Predation is a major biotic force regulating prey populations, as well as exerting strong effects on community structure. Evidence suggests that the sense of smell mediates predatory search in many terrestrial and aquatic animals. Past investigations have not, however, been designed to link either the degree of successful olfactory-mediated search or guidance mechanisms with the production of chemical attractant by prey or the hydraulic environment in which predatory activities naturally take place. Using the predator - prey interaction between blue crabs (Callinectes sapidus) and hard clams (Mercenaria mercenaria), this project will investigate the olfactory basis for predation. Results from assays of crab predatory search behavior will guide chemical fractionation and isolation of peptide attractants being released by intact live clams. Water collected from the excurrent siphons of actively pumping clams will be characterized by partitioning into different molecular weight classes and by using selective enzyme degradation to probe peptide structures. As attractants are isolated and identified, pure compounds will be bioassayed for potency (dose-response) and for potential mixture interactions. In this manner, the natural ligands will be discovered and made available for discriminating between the roles of antennules and legs in chemosensation that mediates crab predatory search behavior. The rates of water exchange and attractant flux will then be measured for clams, and a biomechanical and chemical model of a clam releasing attractants will be engineered. Crab ability to find clam patches will be tested under controlled hydrodynamic conditions in a large (10-m long) flume, simulating flow in the natural habitat. As a final test, selected experiments will be repeated in the field by using live clams and models as attractant sources to free-ranging crabs. This project will thus provide the most rigorous test to date of olfaction as a regulatory factor in predator-prey interactions. It will yield new attractant structures for use in cellular and molecular studies on basic chemosensory processes, while it will provide a critical link between the olfactory sense of predators and prey vulnerability to predation. Finally, it could open significant avenues for future research to explore the mechanisms through which prey might defend themselves by adopting risk-sensitive metabolic strategies in which attractant production stops when nearby predator are detected.
