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 polychaet e tubes in the North Pacific. Experiments were designed to explore the dichotomy, ''dispersal or retention'' of marine invertebrate larval p hases, that seems particularly puzzling in this species which is confi ned within a rare habitat, namely the bioherms built by the worm Sabel laria cementarium. All the four larval stages of this crab are negativ ely buoyant, and their responses to the more conservative variables (g ravity and hydrostatic pressure inducing, respectively, tactic and kin etic responses) seem to suggest that the larvae (especially early zoea e) 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, an d seem less precise in their depth regulation. Other, more refined beh avioral mechanisms (e.g. their responsiveness to stimuli emitted by ei ther sabellarian tubes or adult conspecifics) might play a role in mai ntaining the position of Stage IV larvae close to the parental populat ion.