2000-08 Allied Cowries

Denise Nielsen Tackett and Larry Tackett

At first glance, it’s hard to tell where one animal ends and the other begins. Look closely though and you will spot a small mollusk on the gorgonian. This is an allied cowrie. Allied cowries always live with host organisms such as soft corals or sponges. They come in different sizes and shapes. Each cowrie has a soft mantle (fleshy material that covers the shell) that matches the color and texture of its host although there are notable exceptions.

This species, Aclyvolva lanceolata (one inch long), lives on a Whip Gorgonians and has developed retractable false polyps that imitate the true coral polyps. Gorgonians are generally unpalatable and are thus avoided by most predators. By closely resembling the gorgonian, the mollusk also gains protection from predators. Only the mollusk, however, benefits from this relationship.

Allied cowries are, in fact, parasites that harm their host. They feed on the host’s tissues, mucus and polyps and absorb pigments that enable the cowries to closely match the host’s color. The host continually regrows the lost tissue so the cowrie never runs out of food.

Free-swimming post-larval juveniles probably detect chemical clues that signal a potential host is nearby. Hosts are often home to several individuals. When settled, cowries graze up and down the coral and eventually deposit their eggs on a bare branch, beginning the cycle again.

Allied cowries are hard to find because they are small (two inches or less) and well-camouflaged. Some species have false polyps, some don’t. Although their disguise is good, it is not perfect. If you look closely at the false polyps on this species, you will notice that the tentacles are smooth; all soft coral polyps have branched tentacles.

Allied cowries are found in all waters. It’s easier to spot them when the coral polyps are withdrawn or at night with a good light. Check gorgonians and other soft corals and sponges for any irregularities. This particular species is common in the Indo-Pacific region.