Sea anemones are invertebrate animals belonging to the phylum Coelenterata, a term that means hollow gut. Sea anemones are found in all major oceans from the polar regions to the equator. All are exclusively marine-dwelling with a strong tendency for shallow, warm waters. More than 1,000 species have been described so far. These vary considerably in size, with a body diameter that ranges from just 0.15 in (4 mm) to more than 3.3 ft (1 m), and a height of 0.6 in (1.5 cm) to 2 in (5 cm). Many are strikingly colored with vivid hues of blue, yellow, green, or red or a combination of these, but others may blend into the background through an association with symbiotic algae that live within the body wall of the anemone.
Related to corals and more distantly to jellyfish, sea anemones have a very simple structure, comprising an outer layer of cells which surround the body, an inner layer lining the gut cavity, and a separating layer of jelly-like material that forms the bulk of the animal. The central gut serves as stomach, intestine, circulatory system, and other purposes. The single mouth, through which all materials enter and leave the gut, is typically surrounded by a ring of tentacles that vary in size, appearance, and arrangement according to the species. Many of these tentacles are armed with special barbed stinging cells (nematocysts). These are used both in defense and in capturing prey. Whenever the tentacles come into contact with a foreign object, special capsules in the cell walls are triggered to unleash a number of nematocysts, some of which may carry toxic materials that serve to sting or paralyze the intruding object. Some tentacles produce a sticky mucus substance which serves a similar purpose, repelling potential predators and adhering to any small passing animals.
Unlike their coralline relatives, sea anemones are solitary animals that live firmly attached by a pedal disk to some object, either a branching coral, submerged rocks, or shells. A few species even bury themselves partly in soft sediments. All are free-living species that feed on a wide range of invertebrates; some of the larger species even feed on small fish that are captured and paralyzed by the nematocysts. In general, however, most of the smaller food items are captured by the regular beating movements of the tentacles, which draw small food particles down towards the mouth region. As food such as plankton is trapped on the surface of the tentacles, the latter bend down towards the mouth and deposit the food.
Sea anemones can reproduce by sexual or asexual means. Some species are either male or female, while others may be hermaphroditic. In the latter, eggs and sperm are produced at different times and released to the sea where external fertilization may take place. Another means of reproduction is by fission, with the adult anemone splitting off new daughter cells that, in time, develop to full size.
Some species of anemones have developed specialized living relationships with other species of animals. A number of crabs encourage sea anemones to attach themselves to their shells. Some species, such as the soft-bodied hermit crabs which live inside discarded mollusk shells, even go to the extreme of transferring the sea anemone to another shell when they move into another larger shell. Other crabs have been observed to attach sea anemones to their claws—an adaptation that may help in further deterring would-be predators. While the crabs clearly benefit for additional camouflage and greater security, the anemone is guaranteed of being in a place of clear open water for feeding; it may also benefit from some morsels of food captured by the crab.
An even greater level of cooperation is evident in the relationship that has developed between some species of sea anemones and single species of fishes. Clownfish, for example, are never found in nature without an anemone. For these fish, the anemone, which is capable of killing
fish of a greater size, is its permanent home. Depending on its size, each anemone may host one or two fish of the same species, as well as their offspring. When threatened by a predator, the fish dive within the ring of tentacles, where they are protected by the anemone's battery of stinging cells. Taking further advantage of this safe place, clownfish also lay their eggs directly on the anemone. No direct harm comes to the anemone through this association. In return for this protection, the fish help repel other fish from attacking the anemone and also serve to keep their host clear of parasites and other materials that may become entangled in their tentacles which could interrupt their feeding behavior. No one is quite sure how these fishes avoid the lethal stinging actions of the anemone's tentacles. Some fish are known to have a thicker skin and to produce a mucus covering that may help protect them from being stung. Other species have been seen to nibble tiny parts of the tentacles and, in this way, may be able to develop some degree of immunity to the toxins carried in the nematocysts. Both of these reactions are, however, host specific: an anemone fish placed on a different species of anemone will almost certainly be killed, as it is not recognized by the anemone.