Size matters for predator-prey relationships: The case of swimming crab Portunus trituberculatus preying on Manila clam Ruditapes philippinarum

The effects of size of Manila clam Ruditapes philippinarum on the predation rate, foraging time budget, encounter behavior and shell-breaking techniques of swimming crab Portunus trituberculatus were studied by video recording analysis system in the laboratory. Here, we report the differences between two different sized crabs preying on two different sized clams. 1. Predation rate of large (100-110 mm carapace width, 6.31 clams·d− 1) and small crabs (80-90 mm carapace width, 4.25 clams·d− 1) preying on small sized clams (28-31 mm shell length) was higher than when preying on large sized clams (38-41 mm shell length, 1.81 clams·d− 1, 0.93 clams·d− 1). The predation rate of large sized crabs was higher than that of small sized crabs. 2. The proportion of time that the crabs spent on searching for clams decreased with increasing crab size or clams size. However, the proportion of time crabs spent on handling clams did not show a similar correlation. 3. Encounter rate between crab and clam increased with increasing crab and clam sizes. 4. There were no significant differences in the probability of capture upon encounter between crabs and clams at all size relationships. However, there were significant differences in the probability of consumption upon capture. 5. Behavior components analysis indicated that the probability of capture upon encounter appeared to be important mechanisms determining the predation rate of small crabs preying on both sized clams. The probability of consumption upon capture appeared to be important mechanisms determining the predation rate of large crabs on two sizes clams. Encounter rate and the probability of consumption upon capture were the important mechanisms determining the predation rate of two sizes crab on the same size clams. 6. The handling time per clam increased with increasing clam size. With increased clam size, crabs decreased the crushing of clams but increased cutting, chipping and prying of clams.