Weight difference threshold during shell selection relates to growth rate in the semi-terrestrial hermit crab Coenobita compressus

To minimize the energetic expenditure imposed by carrying a protective gastropod shell, terrestrial hermit crabs should discriminate the weight difference between available shells; if the difference exceeds a certain amount — the threshold — the lighter shell should be accepted because of the cost of growing in a shell that is too heavy, while swapping between similar shells (less different than the threshold) may be inconsequential. We assigned shells of different weight adjusted to the crab's size to individually-housed crabs to (1) establish the weight difference at which crabs systematically reject a heavy shell and (2) compare growth rates of crabs confined to shells that were lighter-or heavier than the weight differential threshold. Crabs were significantly more likely to reject a heavy shell over a lighter one if the difference between the two was > 50%, and when the shell weight exceeded 60% of the crab's weight. Crabs living in shells < 50% heavier than their preferred shell grew larger than crabs living in shells that were > 50% heavier. This suggests that hermit crabs reject a heavier shell if a meaningful threshold is exceeded: this finding should prompt research into the neurophysiologic mechanism involved in shell assessment.