Portunid crab predation on juvenile hard clams: effects of substrate type and prey density

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Abstract
Studies on predation are often based solely on the measurement of predator consuumption rates, providing little information on underlying behavioral mechanisms. In this study, behavioral observations were used in conjunction with consumption rate analyses to investigate the predatory interactions between 2 brachyuran crabs, Ovalipes ocellatus and Callinectes sapidus, and juvenile hard clams Mercenaria mercenaria (15 to 20 mm SL). In laboratory experiments, O. ocellatus was observed foraging on 5 densities of clams (24 to 120 m-2) in sand with and without the addition of natural shell debris. Across all prey densities, crab consumption rates were significantly higher in sand than in the sand/shell substrate. Behavioral observations indicated that crabs foraged less efficiently in sand/shell, particularly at low prey densities, suggesting the existence of a low-density prey refuge in the more heterogeneous substrate. In comparative experiments, where O. ocellatus and C. sapidus were permitted to feed on a single low prey density (24 m-2) in 3 substrates (sand, sand/shell, sand/gravel), both crabs foraged most successfully in sand. Although consumption rates for both species were similar, behavioral observations demonstrate distinct differences in general crab behavior patterns. Direct behavioral observation of predatory interactions can provide critical information often unavailable through the measurement of consumption rates alone. Results suggest that utilization of low density prey refuges in heterogeneous substrates may be an effective strategy for reducing portunid crab predation on juvenile hard clams in mariculture operations.