Larval Wobble: A Novel Vertical Swimming Behavior in Larvae of the Common Slipper Shell, Crepidula fornicata

Swimming behaviors of larvae of benthic invertebrates have important consequences for foraging, predator avoidance, dispersal, and population connectivity. We have observed a novel, unsteady, vertical swimming behavior in larvae of the common slipper shell, Crepidula fornicata, in which individuals tilt about their vertical axis (wobble) as they move upward. This discovery raises questions about what causes the wobble, how it influences the surrounding fluid motion, and what benefit it might confer on the larvae. To tackle these questions, we video-recorded behaviors of free-swimming, precompetent, larvae (0.4 – 0.5 mm shell diameter) in a newly developed jet-stirred turbulence tank. Using particle tracking and particle image velocimetry, we resolved the velocity of the larvae and the surrounding flow in two dimensions. We found that this wobble is a consistent feature of the motion of larvae, detectable in still water and in turbulence ranging from dissipation rates of 0.01 to 1.0 cm² s^-3. The frequency of the wobble is roughly 1 Hz, which is substantially lower than typical gastropod larvae ciliary beat frequencies, and higher than the frequency associated with their helical swimming paths. In these experiments, the larvae were most frequently observed upright while wobbling; however, under the strongest turbulent conditions upside-down larvae were observed for short times. In a prior study, the wobble was also observed in larger C. fornicata larvae that were nearing competency. The uniqueness of this wobble behavior to C. fornicata, and its persistence across different broods, ontogenetic stages, and turbulence intensities, suggests that it may play an integral role in their foraging or dispersal ecology.