Water uptake in terrestrial hermit crabs: a morpho-functional analysis

Coenobita hermit crabs succeeded in their adaptation to land thanks to their shell and to the possibility of carrying water within it. Such a water reserve may play several roles: to compensate for dehydration, to keep the gills and abdomen surface sufficiently wet to allow gas exchange, to reduce temperature through evaporation and to facilitate osmoregulation. The mechanism through which water is absorbed and then imbibed or stored within the shell was studied using transparent shells and dyes. We were able to show that there are two pathways involving different organs. Drinking relies on setae on the 3rd maxillipeds while shell refilling involves the setae of both claws, as well as setae of the 3rd maxillipeds, and inverted scaphognatite beating activity. Shell refilling and drinking are alternated and both are interrupted by grooming activity. Thus, refilling the entire shell takes a variable period of time. The absorbing setae of both the maxilliped and the claw show a particular twisted shape, strongly resembling in both structure and density the absorbing setae at the base of the 3rd and 4th walking legs of the Ocypodidae.