Sea anemone, any member of Actiniaria (class Anthozoa, phylum Cnidaria) invertebrate order, soft-bodied, mainly flowers-like, sedentary marine animals. The ornately colored sea anemone (uh-NEM-uh-nee) takes its name from the equally flashy terrestrial anemone flower. Sea anemones are classified within the phylum, Anthozoa, subclass Hexacorallia. As cnidarians, sea anemones are associated with corals, jellyfish, tube-dwelling anemones, and Hydra. Unlike jellyfish, sea anemones don’t have a medusa stage in their life cycle.
Sea anemones are found from every ocean’s tidal zone to depths of more than 10,000 meters (about 33,000 feet). Some species live in bracketed water. Of warmer seas, they are the largest, the most numerous, and the most colorful. The colorful Tealia are found in temperate regions. A typical anemone could be a single polyp attached to a tough surface by its base, but some species board soft sediment, and some float near the surface of the water. A detailed relative of coral and jellyfish, anemones are stinging polyps that spend most of their time attached to rocks on the bottom of the ocean or on coral reefs watching fish pass close enough in their venom-filled tentacles to urge them to snatch.
Sea anemones breed into the water by mouth releasing sperm and eggs. Their bodies are composed of a pedal or foot adhesive, a cylindrical shell, and a series of tentacles surrounding a central mouth. Even they breed asexually, splitting into half or smaller parts that regenerate into polyps. The tentacles are activated by the slightest contact, a harpoon-like filament is shot into their target and a paralyzing neurotoxin is injected. The helpless prey is then guided by tentacles into the mouth.
The nearly 1,000 species vary in size from some millimeters (a fraction of an inch) in diameter and length to about 1.5 meters (about 5 feet) in diameter. The biggest sea anemones also the biggest cnidarians are of the genus Stichodactyla. Sea anemones are sometimes kept in reef aquariums; for this purpose, global trade in marine ornamentals is expanding and threatens populations of sea anemones in some locations, as the trade is dependent on wild collection.
Many anemones develop symbiotic relationships with green algae, such as their cousins in corals. The anemone provides oxygen and sugar, the bi-products of the photosynthesis of the algae, in return for providing the algae safe harbor and access to sunlight. Actinarians show great variety in shape and habit. Also, the cylindrical body is thick, short, long, and slender. The oral disc encompasses a central mouth, usually slit-shaped, surrounded by one or more whorls of tentacles. The slit ends lead to grooves in the pharynx wall known as siphonoglyphs; there are usually two of these grooves but some groups have more.
Sea anemones are commonly yellow, green, or blue; they are typically attached to a hard surface like a rock, a wharf timber, a seashell, or a crab’s back by the pedal disk, or base. Although most marine anemones are harmless to humans, some extremely toxic species (notably Actinodendron arboreum, Phyllodiscus semoni, and Stichodactyla spp.) have caused serious injury and are potentially lethal. Sea anemones don’t have any solid skeleton but may secrete a horny covering. Some species have adhesive-secreting structures and canopy themselves with grains of sand, bits of shell, or other foreign objects.
A sea anemone can radically change its shape; the column and tentacles have longitudinal, transverse and diagonal muscle sheets, and can lengthen and contract, as well as twist and bend. Some species only eat micro-organisms. Sea slugs, certain starfishes, eels, flounders, and codfish are eaten by anemones. Males may release sperm for sexual reproduction to stimulate females to release eggs, and fertilization occurs either internally in the gastrovascular cavity or in the water column. Reproduction sometimes occurs asexually by longitudinal fission (e.g., in Anemonia); that’s, the animal splits lengthwise into two equal individuals. The pedal disk breaks into fragments that grow into new individuals in some species ( e.g., Metridium)
Sea anemones from another, more famous symbiotic alliance with clownfish which is protected by a layer of mucus which makes them immune to the sting of the anemone. Often they live in close ties with other organisms. The decapod crustacean Pagurus arrosor carries one anemone of the genus Calliactis on the snail shell it uses as a “house.” Although some species of anthozoan burrow in soft sediment, the bulk are mainly sessile, attaching to a tough surface with their pedal disc, and have a tendency to remain within the same spot for weeks or months at a time.
Likewise, the hermit crab Eupagurus Prideaux and the sea anemone Adamsia palliata always find themselves living together, never alone. Normally, marine anemones are hunters, ensnaring prey of suitable size that comes within reach of their tentacles and immobilizing them with the aid of their nematocysts. Fish of the genera Premnas and Amphiprion sometimes live safely among an anemone’s poisonous tentacles, such as Stichodactyla, Radianthus, or Discosoma. Sea anemones are found worldwide in both the deep and shallow coastal waters. The greatest diversity is in the tropics while many species are adapted to relatively cold waters.
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