NONCONVENTIONAL HERMIT-CRABS - PROS AND CONS OF A SESSILE, TUBE-DWELLING LIFE IN DISCORSOPAGURUS-SCHMITTI (STEVENS)

Discorsopagurus schmitti is a 'non-conventional' hermit crab in that i t adopts as shelters the fixed tubes produced by the worm Sabellaria c ementarium Moore. Experiments carried out in northern Puget Sound (Was hington, USA) showed that (1) tubes are limiting resources, (2) both e xploitative and interference competition comes into play when the sour ces of tubes are conspecifics, (3) residents strongly defend their hou sings from invaders, mostly using visual patterns and (4) change of tu bes is related more to the activities of feeding than to growth. Free- choice experiments in the laboratory revealed that adult hermits would prefer to live in loose tubes. On the one hand, a mobile life increas es hermit crabs' foraging efficiency, at least within laboratory condi tions. On the other hand, tubes are more fragile than gastropod shells and thus tube-dweliing hermits are more easily eaten by predators (es pecially brachyurans) than those housed in shells. On the premise of a shell-dwelling ancestry of Discorsopagurus schmitti, one possible sce nario is that, in evolutionary time, this hermit was displaced from ga stropod shells to worm tubes by more competitive species. In tubes, ho wever, D. schmitti makes now 'the best of a bad situation'.