Sea anemones ( ə-NEM-ə-nee) are a group of predatory marine invertebrate animals constituting the order Actiniaria. Because of their colourful appearance, they are named after the Anemone, a terrestrial flowering plant. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Hexacorallia. As cnidarians, sea anemones are related to corals, jellyfish, tube-dwelling anemones, and Hydra. Unlike jellyfish, sea anemones do not have a medusa 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 tentacles and a central mouth. The tentacles can be retracted inside the body cavity or expanded to catch passing prey. They are armed with cnidocytes (stinging cells). In many species, additional nourishment comes from a symbiotic relationship with single-celled dinoflagellates, with zooxanthellae, or with green algae, zoochlorellae, that live within the cells. Some species of sea anemone live in association with clownfish, hermit crabs, 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 larvae which, after being planktonic 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 aquariums; 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
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 1 to 5 cm (0.4 to 2.0 in) in diameter and 1.5 to 10 cm (0.6 to 3.9 in) in length, but they are inflatable and vary greatly in dimensions. Some are very large; Urticina columbiana and Stichodactyla mertensii can both exceed 1 metre (3.3 ft) in diameter and 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 specialised structures; these include solid papillae (fleshy protuberances), adhesive papillae, cinclides (slits), and small protruding vesicles. 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 bone all the 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 cnidocytes, cells that are both defensive and used to capture prey. Cnidocytes contain stinging nematocysts, capsule-like organelles 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, primarily aggregating anemones, 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. Many sea anemones also have acontia, thin filaments covered in cnidae that can be ejected and retracted for defence.
The venom is a mix of toxins, including neurotoxins, that paralyzes the prey so the anemone can move it to the mouth for digestion inside the gastrovascular cavity. Actinotoxins are highly toxic to prey species of fish and crustaceans. However, Amphiprioninae (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 adaptations 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.
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