in many cuisines; in English, the culinary name calamari is often used for squid dishes.'' Oxford English Dictionary'', 3rd edition, 2002''s.v.''/ref> There are many ways to prepare and cook squid. Fried squid is common in the ...
, and deep-fried in olive oil. Anemones are also a source of food for fisherman communities in the east coast of Sabah
Sabah () is a States and federal territories of Malaysia, state of Malaysia located in northern Borneo, in the region of East Malaysia. Sabah borders the Malaysian state of Sarawak to the southwest and the North Kalimantan province of Indone ...
, Borneo
Borneo (; id, Kalimantan) is the third-largest island in the world and the largest in Asia. At the geographic centre of Maritime Southeast Asia, in relation to major Indonesian islands, it is located north of Java, west of Sulawesi, and eas ...
, as well as the Thousand Islands
The Thousand Islands (french: Mille-Îles) constitute a North American archipelago of 1,864 islands that straddles the Canada–US border in the Saint Lawrence River as it emerges from the northeast corner of Lake Ontario. They stretch for abo ...
(as ''rambu-rambu'') in Southeast Asia.
Fossil record
Most Actiniaria do not form hard parts that can be recognized as fossils, but a few fossils of sea anemones do exist; '' Mackenzia'', from the Middle Cambrian
Middle or The Middle may refer to:
* Centre (geometry), the point equally distant from the outer limits.
Places
* Middle (sheading), a subdivision of the Isle of Man
* Middle Bay (disambiguation)
* Middle Brook (disambiguation)
* Middle Creek (di ...
Burgess Shale
The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest foss ...
of Canada, is the oldest fossil identified as a sea anemone.
Taxonomy
Sea anemones, order Actiniaria, are classified in the phylum Cnidaria
Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter.
Their distinguishing feature is cnidocytes, specialized cells that th ...
, class Anthozoa
Anthozoa is a subphylum of marine invertebrates which includes the sea anemones, Scleractinia, stony corals and Alcyonacea, soft corals. Adult anthozoans are almost all attached to the seabed, while their larvae can disperse as part of the plank ...
, subclass Hexacorallia
Hexacorallia is a Class (biology), class of Anthozoa comprising approximately 4,300 species of aquatic organisms formed of polyp (zoology), polyps, generally with 6-fold symmetry. It includes all of the stony corals, most of which are Colony (b ...
. Rodriguez et al. proposed a new classification for the Actiniaria based on extensive DNA results.
Suborders and superfamilies included in Actiniaria are:
* Suborder Anenthemonae
Anenthemonae is a suborder of sea anemones in the order Actiniaria
Sea anemones are a group of predatory marine invertebrates of the order Actiniaria. Because of their colourful appearance, they are named after the ''Anemone'', a terrest ...
** Superfamily Edwardsioidea
** Superfamily Actinernoidea
* Suborder Enthemonae
The Enthemonae is a suborder of sea anemones in the order Actiniaria. It comprises those sea anemones with typical arrangement of mesenteries for actiniarians.Rodríguez, E., Barbeitos, M. S., Brugler, M.R., Crowley, L. M., Grajales, A., Gusm ...
** Superfamily Actinostoloidea
** Superfamily Actinioidea
** Superfamily Metridioidea
Phylogeny
External relationships
Anthozoa
Anthozoa is a subphylum of marine invertebrates which includes the sea anemones, Scleractinia, stony corals and Alcyonacea, soft corals. Adult anthozoans are almost all attached to the seabed, while their larvae can disperse as part of the plank ...
contains three subclasses: Hexacorallia
Hexacorallia is a Class (biology), class of Anthozoa comprising approximately 4,300 species of aquatic organisms formed of polyp (zoology), polyps, generally with 6-fold symmetry. It includes all of the stony corals, most of which are Colony (b ...
, which contains the Actiniaria; Octocorallia
Octocorallia (also known as Alcyonaria) is a class of Anthozoa comprising around 3,000 species of water-based organisms formed of colonial polyps with 8-fold symmetry. It includes the blue coral, soft corals, sea pens, and gorgonians (sea fans ...
; and Ceriantharia
Tube-dwelling anemones or ceriantharians look very similar to sea anemones but belong to an entirely different class of anthozoans. They are solitary, living buried in soft sediments. Tube anemones live inside and can withdraw into tubes, which ...
. These are monophyletic
In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...
, but the relationships within the subclasses remain unresolved.
†= extinct
Internal relationships
The relationships of higher-level taxa in Carlgren's[Carlgren O. (1949). A survey of the Ptychodactiaria, Corallimorpharia and Actiniaria. K Svenska VetenskapsAkad Handl 1: 1–121.] classification are re-interpreted as follows:[
]
See also
* AETX
AETX refers to a group of polypeptide neurotoxins isolated from the sea anemone ''Anemonia erythraea'' that target ion channels, altering their function. Four subtypes have been identified: AETX I, II, III and K, which vary in their structure and ...
* Cangitoxin
Cangitoxin, also known as CGTX or CGX, is a toxin purified from the venom of the sea anemone ''Bunodosoma cangicum'', which most likely acts by prolonging the inactivation of voltage-gated sodium channels (NaV channels).
Sources
Cangitoxin is a po ...
* Halcurin Halcurin is a polypeptide neurotoxin from the sea anemone '' Halcurias'' sp. Based on sequence homology to type 1 and type 2 sea anemone toxins it is thought to delay channel inactivation by binding to the extracellular site 3 on the voltage gated s ...
* Sea anemone dermatitis
* Sea anemone neurotoxin
Sea anemone neurotoxin is the name given to neurotoxins produced by sea anemones with related structure and function.
Sea anemone neurotoxins can be divided in two functional groups that either specifically target the sodium channel or the pota ...
References
External links
Order Actiniaria
Actiniaria.com
* ttp://www.news.ucdavis.edu/search/news_detail.lasso?id=7438 Anemone Armies Battle to a Standoff
Sea anemones look like sea flowers but they are animals of the Phylum Cnidaria
Photographic Database of Cambodian Sea Anemones
{{Good article
Hexacorallia
Negligibly senescent organisms
Symbiosis