Tidal elevation and parasitism: patterns of infection by the rhizocephalan parasite Sacculina carcini in shore crabs Carcinus maenas

While the distinct zonation patterns of benthic organisms along intertidal elevation gradients have been extensively documented, relatively little is known about the impact that tidal elevation has on the distribution and abundance of marine parasites that are common in intertidal ecosystems. In this study, we investigated the distribution of shore crabs Carcinus maenas infected with the rhizocephalan parasite Sacculina carcini at 12 locations and in 3 adjacent habitats (intertidal mussel beds, intertidal bare sand flats and subtidal gullies) along a tidal elevation gradient in the Dutch Wadden Sea. Our sampling revealed that of the 27 629 crabs investigated, most infected crabs were found in the subtidal gullies and almost none on intertidal bare sand flats or mussel beds at all of the 12 locations. This probably resulted from a parasite-induced manipulation of infected crabs to behave like egg-bearing females which migrate towards deeper waters, as the same pattern was observed in the distribution of non-infected ovigerous females. The prevalence of both infected crabs and ovigerous females in the gullies was significantly correlated with water depth, and both tended to increase (albeit not significantly) with increasing salinity. As water depth and salinity are expected to affect larval survival of both parasites and crabs, this suggests that the migration into subtidal habitats may result in favourable conditions for reproduction and dispersal. By using a replicated and nested sampling design as well as a large sample size, our study significantly increases the limited understanding of parasite distributions along tidal elevation gradients.