Behavioural mechanisms of size-selection by crabs, Carcinus maenas (L.) feeding on mussels, Mytilus edulis L.

Abstract

Starved Carcinus maenas (L.) reached virtual satiation within 3 h when fed ad. lib. on medium sized Mytitus edulis L. When presented with mixtures of medium and small or medium and large mussels, crabs indiscriminately took the first few mussels encountered but within 30 min began rejecting certain prey. Although mussels in each size class were eaten in proportion to the rates at which they were picked up, crabs preferred medium mussels, so that these were relatively more abundant in the diet than small or large mussels. Two alternative behavioural hypotheses offer equally plausible explanations for the observed size selection. 1. (1) Prey-evaluation hypothesis: small mussels are rejected on the basis of size, gauged during a brief manipulatory period, whilst medium and large mussels are more often rejected on the basis of a strength, indicated by their resistance to crushing. 2. (2) Relative-stimulus hypothesis: retention or rejection of a mussel grasped in the chelae depends on the strength of the “chela” stimulus relative to stimuli caused simultaneously by other mussels touching the pereiopods. Handling is completed once a mussel begins to fracture but prior to fracture, strong “pereiopod” stimuli may cause a grasped mussel to be rejected. Both hypotheses assume that selectivity increases with decreasing hunger level and both behavioural mechanisms would maximize the average rate of food intake when crabs forage on mussel beds, where prey are numerous but extremely variable in shell strength and accessibility.