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Fluid mechanics produces conflicting, constraints during olfactory navigation of blue crabs, Callinectes sapidus

M. J. Weissburg^1,*, C. P. James², D. L. Smee¹ and D. R. Webster²

¹ School of Biology, Georgia Institute of Technology, Atlanta, GA 30332-0230, USA
² School of Civil & Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0355, USA

View larger version (10K): [in a new window]   Fig. 1. Schematic of the force balance for the drag measurements. The crab was supported at a distance l1 below the pivot point. A weight (W) on the horizontal lever arm, at a distance l2 from the pivot point, provided a moment in the opposite sense to that of the drag force (FD). Fshaft represents the drag force on the submerged section of the support shaft, which is at a distance lshaft below the pivot point.

View larger version (20K): [in a new window]   Fig. 2. Drag coefficient (CD) for five crab specimens at orientation angles () from 0° to 90°. Based on the expected symmetry of the flow patterns around the crab (i.e. roughly symmetrical left to right and front to back), a functional form of cos(2) is assumed for the drag variation with orientation angle. A trend line based on linear regression is shown and yields an r2 value of 0.84. A third-order polynomial fit with respect to yielded a similar r2.

View larger version (134K): [in a new window]   Fig. 3. Flow visualization of odor plumes 0.3 m from the source. Undisturbed plume (A) and a dead crab specimen oriented at 0° (B), 45° (C) and 90° (D). Crab specimen is #2 in Table 1.

View larger version (123K): [in a new window]   Fig. 4. Flow visualization for a live crab located 0.3 m from the source at an orientation of 45°.

View larger version (37K): [in a new window]   Fig. 5. Instantaneous concentration field for (A) an undisturbed plume and (B) a dead crab specimen oriented at 45°. The filament concentration (c) is normalized by the source concentration C0, and the streamwise (x) and cross-stream (y) distances are normalized by the channel depth H.

View larger version (18K): [in a new window]   Fig. 6. Cross-stream profiles of (A) the filament concentration average () and (B) the standard deviation at x/H=2.5 (see Fig. 5) for three crab orientations. Concentration is normalized by the source concentration C0, and the cross-stream distance (y) is normalized by the channel depth H.

View larger version (16K): [in a new window]   Fig. 7. Chemical signal characteristics near appendages of dead crab specimens as a function of body angle. (A) Average filament concentration (c) normalized by the source concentration C0 and (B) proportion of samples above the detection threshold (i.e. the intermittency factor). As shown in Fig. 3, the left leg moves upstream as the body is rotated from 0° to 90°.

View larger version (28K): [in a new window]   Fig. 8. Mean body angle for crabs in different flow (high- or low-velocity) and odor (tracking or non-tracking) conditions. Graph shows mean path angle for crabs in each treatment group ± 95% confidence limit. The measure of angular dispersion, r (the mean vector length), is 0.98, 0.98, 0.99 and 0.99 for high-velocity non-tracking, high-velocity tracking, low-velocity non-tracking and low-velocity tracking, respectively. Higher values indicate less variation, and the statistic has a maximum value of 1. The difference between mean angles of tracking vs non-tracking groups is significant only for the low-velocity flow (Watson—Wheeler test; F1,21=92.24, P<<0.001). The confidence limit is derived from r (Batschelet, 1981). Sample sizes consist of 9, 10, 8 and 14 paths for high-velocity non-tracking, high-velocity tracking, low-velocity non-tracking and low-velocity tracking, respectively.

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