Spatial distribution of shore crabs Carcinus maenas in an intertidal environment in relation to their morphology, prey availability and competition

Abstract

Identifying the key determinants of benthic predator distributions is crucial in under- standing the dynamics of predator and prey populations in intertidal environments. In this paper, we assessed the roles of predator morphology, prey availability and competition in determining the dis- tribution of an important benthic predator, the shore crab Carcinus maenas (L.). The abundance of shore crabs at high tide on 3 intertidal mudflats in the Dutch Wadden Sea was assessed in relation to distance to the nearest gully, size, sex and colour of the crabs. Furthermore, food availability (bi- valves, crustaceans, polychaetes) in the sampling sites was measured. Half of all crabs were found in the gullies adjacent to each mudflat, where the abundance of a prey species (the large brown shrimp Crangon crangon) was also highest. The spatial distribution of crabs between the gullies and the mudflats depended in an interactive way on the colour morph and sex of individuals. Comparing observed distributions of crabs with those predicted from a recently developed ideal free distribution model showed that crabs did not distribute themselves optimally in terms of maximising food uptake and minimising the strength of competition from conspecifics. The results highlight the yet under- rated role of shore crabs as a potentially important predator of shrimps, and the need to incorporate morphological characteristics and different prey types into ideal free distribution models to test the optimal foraging behaviour of benthic crabs.