The Life History of Lernaeodiscus Porcellanae (Cirripedia: Rhizocephala) and Co-evolution With Its Porcellanid Host

Abstract

The elaborate life history of the rhizocephalan parasite Lernaeodiscus includes a number of counterdefensive measures specifically adapted to circumvent defense systems of the host crab Petrolisthes. Lernaeodiscus is dioecious. The externa produces large male and small female cyprids. The female cyprids invade the host by means of a kentrogon stage. The sole site of invasion is the gills. The female parasite later erupts on the ventral side of the abdomen as a virgin externa. This has to be hyperparasitized by a male cyprid in order to reach sexual maturity. The life cycle is completed in about 5 months. Autogrooming is the crab's primary defense against infestation and ordinarily only crabs slightly deficient in this regard become infested. Once inside, the parasite gains control of the host crab and induces it to accept the parasite as "self"—in effect, as its own reproductive system. This nullifies all remaining defenses the crab might have against the parasite. In addition, infested male crabs become behaviorally and morphologically female, so that they provide maternal care for the external reproductive body of the parasite, as do infested female crabs, rather than attempting to remove the parasite from their body. While the parasite has diverted all of the reproductive resources and capabilities of the crab to its own use, the crab otherwise carries out its life as a normal member of the intertidal community.