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Biology/Natural History: Isopods of family Bopyridae live parasitically on the gills of other crustaceans such as shrimp. Typically a large female and a smaller male are found together on the same individual (photo). Usually they parasitize only one side of the host--rarely is a shrimp parasitized on both sides by bopyrids. The females are usually asymmetrical (see photo above), while the males are symmetrical (photo). They seem to enter the shrimp early in the shrimp's life and remain through molts. In Pandalopsis dispar they appear to retard growth and delay sex change in the host shrimp. Bopyrids develop to young larvae within the mother's marsupium formed by her oostegites. In this individual, the mother's ventral side was pressed against the medial surface of the gill cover instead of against the gill itself (photo). The oostegites did not overlap, but simply formed a wall around the eggs while the host's gill cover formed the floor of the chamber the eggs were within. When I removed the female from the gill chamber her eggs mostly spilled out because they were not fully enclosed by the ooostegites. The first larval stage is 2-3.5 mm long and is called an "epicaridium". It has six free pereonites, each of which bears a pair of subchelate pereopods which it uses to attach to an intermediate planktonic host such as a copepod. On that host it molts into a "microniscus" or "microniscum" larva, then later to a "cryptoniscum" or "cryptoniscus", which is up to 7 mm long and has a 7th pereonite. At this stage it releases from the intermediate host, becomes benthic, and finds its final crustacean host. Within the host it metamorphoses into either a large, asymmetrical female or into a smaller, symmetrical male (apparently it can become either sex). The adult form is called a "bopyridium".