The Role of the Predatory Crab, Liocarcinus depurator, in a Marine Food Web

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Abstract
The importance of large epibenthic predators in structuring marine sublittoral assemblages is unclear. Several previous investigations have yielded conflicting results, possibly because of limitations in experimental design. The effects of predation by the crab Liocarcinus depurator on prey assemblages are examined in a field experiment in a subtidal habitat on the west coast of Scotland [UK]. Cages covering 1 m2 maintained crab densities from zero to eight crabs m-2 (0-32 times natural densities) for a period of 24 days. The total number of prey individuals, the number of species in samples and abundance of selected species were compared between treatments using ANOVA. Overall differences in assemblages were explored using multidimensional scaling. Effects on population size structure for four common species were also examined. Contrary to expectation, there was little evidence that predation by Liocarcinus had any significant effect on the abundance of benthic species. The number of species in samples and the total number of individuals in samples were not significantly different between treatments and there were no effects on individual species which could be interpreted in terms of predation. The multivariate analysis, however, suggested some effects of caging, possibly due to the exclusion of other predators in addition to crabs in the case of infauna. Potentially confounding process such as prey mobility, crabs not feeding normally and insufficient power in the experiment design appear not to be significant features of our results. Therefore, we conclude that Liocarcinus has trivial effects on the benthic assemblages in the study area.

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