Is the seasonal change of sexual differences in shell use by the hermit crabPagurus minutusconsidered to be driven by growth or reproduction?

Abstract

Sexual differences in behaviours are often affected by the difference in individual interests between the sexes: growth in males and egg production in females. Some hermit crabs show sexual differences in shell use patterns during the reproductive season. In the non-reproductive season, however, when both sexes are focused on increasing growth, this sexual difference is expected to be reduced. In this study, we compared the pattern of shell use in the hermit crab Pagurus minutus between seasons, while focusing on the effects of shell shape on growth or egg production. As we predicted, sexual differences in shell use in P. minutus showed seasonal change. In the non-reproductive season, both sexes appeared to use shells well suited for growth. In the reproductive season, sexual differences became more evident, especially in larger solitary crabs and guarding pairs; males monopolized round-type shells such as those of Umbonium moniliferum, whereas more than 80% of females relied on high-spired Batillaria-type shells such as those of Batillaria zonalis. A lack of advantage for egg number in females using Batillaria-type shells suggests that female shell use is explained by factors other than maximizing clutch size. Both sexes can moult during the reproductive season, and larger body size is advantageous for reproduction. Given that Batillaria-type shells resulted in a lower growth increment and males have an advantage in shell fights in congeneric crabs, our findings suggest the importance of intersexual competition for shells and female compromise in determining the seasonal change of shell use patterns in P. minutus.