On the Question of Sex Reversal in Mole Crabs (Crustacea, Hippidae)

Abstract

Some early field studies of population structure in hippid crabs (Emerita analoga Stimpson and Hippa pacifica Dana) provided evidence in support of a protandry hypothesis for these animals. However, other lines of evidence could lead to the opposite interpretation. Evidence largely supportive of the sex reversal hypothesis is threefold. 1) Sex ratio curves (sex ratio plotted as a function of size), once believed characteristic of a species and/or population and perhaps indicative of sex reversal, are now seen to vary with season, between populations, and with the method or place of capture, 2) A removal of nearly all Hippa pacifica individuals from a beach at Enewetak Atoll, with replacement by a bimodal distribution of males, yielded a bimodal distribution 10 months later. Females comprised the upper size mode, but a possible emigration could not be excluded as the cause. 3) Results of early experiments with contained animals suggested a transition from male to female for some individuals. Repeats of those experiments with better controls have yielded no supportive evidence. Three other lines of evidence largely negated the sex reversal hypothesis. 1) Females and males apparently grow at different rates in both species; in laboratory conditions, males reached a maximum size, continued molting, but ceased growth. 2) Gross examination of internal structure revealed no sexually intermediate forms, except for Emerita asiatica Milne Edwards. 3) The mean size at which female Hippa pacifica individuals begin producing eggs is smaller than the supposed mean size of sex reversal, a severe contradiction for that species.