Geographic variation in decoration selectivity of Micippa platipes and Tiarinia cornigera in Japan

Relatively few studies have compared behavioral variation of more than one species among different locations; yet examining geographic variation in behavior in a comparative context is crucial to help elucidate the generality of geographic patterns. We studied geographic variation in decoration preferences for 2 spider crabs (Brachyura: Majoidea), Micippa platipes and Tiarinia cornigera, which co-occur in intertidal areas along the coast of Japan. Both M. platipes and T. cornigera primarily feed on algae and use them for 'decoration' (i.e. attach them to hooked setae on their carapace). We tested whether individuals from 3 locations along the east coast of Japan differed in their decoration and consumption selectivity for certain algae in the field, and also tested decoration and consumption preferences of M. platipes in laboratory assays. At the 2 more northern locations, both species showed little selectivity among algae, primarily using them for decoration in proportion to their abundance. Conversely, at the southern location, M. platipes exhibited strong preferences for the alga Hypnea pannosa in both field and laboratory assays, and both crab species strongly avoided the alga Corallina pilulifera. Using both an overall selectivity index (SE sub(o)) and a proportional selectivity index (PSE) calculated for each alga, we compared algal use by the crabs vs. algal availability at the 3 locations. Both indices indicated that both species were more selective (had stronger preferences or avoidances of available algae) at the southern location than at the 2 more northern locations. Potential factors driving this latitudinal gradient in selectivity include seasonal and geographic variability in both decoration availability and crab decoration preferences.