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First published online March 8, 2005
Journal of Experimental Biology 208, 809-819 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01438

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Slow-moving predatory gastropods track prey odors in fast and turbulent flow

Matthew C. Ferner^1,2,* and Marc J. Weissburg¹

¹ School of Biology, Georgia Institute of Technology, Atlanta, GA 30332-0230, USA
² Skidaway Institute of Oceanography, Savannah, GA 31411-1011, USA

View larger version (11K): [in a new window]   Fig. 1. Hydrodynamic characteristics of the four unobstructed flow treatments. (A) Profiles of horizontal flow speed (Uz) at various heights (z) above the sediment. ADV measurements were recorded in the center of the flume at the location of stimulus release. Each data point represents a 4 min average (mean) of instantaneous velocities collected at a frequency of 10 Hz. Precise replication of measurements heights was not possible due to slight differences in signal resolution across treatments. (B) Vertical profiles of RMS turbulence intensity corresponding to each of the velocity records in the unobstructed flow treatments. Turbulence increased with velocity, such that the slowest flow was least turbulent and the fastest flow most turbulent. In all four unobstructed conditions, turbulence intensity was greatest from 1–2 cm above the sediment and decreased with height (z) until boundary effects were negligible.

View larger version (18K): [in a new window]   Fig. 2. Vertical profiles of RMS turbulence intensity in the three flow treatments having a free-stream velocity of U=5 cm s–1. Turbulence intensities were derived from ADV measurements of horizontal velocity at (A) the location of stimulus release and (B) the starting position of test animals, which was 1.5 m downstream from the stimulus source. Data for the smooth condition is the same as that shown in Fig. 1 and is included here for the sake of comparison. The bump and cylinder obstructions increased turbulence intensity near the sediment surface relative to unobstructed flow, an effect that persisted throughout the test section.

View larger version (9K): [in a new window]   Fig. 3. Conductivity data representing the number of stimulus peaks detected per second (open circles) and the relative peak concentrations (closed circles) for (A) the three fastest unobstructed flows and (B) the three flow treatments having a free-stream velocity of 5 cm s–1. Values for the smooth condition (U=5 cm s–1) are included in both graphs for the sake of comparison. Data points represent an average of three replicates (±S.E.M.) in which conductivity was recorded for 30 s at a frequency of 10 Hz. Peaks were identified as bursts of concentration above a baseline that was established from background measurements collected prior to each trial. Peak concentrations (C) include all measurements that exceeded baseline and are normalized to source concentration (C0).

View larger version (16K): [in a new window]   Fig. 4. Proportion of motivated whelks that successfully tracked prey chemicals in each flow condition. Success rates were independent of flow treatment for both unobstructed and obstructed flows (P>0.25). (A) Sample sizes for the four unobstructed treatments of 1.5, 5, 10 and 15 cm s–1 were 17, 19, 19 and 16, respectively. (B) Sample sizes for the bump and cylinder obstruction treatments (at U=5 cm s–1) were 14 and 17, respectively. No animals in any flow treatment tracked to the delivery nozzle in response to unscented control plumes.

View larger version (21K): [in a new window]   Fig. 5. Examples of whelk tracking behavior in the two most turbulent treatments: the cylinder obstruction in a flow of 5 cm s–1 (top panel) and an unobstructed flow of 15 cm s–1 (bottom panel). Trials were filmed with a CCD camera mounted directly above the flume and paths show motion of the anterior tip of an individual knobbed whelk. Images of animal location were collected at a frequency of 2 Hz, smoothed over 8 s bins and downsampled to a frequency of 0.125 Hz. Jagged lateral motions represent siphon casting as whelks followed odor plumes upstream and asterisks represent the stimulus source.

View larger version (14K): [in a new window]   Fig. 6. Average search time (±S.E.M.) required for successful whelks to navigate from the starting cage to the odor source located 1.5 m upstream. (A) Compared with search times in the slowest unobstructed flow, knobbed whelks tracked more efficiently (i.e. reduced search time) in the two fastest flows. Seven tracks were analyzed for each of the unobstructed treatments and asterisks indicate a significant reduction in search time compared with the 1.5 cm s–1 treatment (P<0.05). (B) Compared with search times in unobstructed flow of the same velocity, knobbed whelks tracked more efficiently when the cylinder obstruction introduced turbulent mixing at the odor source. Five tracks were analyzed for each of the obstruction treatments and asterisks indicate a significant reduction in search time compared with the unobstructed (smooth) condition (P<0.05).