J. D. Wheeler, K. R. Helfrich, E. J. Anderson and L. S. Mullineau
Understanding the behavior of larval invertebrates during planktonic and settlement phases remains an open and intriguing problem in larval ecology. Larvae modify their vertical swimming behavior in response to water column cues in order to feed, avoid predators, and search for settlement sites. The larval eastern oyster (Crassostrea virginica) can descend in the water column via active downward swimming, sinking, or “diving”, which is a flick and retraction of the ciliated velum to propel a transient downward acceleration. Diving may play an important role in active settlement, since diving larvae move rapidly downward in the water column and may regulate their proximity to suitable settlement sites. Alternatively, it may function as a predator-avoidance escape mechanism. We examined potential hydrodynamic triggers to this behavior by observing larval oysters in a grid-stirred turbulence tank. Larval swimming was recorded for two turbulence intensities and flow properties around each larva were measured using particle image velocimetry. The statistics of flow properties likely to be sensed by larvae (fluid acceleration, deformation, vorticity, and angular acceleration) were compared between diving and non-diving larvae. Our analyses showed that diving larvae experienced high average flow accelerations in short time intervals (approximately 1-2 seconds) prior to dive onset, while accelerations experienced by non-diving larvae were significantly lower. Further, the probability that larvae dove increased with the fluid acceleration they experienced. These results indicate that oyster larvae actively respond to hydrodynamic signals in the local flow field, which has ecological implications for settlement and predator avoidance.