Behavioral mechanisms underlying the functional response of the swimming crab Portunus trituberculatus preying on the Manila clam Ruditapes philippinarum

Abstract

The rate and behavioral parameters of predation by the swimming crab Portunus trituberculatus on Manila clams Ruditapes philippinarum were measured with a video recording analysis system in aquaria. The type of functional response was determined, and the underlying behavior mechanisms displayed by swimming crabs preying on Manila clams were examined by linking the functional response parameters (a and Th) to foraging behaviors. Significant differences were observed in the swimming crab predation rate among different Manila clam densities (P < 0.001); predation rates of individual crabs offered clam densities of 2–8 clams/0.36 m−2 reached 1.37–2 clams/day−1, which increased with increasing clam density, but when crabs were offered clam densities of 10–50 clams/0.36 m−2, crab predation rates tended to be higher, at 2.13–2.25 clams/0.36 m−2. The parameters of functional response models, including a (rate of successful search, P = 0.455) and Th (handling time per prey, P = 0.472), were not significantly affected by clam density. The estimated a values were consistent with values derived from available behavioral data, whereas the Th values exhibited slight differences. Moreover, clam density significantly influenced crab foraging time budgets, encounter rates, the probability of capture upon encounter, and the probability of consumption upon capture (P < 0.001). The proportion of time crabs allocated to searching for and handling clams, and the probability of capture upon encounter, decreased with increasing clam density. A positive trend was detected for the encounter rate and the probability of consumption upon capture as clam density increased. These results, particularly in regard to the a and Th values, indicated that swimming crabs exhibit a type II functional response when preying on Manila clams in aquaria.