Thirty-seven crabs (8 males and 29 females) of the genus Arcotheres, which inhabit the sessile bivalve Barbatia virescens (Reeve, 1844), were reared for 135 days in the laboratory and their morphological development was described. There was a strong size correlation between female crabs and their host bivalves. During laboratory culture, seven males and 18 females molted once, and one male and four females molted twice. The crabs showed little growth, perhaps because the crabs and bivalves were not fed during the study and because crab growth may be restricted by the host size. One small female molted from pre-hard stage to hard stage and progressed further to Stage II. Some adult females in Stage III or IV developed swimming setae following the molt. In past studies, only one metamorphosis was detected within a crab, and adult females were not thought to develop swimming setae. Thus, our study is the first to detect successive changes in body form within a crab. Furthermore, this study confirmed that adult females can develop swimming setae.