The slipper limpet, Syphopatella walshi (Reeve), is commensal with hermit crabs. S. walshi obtains benefit from a hermit crab while it is considered neither beneficial nor harmful to hermit crabs. S. walshi attaches itself to a gastropod shell occupied by a hermit crab, but not directly to the hermit crab’s body. The objective of the present study was to clarify which factors might be more important for the settlement of S. walshi, the hermit crab or the shell.

Hermit crabs were collected using transects and quadrats at 40cm, 70cm and 100cm tidal levels on the stony shore of Magarisaki in Amakusa Shimoshima Island, south-western Japan. All hermit crabs and their shells were identified, and the carapace length of hermit crabs and the aperture size and height of shells were measured. In addition, the presence/absence of S. walshi was checked. Larger hermit crabs with larger shells were often selected by S. walshi, suggesting the size-dependent host select by S. walshi. Moreover, the result of GLMM indicated that the aperture size had a greater effect on selection than other two characteristics. The study has revealed that the size of host shell is more important than the hermit crab’s identity for host selection by S. walshi.