Organisation of Hermit Crab Behaviour: Responses To Multiple Chemical Inputs

Abstract

To characterize the organisation of behaviour, the responses to chemical inputs of individuals of the diogenid hermit crab Clibanarius vittatus were tested in the laboratory. Stimuli were introduced both singly and in combinations on three days (one, four, and seven days after the crabs were last fed). First, responses to single inputs were characterized: (a) odour of a predator elicited an increase in locomotion; (b) odour of food also elicited an increase in locomotion; (c) odour resulting from snail predation (a signal of availability of an empty shell) elicited brief grasps of the shells of other crabs and (d) the odour of conspecific blood elicited long grasps of shells. Individual crabs differed significantly in their tendencies to respond to chemical signals. Those responding strongly to one cue responded strongly to other cues. Those responding to one shell-related cue (snail odour) responded to the other such cue (conspecific blood) and individuals showing a stronger response to food odours the first day did so on subsequent days. Combinations of cues yielded examples of both summation of inputs (rate of locomotion when snail and predator odours were combined) and inhibition of the responses to one input by the detection of the second odour (brief grasps typical of the response to snail odour were eliminated by addition of predator odour). Responses to conspecific blood were unaffected by predator odour detection. Increasing levels of hunger resulted in increases in locomotion in response to some chemical signals (control, snail odour, and conspecific blood) and a decrease in the number of brief grasps elicited by snail odour. The combination of increased hunger level and multiple chemical inputs yielded patterns not predictable from the two-factor combinations. The rate of locomotion decreased over time for snail + predator odours together and for blood + predator odours together, whereas the single odours resulted in either an increase or no change over time. Food + predator odours together elicited an increase in both long grasps of shells and feeding movements over days without food. While most instances of summation or inhibition of responses seemed biologically reasonable, other multi-input responses observed were difficult to explain. Thus, it is difficult to make generalisations about the interactive effects of various inputs.