Life and Death in Moving Fluids: Hydrodynamic Effects on Chemosensory‐Mediated Predation

Abstract

Previous studies in taxonomically diverse marine animals have established the general existence, and importance, of the olfactory sense in a wide variety of behavioral processes. Evidence suggests that the sense of smell mediates predatory search in many marine animals. Past investigations have not, however, been designed to link either the degree of successful olfactory—mediated search or guidance mechanisms with the hydraulic environment in which predatory activities naturally take place. In an effort to examine the interaction between hydrodynamics and chemoreception, we investigated predatory success and search strategies of blue crabs foraging in controlled hydrodynamic environments generated in a flume. Hydrodynamics were characterized by quantifying boundary layer shear velocity (°) and roughness Reynolds number (Re°), two measures that describe the structure of boundary layer flows. Flow properties affected the ability of crabs to orient to odor plumes emanating from actively pumping infaunal bivalves (Mercenaria mercenaria). High flow speed or large sediment particle size increased boundary layer turbulence, thereby decreasing the success of crab chemo—orientation ability. In addition, high flow speed also lessened the probability that crabs contacted odor plumes. Thus, habitats with high flows can provide hydrodynamic refuges from olfactory—mediated predation. Because search ability is contingent on the magnitude of boundary layer turbulence, olfactory—mediated predation may also be ineffective in slow flows, if bottom roughness elements can generate sufficient turbulence. Further, search ability in blue crabs is extremely sensitive to small changes in benthic boundary layer structure. The presence of a thick viscous sublayer, dominated by quasilaminar flow, seems especially critical for successful location of an odor source by crabs. Benthic estuarine crustaceans inhabit an environment where flows are transitional between smooth—and rough—turbulence conditions. Accordingly, chemosensory systems appear geared primarily to extracting information from hydraulically smooth flows. These results indicate that mechanisms governing the physical transport of odor signals can have profound influences, not only on the development of sensory and behavioral mechanisms, but also on biotic interactions such as predation, which, in turn, can mediate community structure.