Foraging behaviour of the crab Cancer pagurus feeding on the gastropods Nucella lapillus and Littorina littorea: comparisons with optimal foraging theory

Abstract

Juvenile Cancerpagurus used the left and right chelae with equal facility to immediately crush small Nucella lapillus and Littorina littorea, and to gradually chip away the shells of larger snails. Due to slippage of the chela over the apical whorls, attacks directed towards the shell aperture of larger prey were more successful than those applied to the spire. Attack success rate decreased from unity at the critical size for crushing to zero at the critical size for apertural breakage, falling more rapidly for N. lapillus than for L, littorea. Handling times rose rapidly for N. lapillus as prey exceeded critical size for crushing and consequently the maximum yield per unit handling time coincided with this critical size. Maximum yield for L. littorea was within the range of sizes that were opened by apertural breakage. Profitability curves derived from single regressions of yield and of handling time on prey size failed to predict profitability accurately. Satisfactory curves were obtained by combining separate handling time functions for prey below and above the critical size for immediate crushing. When offered a range of prey sizes, C. pagurus attacked all size classes, but incorporated smaller prey into the diet first as they succumbed more frequently to attack. The greater susceptibility of L. littorea was reflected in differential predation on this species when offered together with N. lapillus. Contrary to optimal foraging expectations of short persistence times when prey are of variable and relatively unpredictable profitability, C. pagurus persisted for up to several hours with large prey. Thus prey-size selectivity appeared to have a passive mechanical, rather than an active behavioural basis.