Spatial and temporal variation in the predation risk for hermit crabs in a subtropical bay

• Tethering can be used to investigate predation risk in turbid coastal waters.
• The intertidal fringe is a risky microhabitat for hermit crabs and gastropods.
• Predation outcomes directly relate to portunid predator abundance.
• Season strongly influences predation risk (summer > winter).
• Shoreline hardening may alter predation risk for benthic prey.

Comparing the spatial and temporal dynamics of predation in turbid coastal waters can be notoriously difficult. We used a tethering approach to directly assess variation in predation risk for the intertidal hermit crab Pagurus criniticornis and the gastropod Cerithium atratum in a subtropical bay. Predation outcomes were used to test the effects of; (1) microhabitat (i.e., intertidal mud vs. intertidal fringe), (2) season (summer vs. winter), and (3) the type of domicile shell occupied (C. atratum vs. Tegula viridula). Predation varied across microhabitat for both species, with unambiguous cases more prevalent at the intertidal fringe than the mud habitat (20.6% and 11.8% greater for the hermit crab and gastropod, respectively). This pattern related to the abundance of the portunid crab predator Callinectes danae (~ 2.2 times greater in the fringe vs. mud). There was similar variation among seasons for P. criniticornis, with cases of predation 51.3% more prevalent in summer than winter, again matching the abundance of C. danae (greater in summer than winter). Closer examination of the type of shell damage suggests differences in the way that predators might access prey across spatial and temporal scales. The high prevalence of damaged shell apertures (indicative of peeling) suggests possible targeting of this species by smaller individuals or females. Although there was no seasonal difference in the size of predators, the proportion of males was greater during summer and this may constitute a driver of variable predation pressure. Lastly, the risk of predation was also related to domicile shell morphology, with crabs inhabiting C. atratum shells 27.3% more likely to be predated upon than those inhabiting T. viridula shells. Combining the outcomes of our experiments, we suggest that changes to intertidal habitat configuration (e.g., the proliferation of hard substrates through, erosion or harbor and break wall construction) as well as factors that alter the supply of appropriate domicile shells (e.g., effects of acidification on calcifying organisms) could alter predation risk for hermit crabs and other soft bottom species.