You’d recognize a snail if you saw one, right? Crawls along on a big muscular
foot, its body stuffed inside a hard calcium carbonate shell? Think again. When an animal becomes parasitic, sometimes its most characteristic features simply disappear. Take the snail Asterophila japonica, from the Sea of Japan. It’s a member of the Eulimidae, a family of parasitic sea snails that typically prey on echinoderms -- animals like sea urchins and starfish. Although most of the Eulimidae are very small, they still look like typical snails, with shells and a foot. Asterophila is different. This snail lives its life inside a starfish, squeezed between its epithelium and the lining of its coelomic cavity. It has simplified or lost most of the organ systems found in a typical snail -- Asterophila has no shell, and only a small rudimentary foot and mantle, giving it a beanlike appearance. It has a striking sexual dimorphism – males are tiny, only a couple of millimeters long, and live attached to the much larger (as much as 20 mm wide) females. Because both sexes live inside of starfish, no one yet knows how their larvae make their way into the open ocean to parasitize new hosts.
References:
Randall, J. and H. Heath 1912. Asterophilla, a new genus of parasitic gastropods. Biol. Bull. 22(2): 98-106.
Sasaki T, Muro K, Komatsu M. 2007. Anatomy and ecology of the shell-less endoparasitic gastropod Asterophila japonica Randall and Heath, 1912 (Mollusca: Eulimidae). Zoolog Sci. 24(7):700-13.
Image from Randall and Heath 1912: Diagram of the external surface of Asterophila.
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