Foraging behaviour of an excavating predator, the red rock crab (Cancer productus Randall) on soft-shell clam (Mya arenaria L.)

We investigated the influence of soft-shell clam (Mya arenaria) burial dept h on red rock crab (Cancer productus) foraging behaviour. In laboratory tan ks, crabs searched for and excavated clams buried at depths of 5, 10, and 1 5 cm. Handling time of prey was partitioned into excavation time, breaking time, and eating time. Prey excavation formed 23% of mean total handling ti me (mean 8.2 min total 35.7 min(-1)) at 5 cm prey depth, 62% at 10 cm depth (mean 49.3 min total 79.5 min(-1)), and 73% at 15 cm depth (mean 93.5 min total 128.2 min(-1)). Mean excavation time for clams increased at a rate of 8.3 min cm(-1) of depth; clam profitability (energy intake per unit handli ng time (J s(-1))) decreased with burial depth. Predation rates decreased w ith prey burial depth. Often crabs gave up attempts to excavate a clam, and abandonment frequency and excavation time before abandonment increased sig nificantly (p < 0.05) with burial depth. Our findings confirm earlier studi es which indicate that burial depth affords an important refuge from crab p redation, significantly increasing prey handling time and decreasing the pr ofitability of individual prey. (C) 1999 Elsevier Science B.V. All rights r eserved.