Differential regulatory roles of crustacean predators in a sub-arctic, soft-sediment system

The role of predation in structuring soft-sediment communities varies as a function of the number and composition of predators that co-occur in a given habitat. In Bonne Bay, Newfoundland, contrasting abundances or predators in different areas of the bay may contribute to different regulatory roles of predators on infauna. To test this hypothesis, results from a field exclusion experiment were compared with 5 laboratory experiments that measured the individual effects of the main crustacean predators of the bay: snow crab, rock crab, and toad crab. In the field experiment, the exclusion of predators generated clear differences in infaunal composition, and 2 species (the polychaete Pholoe tecta and the clam Macoma calcarea) dominated exclusion treatments. Predator exclusion also resulted in a significant increase in density, but only a modest increase in infaunal diversity. In the laboratory, fresh, undisturbed sediment cores were paired with similar cores, protected by mesh and exposed to each crab species in order to test for their potential effects on infaunal communities. Results indicate that snow crab and rock crab have clear effects on species composition and, as was the case with the field experiment, the infaunal species P. tecta and M. calcarea dominated exclusion treatments for both predatory crabs. These predators also reduced total infaunal density, but only rock crab significantly reduced species richness. In contrast, toad crab effects were not significant. Given that snow crab and rock crab are both targeted by commercial fisheries in Atlantic Canada, our results suggest that crab fishery removal may have multiple indirect effects on infaunal communities.