Hermit crab larval behavior: depth regulation in Discorsopagurus schmitti (Stevens).

The behavioral basis of depth regulation has been investigated in the larvae of Discorsopagurus schmitti, a hermit crab inhabiting polychaete tubes in the North Pacific. Experiments were designed to explore the dichotomy, “dispersal or retention” of marine invertebrate larval phases, that seems particularly puzzling in this species which is confined within a rare habitat, namely the bioherms built by the worm Sabellaria cementarium. All the four larval stages of this crab are negatively buoyant, and their responses to the more conservative variables (gravity and hydrostatic pressure inducing, respectively, tactic and kinetic responses) seem to suggest that the larvae (especially early zoeae) migrate in a pattern which keeps them, on average, near the bottom. Older zoeae are more variable in their behavioral responses; they are less sensitive to pressure changes than those of the early stages, and seem less precise in their depth regulation. Other, more refined behavioral mechanisms (e.g. their responsiveness to stimuli emitted by either sabellarian tubes or adult conspecifics) might play a role in maintaining the position of Stage IV larvae close to the parental population.