Influence of infection by Sacculina carcini (Cirripedia, Rhizocephala) on consumption rate and prey size selection in the shore crab Carcinus maenas

Abstract

Parasites generally influence the feeding behavior of their host and may therefore indirectly impact ecosystem structure and functioning if the host plays an ecological key role. The ecologically important shore crab (Carcinus maenas) is commonly infected by the rhizocephalan parasite Sacculina carcini that aside from inflicting behavioral change, castration and ceased molting, also feminizes its male host morphologically. The latter results in reduced cheliped size, and, together with the other parasite-induced effects, this may potentially impact host feeding behavior. In two separate laboratory experiments, we offered infected and uninfected adult male crabs respectively ad libitum small, easy-to-handle blue mussels (Mytilus edulis) (10–15mm in shell-length), and a limited, size-structured prey population (15–45mm in shell-length; seven size-classes, ten mussels per class) during 10–15days. Corrected for carapace width, the per capita consumption rate of the infected and uninfected crabs was similar in either experiment, both regarding number of mussels and amount of tissue dry-weight consumed. Also, the median mussel size preyed upon when exposed to the size-structure prey population was unaffected by infection. However, infected crabs preyed less frequently (26%) on intermediate mussel sizes (25–30mm) than uninfected crabs. For both infected and uninfected crabs the median prey size increased linearly with maximum claw height. Host dry weight was significantly reduced by infection, assumed to be the result of the morphological feminization (reduced appendage size) rather than reflecting poorer general condition of infected individuals. Infected crabs were nonetheless subjected to a higher mortality rate than uninfected crabs during the experimental period. We conclude that Sacculina infection has a very limited effect on its host crabs' feeding biology and that any measurable ecosystem ramifications of the host–parasite association must originate from other processes; for instance reduced mean size (infection inhibits molting) and density (infection increases mortality) of the crab population where parasitism is high.