The organisation of behaviour in hermit crabs: Responses to variation in stimulus strength

Chemical stimuli were presented to individuals of the hermit crab Diogenes avarus to determine the nature of the interactions between stimuli that elicit conflicting responses. The odour of degraded snail flesh (a signal associated with potential empty shells) elicited an increase in both locomotion and the rate of grasping of gastropod shells. The odour of a visual predator (Matuta lunaris) elicited a cessation of locomotion by the hermit crabs. When snail flesh odour was presented in combination with various strengths of the predator odour (5% to 100%), the responses tended to show a step-function relationship to stimulus strength. Predator inhibition of snail-induced grasping dominated until predator strength was just 5% of full strength odour. However, 5% predator odour alone induced a response similar to full strength predator odour. In the case of locomotion, snail-induced increases predominated no matter what the strength of the predator odour. For both behaviours, responses of hermit crabs tended to be hierarchical rather than graded.