Effects of body size and shell availability on the shell utilization pattern of the hermit crab Pagurus filholi (Anomura : Paguridae)

Abstract

Shell availability and shell utilization patterns of the hermit crab Pagurus filholi (de Man) were studied on a rocky shore at Kattoshi, Hakodate Bay, Japan. Quantitative sampling revealed that the proportion of Reticunassa and Littorina shells used by hermit crabs was significantly higher than the relative abundance of these gastropods. Since shell-species preference tests revealed that P. filholi preferred Reticunassa shells, it is suggested that shell-species preference is one of the factors that influence shell utilization. Shell adequacy index (SAI) decreased with increasing crab size for each shell type, and hermit crabs used relatively suitable shells (SAI=1) when shell availability was high. Larger crabs used smaller than preferred shell sizes (SAI<1) and smaller crabs used larger than preferred shell sizes (SAI>1) with decreasing shell abundance. Shell species used by the hermit crab changed with increasing crab size. Therefore, it is suggested that hermit crabs obtain relatively suitable shells by changing shell species.