Cannibalism and inter-specific predation in early stages of intertidal crab species that compete for refuges

• We measured rates of intra and inter-cohort cannibalism and inter-specific predation.
• Rates of intra-cohort cannibalism can be as high as inter-cohort cannibalism.
• No density-dependent effect was observed for either form of cannibalism.
• Juveniles show type III functional response when consuming recruits of either species.
• Recruits of the most abundant species suffer higher post-settlement mortality.

Most benthic species with dispersive life stages suffer high mortality rates in hours to days following settlement. Predation exerted by other species and predation by self (cannibalism) are commonly the chief sources of post-settlement mortality and can profoundly influence population dynamics and adult spatial distribution. In Chile, two predatory crab species (Acanthocyclus gayi and Acanthocyclus hassleri) coexist in the intertidal zone. Both settle preferentially in mussel beds, but adults show remarkable spatial segregation, apparently as a result of asymmetric competition for refuges. Although early recruits of A. gayi are an order of magnitude more abundant than A. hassleri, late juveniles are similarly abundant. Recruits of A. gayi are probably subjected to higher mortality before competition for refuges intensifies. Here, through laboratory experiments, we quantified the strength of intra and inter-cohort cannibalism and inter-specific predation as probable sources of differential post-settlement mortality. Intra-cohort cannibalism (among recruits of same size) accounted for the mortality of up to 30% of recruits in both species, with no evidence of density-dependent effects on mortality. Rates of cannibalism between juveniles and recruits (inter-cohorts) were also similar between the two species. Both species exhibit type III functional responses of juvenile predators with a tendency to consume heavily upon the most abundant recruits (A. gayi in the field), which could potentially provide A. hassleri recruits with a “virtual refuge” from the inter-cohort predation in the field. The combination of these different sources of mortality might contribute to the large reduction in abundance of A. gayi recruits by the time they reach juvenile stages. Our results illustrate the complexity of mechanisms that can underlay patterns of distribution and relative abundances among competitors through different life stages, especially among coexisting species in which attacking peers can provide higher rewards later in development than just the energy obtained from other food.