Allopetrolisthes spinifrons has been reported as a commensal crab of sea anemones, sea stars, and gastropods. Previous studies had shown that small crabs (carapace length, CL < 4.0 mm) inhabit various hosts, while adults only dwell on sea anemones. This last observation suggests that these commensal crabs may change their host-use pattern during ontogeny. We examined the host-use pattern of A. spinifrons in the field and the host selection behaviour in the laboratory to examine if different hosts are used during ontogeny. Allopetrolisthes spinifrons was found on 77% and 58% of collected sea anemones and sea stars, respectively. On gastropod hosts, frequency of occurrence of A. spinifrons was low (< 20%). The occurrence of the association was affected by the size of the host, large hosts being more frequently inhabited by commensal crabs. In general, one or two crabs were observed per host. Usually no more than five crabs were found on a single host, with the exception of one large sea star that hosted a maximum of 12 crabs. The analysis of the host-use pattern demonstrated that all the life stages (including mature crabs) of A. spinifrons inhabit sea anemones while on the remaining hosts only megalopa larvae and small juveniles (crabs < 4.3 mm CL) were found. In laboratory experiments, crabs of all sizes preferred sea anemones over limpets (Fissurella spp.). Most of the small commensal crabs (2.0 < CL< 4.0 mm) migrated from limpets to sea anemones during a 10 experiment day. These results indicate that the non-anthozoan invertebrates studied herein are used as alternative hosts by A. spinifrons during larval settlement possibly as a result of the limited availability of sea anemones. When crabs attain a size between 2.0-4.0 mm CL they abandon these alternative hosts in order to rnd their preferred hosts, sea anemones. Thus, habitat restrictions during the settlement process may explain the existence of various alternative hosts utilized by recruits compared to the high host-specificity shown by adult crabs of A. spinifrons.
