Alarm/investigation responses of hermit crabs as related to shell fit and crab size

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Abstract
Hermit crabs may acquire shells by trading with other crabs or at predation events that make new shells available. Predation events are located chemically through odors associated with the death of gastropods and the death of conspecifics. Crabs respond identically to either source of cues. Here we report that hermit crabs, Clibanarius vittatus from the field respond in one of three distinct ways to chemicals from gastropod flesh and from conspecific hemolymph: (1) crabs do not respond (23%), (2) crabs flee (40%), and (3) crabs investigate neighboring shells (37%). Changing the size of the shell can cause crabs to change behavior. Altering shell size by clipping down shells resulted in high (approx. 80%) response by investigation. Crabs given a free choice of unoccupied shells fled (86%) in response to chemical stimulation. In a series of experiments small crabs were tested for chemically‐stimulated behavior and then given access to excess empty shells. Chemically‐stimulated behavior was assessed after 24 hours, the majority of crabs fled upon stimulation. Shells occupied by crabs were then ordered from largest to smallest and crabs moved up or down one to the shells. Crabs tested after being moved into new shells routinely fled upon chemical stimulation. After approximately 30 minutes, reproducible behaviors related to the new shell were expressed. Most crabs moved to the next smaller shell investigated. Most crabs moved to one shell larger fled. Most crabs moved to two shells larger withdrew upon chemical stimulation. Hermit crab behavioral responses to chemical cues include escape, investigation, and withdrawal and are dependent upon the relative size of shell inhabited.