Effect of turbulence on oyster larvae

Investigating settlement responses in the transitory period between planktonic and benthic stages of invertebrates helps shape our understanding of larval dispersal and supply, as well as early adult survival. Turbulence is a physical cue which has been shown to induce sinking and potentially settlement responses in mollusc larvae. In this study, we determine the effect of turbulence on vertical swimming velocity and diving responses in competent eastern oyster larvae (Crassostrea virginica). We quantified the behavioral responses of larvae in a moving flow field by measuring and analyzing larval velocities in an relative framework (where local flow is subtracted away, isolating the behavioral component) in contrast to the more common absolute framework (in which behavior and advection by the flow are conflated). We achieved this separation by simultaneously and separately tracking individuals and measuring the flow field around them using particle image velocimetry (PIV) in a grid-stirred turbulence tank. Contrary to our expectations, larvae swam upward even in highly turbulent flow, and the dive response became less frequent. These observations suggest that oyster larvae are stronger

swimmers than previously expected and provide evidence that turbulence alone may not always be a sufficient cue for settlement out of the water column. Furthermore, at a population level, absolute velocity distributions differed

significantly from isolated larval swimming velocities, a result that held over increasing turbulence levels. The absolute velocity distributions indicated a strong downward swimming or sinking response at high turbulence levels, but this observation was in fact due to downwelling mean flows in the tank within the imaging area. Our results suggest that reliable characterization of larval behavior in turbulent conditions requires the subtraction of local flow at an individual level, imposing the technical constraint of simultaneous flow and behavioral observations.

Wheeler, J. D., Helfrich, K. R., Anderson, E. J., McGann, B., Staats, P., Wargula, A. E., Wilt, K. and Mullineaux, L. S. (2013). Upward swimming of competent oyster larvae (Crassostrea virginica) persists in highly turbulent flow as detected by PIV flow subtraction. Mar. Eco. Prog. Ser. 488, 171-185.

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