Responding to the signal and the noise: behavior of planktonic gastropod larvae in turbulence

Swimming organisms may actively adjust their behavior in response to the flow around them. Ocean flows are typically turbulent and are therefore characterized by chaotic velocity fluctuations. While some studies have observed planktonic larvae altering their behavior in response to turbulence, it is not always clear whether a plankter is responding to an individual turbulence fluctuation or to the time-averaged flow. To distinguish between these two paradigms, we conducted laboratory experiments with larvae in turbulence. We observed veliger larvae of the gastropod Crepidula fornicata in a jet-stirred turbulence tank while simultaneously measuring two components of the fluid and larval velocity. Larvae were studied at two different stages of development, early and late, and their behavior was analyzed in response to different characteristics of turbulence: acceleration, dissipation and vorticity. Our analysis considered the effects of both the time-averaged flow and the instantaneous flow, around the larvae. Overall, we found that both stages of larvae increased their upward swimming speeds in response to increasing turbulence. However, we found that the early-stage larvae tended to respond to the time-averaged flow, whereas the late-stage larvae tended to respond to the instantaneous flow around them. These observations indicate that larvae can integrate flow information over time and that their behavioral responses to turbulence can depend on both their present and past flow environments.