Family group structure in mysids, commensals of hermit crabs (Crustacea)

Abstract

Two commensal crustaceans, Heteromysis harpax Hilgendorf 1878 (Mysidacea) and Aretopsis amabilis De Man 1910 (Decapoda Alpheidae) were found inhabiting the shells of hermit crabs (five species of Dardanus Paulson 1875) collected in Somalia and Kenya. The two species were never found together in the same shell. In Kenya, where alpheids seem to be absent, the relative frequency of shells inhabited by mysids is greater than in Somalia. These findings suggest that the two commensals are in competition with each other, at least in sympatric areas. Mysids were particularly common in Kenya and detailed observations were made on this species. No relationship seems to exist between the sex, size and species of hermit and the probability of its shell being inhabited by the mysids. Several females were ovigerous, the average brood size being about 7. 7 with a positive correlation between the size of the female and number of eggs. The mysids were usually found living as a pair of adults, or in about in 1/8 of cases, together with a large crowd of smaller individuals (up to 77). In adult pairs, the male was often smaller than the female. No sexual dimorphism seems to exist regarding the large claws, typical of this genus of mysid. The crowds of young were composed of individuals belonging to several distinct age classes, which presumably correspond to separate broods of similar or different age produced by the resident adult pair, thus forming a true family.