ODOR PLUMES AND HOW BLUE CRABS USE THEM IN FINDING PREY

Orientation of animals using chemical cues often takes place in flows, where the stimulus properties of odorants are affected by the charact eristics of fluid motion. Kinematic analysis of movement patterns by a nimals responding to odor plumes has been used to provide insight into the behavioral and physiological aspects of olfactory-mediated orient ation, particularly in terrestrial insects. We have used this approach in analyzing predatory searching by blue crabs in response to plumes of attractant metabolites released from the siphons of live clams in c ontrolled hydrodynamic environments. Crabs proceed directly upstream t owards clams in smooth-turbulent flows and show high locomotory veloci ties and few periods of motionlessness. Crabs assume more indirect tra jectories and display slower locomotion and more stopping in rough-tur bulent flows. This degradation of foraging performance is most pronoun ced as flow shifts from a smooth- to a rough turbulent regime, where t he change in hydraulic properties is associated with contraction of th e viscous sublayer region of the boundary layer. Because flow in this region is quasi-laminar, the viscous sublayer may be a particularly ef fective vehicle for chemical stimulus transmission, such that orientat ion is severely compromised when it is reduced or removed. Our results also suggest that rheotactic and chemical information, are both neces sary for successful orientation. Perception of chemical cues acts to b ias locomotion upcurrent, and feedback from odorant stimulus distribut ions appears directly to regulate subsequent stopping acid turning en route to prey. Although the mechanisms of orientation to odorant plume s displayed by insects and blue crabs are largely similar, blue crabs appear to rely more heavily on spatial and/or temporal aspects of chem ical stimulus distributions than has been suggested for insect systems .