Infestation of shore crab gills by a free-living mussel species.

Parasitic and commensal species can impact the structure and function of ecological communities and are typically highly specialized to overcome host defences. Here, we report multiple instances of a normally free-living species, the blue mussel Linnaeus, 1758, inhabiting the branchial chamber of the shore crab (Linnaeus, 1758) collected from widely separated geographical locations. A total of 127 were examined from four locations in the English Channel, one location in the Irish Sea and two locations at the entrance of the Baltic Sea.