Prey selection of the swimming crab Portunus trituberculatus (Miers, 1876) (Brachyura: Portunidae) foraging on bivalves

Abstract

Predators may be exposed to different prey types simultaneously, and so may select certain prey types over others. We examined prey selection, predation rate, and foraging behaviour of the swimming crab Portunus trituberculatus (Miers, 1876) provided with three types of clams, the Manila Ruditapes philippinarum (Adams & Reeve, 1850), bloody Scapharca subcrenata (Lischke, 1896), and hard Meretrix meretrix (Linnaeus, 1758) clams in laboratory experiments. When provided will all three possible prey simultaneously, crabs exhibited higher preference for R. philippinarum over S. subcrenata and M. meretrix (Chesson’s selectivity index; P = 0.003). In the single-prey experiments, predation rates were significantly higher on R. philippinarum and S. subcrenata than on M. meretrix (P = 0.002). Video analysis revealed that prey type significantly affected both the proportion of time crabs spent on searching, and the probability of consumption upon capture. The proportion of time crabs spent on handling (P = 0.171), the encounter rate (P = 0.918), and the probability of capture upon encounter (P = 0.456), however, were not significantly affected by prey types. Handling time per prey was not significantly different among clam species. For the crab, prey profitability (energy intake per unit handling time) of R. philippinarum was similar to that of S. subcrenata, in both cases being significantly higher than that of M. meretrix (P ﹤ 0.001). The relative frequencies of changing from searching to handling were significantly higher for M. meretrix than for R. philippinarum and S. subcrenata (P = 0.007). These results suggest that the probability of consumption upon capture explained the observed selection by crabs. Furthermore, prey profitability, shell strength, and shell width, are important elements to affect prey selection of the crab.