Behavioral mechanisms of swimming crab (Portunus trituberculatus) preying on Manila clams (Ruditapes philippinarum): effects of substrate and competition

Although constructions for marine ranching may influence the local habitat, few studies have focused on the effect of substrate changes on the predator–prey relationship in the benthic community. A predator–prey system was established in the laboratory to investigate how substrate type and conspecific competitor affect the behavioral mechanisms of swimming crab (Portunus trituberculatus Miers) foraging on Manila clams (Ruditapes philippinarum Adams and Reeve). Specifically, we calculated predation rate and quantified foraging behaviors and encounter behaviors between crabs. Circadian differences in foraging behaviors and intraspecific interactions were observed as a result of the burying behavior of crab in the substrate. Spatial refuges produced by the substrate reduced predation rate significantly. The effects of substrates were enhanced gradually by increasing particle size, reflected by continuously decreasing foraging efficiency. Crabs spent considerable time searching, but rarely encountered with a clam. The decline in foraging efficiency intensified the interactions between crabs (e.g., kleptoparasitism, and fighting), and hindered further foraging, indicated by an increase in proportion of handling time and decrease in probability of consumption upon capture. Additionally, crabs raised the probability of capture upon encounter in response to the decline of prey accessibility. The dual protections provided by substrate and intraspecific interactions decreased the predation risk of clams and the foraging efficiency of crabs. These mechanisms might help to reveal the effects of substrate changes on crabs’ predation and competition in marine ranching.