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.