Bed-roughness effects on boundary-layer turbulence and consequences for odor-tracking behavior of blue crabs (Callinectes sapidus)

We employed laser-induced fluorescence (LIF) measurements of odorant plume structure and behavioral observations to examine how turbulence affects the three-dimensional structure of odorant plumes and subsequently mediates olfactory search efficiency and success in our model organism, the blue crab (Callinectes sapidus). The turbulent characteristics of two laboratory flumes (one for behavioral quantification, one for signalstructure quantification) were systematically varied by changing the bed substrate roughness to create smooth, transitional, and fully rough flow conditions. Generally, increasing bed roughness caused greater mixing, decreased the time-averaged odorant concentration and concentration variance, and increased the plume width and homogeneity. Foraging success and speed of blue crabs attempting to locate the odorant source both declined consistently with increasing bed roughness. The variation in signal structure at the height of the antennules among bed-roughness treatments explains the observed behavior differences in crab foraging speed. In contrast, steering (path linearity) appeared to be unaffected by bed-roughness–induced turbulence. The transverse correlation function for odorant concentration at sensors separated across the width of the plume was examined among bedroughness treatments, and, ultimately, the correlation function was found to be related to the spatial position of tracking blue crabs. The spatial arrangement of blue crab chemosensors combined with the three-dimensional structure of odor plumes account for the differential effects of turbulence on the speed and success of crabtracking behavior. The importance of flow and turbulence to the ecology of aquatic benthic organisms has been widely reported (e.g., Nowell and Jumars 1984; Hart et al. 1996; Keller and Weissburg 2004), but the mechanisms linking the flow characteristics to ecological processes remain poorly quantified. Bed roughness is one environmental variable that influences turbulence and is known to be important in shaping the character of boundary-layer flows in aquatic systems (Nowell and Jumars 1984). Roughness has also been of interest to engineers and atmospheric scientists for many decades and is known to influence the intensity and spatial distribution of turbulence (Jiménez 2004). Mobile benthic organisms moving through an estuarine environment encounter a variety of substrates that introduce various levels of turbulence and consequently influence the mixing of chemicals and the structure of ambient chemical signals. Turbulence-induced changes in chemical signal structure are an important determinant of chemosensory abilities in the laboratory (Moore and Atema 1991; Weissburg and Zimmer-Faust 1993; Ferner and Weissburg 2005) and field (Finelli et al. 2000; Smee and Weissburg 2006), and thus, the environmental characteristics that alter plume structure may exert significant ecological effects. The goal of the current study was to explore the consequences of the physical characteristics of the flow environment for chemical signal structure in a benthic boundary layer and animal behavior. Chemical signaling plays an important role in aquatic systems by mediating interactions between organisms, including mate location, prey tracking, or predator identification (Weissburg 2000; Koehl 2006; Vickers 2006). The proficiency with which organisms locate odorant sources, such as prey or mates, has been shown repeatedly to be contingent on the flow environment (Weissburg and Zimmer-Faust 1993; Mafra-Neto and Cardé 1994; Belanger and Willis 1996), which clearly links the ecology of these organisms to hydrodynamic mechanisms. Navigation toward an odorant source relies on an animal’s ability to encode plume signal structure conveyed by spatial and temporal patterns of chemical stimulus intensity (Moore and Atema 1991; Weissburg and ZimmerFaust 1994; Finelli et al. 2000). This is a challenging task since turbulent mixing creates a signal structure that exhibits great spatial and temporal variability (e.g., 1 Present address: Department of Civil and Environmental Engineering, Cal Poly State University, San Luis Obispo, California 93407. Acknowledgments This research was funded by a National Science Foundation grant (IBN-0321444) to M.J.W. and D.R.W. and a NSF Integrative Graduate Education and Research Traineeship Program fellowship awarded to J.L.J. Thanks are due to W. Alex Berry for help with the velocity measurements. Limnol. Oceanogr., 52(5), 2007, 1883–1897 E 2007, by the American Society of Limnology and Oceanography, Inc.