As our knowledge of bivalves has increased, so has information about their parasites. Most studies of bivalve parasites have been devoted to groups infesting commercially important mollusks, notably trematodes and copepods. Another group of bivalve symbionts which has been known since the early nineteenth century is the brachyuran genus Pinnotheres Latreille, 1802 (commonly known as pea crabs). The generic name was given because of the belief that the crabs were predators upon the scallops in which they were most frequently found (Pearce, 1966). However, Stebbing (1893) suggested that instead of acting as predators, the crabs were in fact serving as guards for their hosts.