Sexual difference in shell use in the hermit crab Pagurus filholi (de Man) from northern Japan

Abstract

The shell use pattern between sexes of Pagurus filholi was described, at Kattoshi, northern Japan. Line transect sampling showed that the shells available to P. filholi in this area were mainly composed of four species, Reticunassa fraterculus, Homalopoma sangarense, Batillaria cumingi and Littorina brevicula. The former three shell species were used by small crabs, and the pattern of shell use significantly differed between sexes. Almost all large crabs of either sex used Littorina brevicula shell. The abundance of empty shells was about 10% of total hermit abundance. Shell size suitability of crabs decreased as crab size increased, suggesting that shell limitation was more severe in large-sized crabs than in small ones. However, this trend differed between sexes. Males used unsuitable-sized shell relative to similar-sized females in Reticunassa and Homalopoma species. The population sex ratio was biased towards females, and showed an anomalous pattern as a function of crab size that was initially from male bias to female bias with increasing size, but was biased towards males again with a further increase in size. Sexual differences in shell preference, competitive ability and growth rate were discussed as causal explanations for the present pattern.