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Anthozoa

With about 6,000 species, Class Anthozoa is the largest class of cnidarians. Anthozoans are known as flower animals for their lustrous, rainbow-like colors and include animals such as the sea anemone, coral, sea fan, and sea pen.

Take a trip to the Invertebrate Exhibit at the Zoo to get up close and personal with many of these marine animals, including the Giant green anemone that makes its home on rocky shorelines, the Frilled anemone that swims in spirals and crawls on its pedal disc at about 4 centimeters per hour, and Elegance coral that dwells in the warm waters of Indo-Pacific reefs and dines on plankton and the byproducts of symbiotic algae.

Living Arrangements

Corals often live together in reefs. Sea anemones are largely solitary animals, although some species may be colonial.

Partners in Crime

Some sea anemones and algae enter into a symbiotic relationship. This type of close or long-term relationship may or may not be beneficial to both members of the partnership. In the case of anemones and algae, the benefits usually go both ways. Algae will attach to an anemone and stimulate it to produce a pigment that will protect the tiny symbiont from too much sunlight. In return for a place to live and some organic sunscreen, algae lends the anemone its own golden sheen, camouflaging it so it blends in with its surroundings.

Ticket to Ride

Other symbiotic relationships are a little more moving, such as the partnership between the hermit anemone (Calliactis tricolor) and the hermit crab (Coenobita perlatus). When a crab scuttles by, an anemone may decide to go along for the ride. By attaching to the crab’s shell the anemone gets on the fast track to a new location and easy access to the errant scraps of food that escape the jaws of its host.

A Roof for Some Rations

Sometimes an anemone seizes an opportunity for a symbiotic relationship and other times it happens effortlessly, as in the case of the clownfish (Amphiprion sp.). Clownfish seek out the protection of the anemones’ tentacle forest—they merely show-up and slip in among the waving wands of color and light. In return, the anemone gets first dibs on any aquatic leftovers. Although the tentacles of the anemone are equipped with the highly efficient nematocysts, clownfish are up for the challenge.

Although scientists are not certain how clownfish stay safe among the tentacles, there are a couple of theories. One theory suggests that this relationship is made possible by protective mucus that coats the clownfish and prevents the anemone from discharging its nematocysts. This slimy shield may be made of sugar rather than proteins to distinguish the clownfish from normal anemone fare. Another theory suggests that the mucous coating is similar to the coating of the anemone itself, so the fish appears to be part of the anemone’s own body.

Caveat emptor

Not all fish-anemone pairings are meant to be. Only particular species of clownfish can inhabit particular species of sea anemone. Otherwise, the fish’s mucous covering does little in the way of protection. The careless anemone fish that tries out a non-compatible host may find himself on the receiving end of a stinging nematocyst or worse, a free lunch.