Rock crab predation of juvenile sea scallops: the functional response and its implications for bottom culture

The functional response describes how consumption rate of individual predators changes as prey density changes, and can have important implications for the bottom culture of scallops. We examined (i) the functional response of rock crabs (Cancer irroratus) preying on juvenile sea scallops (Placopecten magellanicus); (ii) the effect of substrate type and scallop size on the functional response; and (iii) the underlying behavioural mechanisms of observed functional responses. Specifically, we quantified predation rate and behaviours, such as the proportion of time spent searching for prey, encounter rate between predators and prey and the outcomes of encounters, when individual rock crabs were offered a range of scallop density (2–50 or 11–111 scallops m⁻²) and two size classes of scallops (∼ ∼25 and ∼ ∼35 mm shell height) on two different substrate types (“glass-bottom” and “granule”). We found that crab predation rate on small juvenile scallops increased at a decelerating rate with prey density to a plateau at high prey density on both substrates, indicating a hyperbolic (type II) functional response. Crab predation rate on large juvenile scallops was independent of prey density (i.e., no functional response evident), suggesting that crabs were at their satiation level. Prey density did not influence any behaviour except encounter rate on small juvenile scallops, which increased as prey density increased. Substrate type influenced crab predation: maximum predation rate of crabs on small juvenile scallops and encounter rate with either size of juvenile scallops was lower on granule than on glass-bottom. Our results in the laboratory suggest that crabs could potentially be swamped if scallops are seeded at a high density in the field. However, many factors in the field may influence the functional response. For example, the presence of multiple prey types may lead to sigmoid functional responses, while the presence of many crab individuals may lead to aggregation of crabs in areas of high prey density.