Cannibalism, post-settlement growth rate and size refuge in a recruitment-limited population of the shore crab Carcinus maenas

Previous studies on intra-specific interactions among green shore crabs have been undertaken in very large populations, in which individuals concentrate at high densities in physically complex substrates. Under such conditions, cannibalism and interference competition often override initial density patterns delivered by larval supply. In this study, we focused on a Carcinus maenas population receiving a low supply of settlers. We reported crab abundance in habitats of different physical structure, and measured cannibalistic interactions, in different substrates, at a prey density expected only after a major recruitment event. Different predator and prey sizes were used to verify if juveniles from a critical range attain a size refuge when coexisting with sub-adults, which are still commonly found in the nursery habitat. Virtually no juvenile shore crabs were found on sandy habitats, and differences between sparsely and densely vegetated cover were only detected shortly after a settlement pulse, indicating that nursery habitats are under their carrying capacity most of the time. Even under extreme high densities, cannibalism on juveniles of 10 mm carapace width (CW) remained undetected, and predation on crabs half this size was only significant in low and medium Zostera cover, when larger predators (20–25 mm CW) were included in experimental plots. The allometric model predicted the non-linear decrease of predation rate as a function of relative prey size, with a nearly asymptotic value of one prey consumed each 5 d by one predator in a square metre, when relative prey size attains 0.6. Such a relationship is expected for ‘cruising’ predators which rely on encounter rate. Growth estimates obtained using time series of the catch of distinct juvenile stages in artificial collectors indicated that crabs surviving 50 d after settlement have attained a size refuge from predation by larger conspecifics.

► Low densities of juvenile crabs suggest recruitment limitation at the Ria de Aveiro.
► Habitat-driven density limitation was only evident after a major settlement event.
► When vegetation cover was high, cannibalism on stages 4–7 was not significant.
► In less structured habitats, relative prey size predicted predation rate.
► Even in highest prey densities, a size refuge is attained 50 d after settlement.