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Sea anemones are the marine,
predatory Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical envi ...

predatory
animals of the
order Order, ORDER or Orders may refer to: * Orderliness Orderliness is a quality that is characterized by a person’s interest in keeping their surroundings and themselves well organized, and is associated with other qualities such as cleanliness a ...
Actiniaria. They are named after the
anemone ''Anemone'' () is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circ ...

anemone
, a terrestrial
flower A flower, sometimes known as a bloom or blossom Image:Cerisier du Japon Prunus serrulata.jpg, Cherry blossoms in Paris in full bloom. In botany, blossoms are the flowers of stone fruit fruit tree, trees (genus ''Prunus'') and of some other plan ...

flower
ing plant, because of the colourful appearance of many. Sea anemones are classified in the phylum
Cnidaria Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, ...

Cnidaria
, class
Anthozoa Anthozoa is a 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 co ...

Anthozoa
, subclass
Hexacorallia Hexacorallia is a class of Anthozoa comprising approximately 4,300 species of aquatic organisms formed of polyps, generally with 6-fold symmetry. It includes all of the stony coral Corals are marine invertebrates within the class (biology), ...
. As
cnidaria Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, ...

cnidaria
ns, sea anemones are related to
coral Corals 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 ver ...

coral
s,
jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum In zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Con ...

jellyfish
, tube-dwelling anemones, and '' Hydra''. Unlike jellyfish, sea anemones do not have a
medusa In Greek mythology Greek mythology is the body of myth Myth is a folklore genre Folklore is the expressive body of culture shared by a particular group of people; it encompasses the tradition A tradition is a belief A b ...
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 Zoology ()The pronunciation of zoology as is usually regarded as nonstandard, though it is not uncommon. is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy ...

tentacle
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 giant secretory organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural scien ...

cnidocyte
s (stinging cells). In many species, additional nourishment comes from a
symbiotic relationship Symbiosis (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approx ...

symbiotic relationship
with single-celled
dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 m ...
s,
zooxanthellae Zooxanthellae is a colloquial term for single-celled dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a count ...

zooxanthellae
or with green algae, zoochlorellae, that live within the cells. Some species of sea anemone live in association with
hermit crab Hermit crabs are anomuran decapod crustacean Crustaceans (Crustacea ) form a large, diverse arthropod An arthropod (, (gen. ποδός)) is an invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (c ...

hermit crab
s, small fish or other animals to their
mutual benefit The Mutual Benefit Life Insurance Company was a life insurance company that was chartered in 1845 and based in Newark, New Jersey, Newark in Essex County, New Jersey, Essex County, New Jersey, United States. The company was headed by Frederick ...
. 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 cnidaria Image:Sea nettles.jpg, Chrysaora fuscescens, Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum under kingdom Animal ...

planula
larva A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that f ...
e which, after being
plankton Plankton are the diverse collection of organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology ...

plankton
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 s in a marine aquarium A marine aquarium is an aquarium An aquarium (plural: ''aquariums'' or ''aquaria'') is a vivarium of any size having at least one transparent side in which aquatic ...

reef aquarium
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, 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
'' and ''
Stichodactyla mertensii
Stichodactyla mertensii
'' can both exceed a metre in diameter and ''
Metridium farcimen
Metridium farcimen
'' 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 specialist 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 of ...
. 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 Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A give ...
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 siphonoglyphs; 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 giant secretory organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural scien ...

cnidocyte
s, cells that are both defensive and used to capture prey. Cnidocytes contain stinging nematocysts, capsule-like
organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
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 harmful substance produced within living cells or organisms; synthetic toxicants created by artificial processes are thus excluded. The term was first used by organic chemist Ludwig Brieger (1849–1919), derived from the word toxic ...
s, including
neurotoxin Neurotoxins are toxins that are destructive to nervous tissue, nerve tissue (causing neurotoxicity). Neurotoxins are an extensive class of exogenous chemical neurological insult (medical), insultsSpencer 2000 that can adversely affect function in ...
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 Digestion is the breakdown of large insoluble food Food is any substance consumed to provide Nutrient, nutritional support for an organism. Food is usually of plant, animal or Fung ...
. Actinotoxins are highly toxic to prey species of fish and
crustacean Crustaceans (Crustacea ) form a large, diverse arthropod taxon which includes such animals as crabs, lobsters, crayfish, Caridea, shrimp, krill, Dendrobranchiata, prawns, woodlice, barnacles, copepods, amphipoda, amphipods and mantis shrimp. The ...
s. However,
Amphiprioninae Clownfish or anemonefish are fishes from the subfamily Amphiprioninae in the family Pomacentridae. Thirty species are recognized: one in the genus ''Maroon clownfish, Premnas'', while the remaining are in the genus ''Amphiprion''. In the wild ...
(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 Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science which deals with the structural organization ...

mouth
and
anus The anus (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

anus
. 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 (; the plural is cilia) is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, ...
, 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 human mouth, mouth and nasal cavity, and above the esophagus and trachea – the tubes going down to the stomach and the lungs. It is found in vertebrates and invertebrates, thou ...

pharynx
. 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 Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
. 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 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 tissue found in cnidaria Image:Sea nettles.jpg, Chrysaora fuscescens, Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found bot ...
, and include filaments of tissue specialised for secreting
digestive enzyme Digestive may refer to: Biology *Digestion Digestion is the breakdown of large insoluble food Food is any substance consumed to provide nutritional Nutrition is the biochemical Biochemistry or biological chemistry, is the study of c ...
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 Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...
, 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 Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology) ...
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 (
gastrodermisThe 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 Ctenophores. It has been shown that the gastrodermis is amo ...
) 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 Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A give ...
. 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.


Lifecycle

Unlike other cnidarians, anemones (and other
anthozoa Anthozoa is a 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 co ...

anthozoa
ns) entirely lack the free-swimming medusal stage of their lifecycle; the polyp produces eggs and sperm, and the fertilized egg develops into a which develops directly into another polyp. Both sexual and
asexual reproduction Asexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Gree ...
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 (sex cells) and sex hormones of an organism. In the female of the species the reproductive cells are the egg cells, and in the male the reproductive cells are t ...
s are strips of tissue within the mesenteries. 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 concept Concepts are defined as abstract ideas A mental representation (or cognitive representation), in philosophy of mind Philosophy of mind is a branch of philosophy that studies the ontology and nature of the mind ...
. The eggs and sperm, or the larvae, usually emerge through the mouth, but in some species such as '''', 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 Biological processes are those processes that are vital for an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are ...
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 Syntrophy, syntrophic consortium of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix th ...

biofilm
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 is a type of asexual reproduction Asexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism ...

budding
, fragmentation, or by longitudinal or transverse
binary fission Binary may refer to: Science and technology Mathematics * Binary number In mathematics and digital electronics Digital electronics is a field of electronics The field of electronics is a branch of physics and electrical engineeri ...
. Some species such as certain '' Anthopleura'' 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 at the base of the column which then fragments, the pieces regenerating into new individuals. Alternatively, fragments detach separately as the animal creeps across a surface. In '''', fragmentation rates were higher in individuals living among live
mussel Mussel () is the common name Common may refer to: Places * Common, a townland in County Tyrone, Northern Ireland * Boston Common Boston Common (also known as the Common) is a central public park in downtown Boston, Massachusetts. It is som ...

mussel
s than among dead shells, and all the new individuals had tentacles within three weeks. The sea anemone ''
Aiptasia diaphana ''Aiptasia diaphana'', commonly known as the yellow aiptasia or glasrose, is a species of sea anemone native to shallow waters in the temperate eastern Atlantic Ocean and the Mediterranean Sea. It has been introduced into the Red Sea. Description ...
'' 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 Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are ...

zygote
s derived from "selfing" (within the founding clone) or out-crossing, that then develop into swimming planula larvae. Anemones tend to grow and reproduce relatively slowly. The (''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, 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 Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may ...
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. '''' 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 A water column is a concept Concepts are defined as abstract ideas A mental representation (or cognitive representation), in philosophy of mind Philosophy of mind is a branch of philosophy ...
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 anemones which live in parchment-like tubes, are in the anthozoan subclass , 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 List of feeding behaviours, feeding behaviours that includes parasitism and micropredation (which ...

predators
, 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 (biology), cell containing one giant secretory organelle called a cnidocyst (also known as a cnida (plural ''cnidae'') or nematocyst) that can deliver a sting to other or ...
. 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 crustacean Crustaceans (Crustacea ) form a large, diverse arthropod taxon which includes such animals as crabs, lobsters, crayfish, Caridea, shrimp, krill, Dendrobranchiata, prawns, woodlice, barnacles, copepods, amphipoda ...

crab
s, dislodged
mollusc Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number ...
s and even small fish. '''' is reported to trap
sea urchin Sea urchins () are spine (zoology), 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, ha ...

sea urchin
s by enfolding them in its carpet-like oral disc. A few species are
parasitic Parasitism is a Symbiosis, symbiotic biological interactions, relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is adaptation (biology), ad ...

parasitic
on other marine organisms. One of these is '''', the larvae of which develop inside the
medusa In Greek mythology Greek mythology is the body of myth Myth is a folklore genre Folklore is the expressive body of culture shared by a particular group of people; it encompasses the tradition A tradition is a belief A b ...
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 Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s and therefore incapable of
photosynthesis Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

photosynthesis
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 Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
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 dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a count ...

zooxanthellae
, zoochlorellae or both. The sea anemone benefits from the products of the algae's photosynthesis, namely
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
and food in the form of
glycerol Glycerol (; also called glycerine in British English and glycerin in American English) is a simple polyol 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 with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

glucose
and
alanine Alanine (symbol Ala or A) is an α-amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are ...

alanine
; 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 (''Anthopleura elegantissima''), the colour of the anemone is largely dependent on the proportions and identities of the zooxanthellae and zoochlorellae present. The (''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 animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebra ...
s live in symbiotic or mutualistic relationships with sea anemones, most famously the
clownfish '') in their anemone home File:Clown Fish Swimming.jpg, A clownfish swimming Clownfish or anemonefish are fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. ...

clownfish
. 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 (such as ), juvenile threespot dascyllus, , juvenile
painted greenling The painted greenling (''Oxylebius pictus'') is a marine fish native to the northeast Pacific Ocean. Its range is from Kodiak Island, Alaska to central Baja California. It can reach a total size of and has seven vertical dark bands. It inhabits r ...
, various crabs (such as '' Inachus phalangium'', '' Mithraculus cinctimanus'' and ''Neopetrolisthes''), shrimp (such as certain ''Alpheus (genus), Alpheus'', ''Lebbeus'', ''Periclimenes'' and ''Thor (shrimp), Thor''),Debelius, H. (2001). ''Crustacea Guide of the World.'' opossum shrimp (such as ''Heteromysis'' and ''Leptomysis''), and various marine snails.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'' and ''Neoaiptasia'') and
hermit crab Hermit crabs are anomuran decapod crustacean Crustaceans (Crustacea ) form a large, diverse arthropod An arthropod (, (gen. ποδός)) is an invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (c ...

hermit crab
s or snails, and ''Bundeopsis'' or ''Triactis'' anemones and ''Lybia'' 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 zone, pelagic.


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 (''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 and deep-fried in olive oil. Anemones are also a source of food for fisherman communities in the east coast of Sabah, Borneo and the Thousand Islands (Indonesia), Thousand Islands (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 Burgess Shale of Canada, is the oldest fossil identified as a sea anemone.


Taxonomy

Sea anemones, order Actiniaria, are classified in the phylum
Cnidaria Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, ...

Cnidaria
, class
Anthozoa Anthozoa is a 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 co ...

Anthozoa
, subclass
Hexacorallia Hexacorallia is a class of Anthozoa comprising approximately 4,300 species of aquatic organisms formed of polyps, generally with 6-fold symmetry. It includes all of the stony coral Corals are marine invertebrates within the class (biology), ...
. Rodriguez et al. proposed List of sea anemone families, a new classification for the Actiniaria based on extensive DNA results. Suborders and Superfamilies included in Actiniaria are: * Suborder Anenthemonae ** Superfamily Edwardsioidea ** Superfamily Actinernoidea * Suborder Enthemonae ** Superfamily Actinostoloidea ** Superfamily Actinioidea ** Superfamily Metridioidea


Phylogeny


External relationships

Anthozoa Anthozoa is a 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 co ...

Anthozoa
contains three subclasses:
Hexacorallia Hexacorallia is a class of Anthozoa comprising approximately 4,300 species of aquatic organisms formed of polyps, generally with 6-fold symmetry. It includes all of the stony coral Corals are marine invertebrates within the class (biology), ...
, which contains the Actiniaria; Octocorallia; and . These are monophyletic, but the relationships within the subclasses remain unresolved.


Internal relationships

The relationships of higher-level taxa in Carlgren'sCarlgren 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 * Cangitoxin * Halcurin * Sea anemone dermatitis * Sea anemone neurotoxin


References


External links


Order Actiniaria

Actiniaria.com


* [http://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 Actiniaria, Hexacorallia Negligibly senescent organisms Symbiosis