Simultaneous measurement of larval behavior and turbulence in the laboratory

Lauren Mullineaux, Biology
Karl Helfrich, Physical Oceanography


Project Funded 2008:

Species living on the seafloor in the coastal zone typically have a two-stage life cycle that alternates between the sedentary adult and a highly mobile larval form.  The larva is able to disperse away from the natal habitat, providing a conduit for the exchange of individuals and genes between populations. The mobility of this stage gives the population an ability to colonize new sites and persist despite disturbance at the natal site.  Tracking the movement of larvae, however, is difficult and presents a challenge to researchers and managers who would like to predict the dynamics of coastal populations.

Our overall goal is to characterize larval behavior in realistic turbulent conditions in order to understand how the larvae disperse and settle into benthic habitats.  We know that larvae of some species have a dive response in which they stop swimming and drop to the seafloor in highly turbulent conditions.  This response may help larvae identify and settle into suitable habitat.  Our specific objectives in this project are to determine the turbulence threshold that elicits a dive response in larvae of the eastern oyster (Crassostrea virginica), and to explore what aspect of turbulence (e.g., shear? or acceleration?) the larvae sense.

We measure the spatial structure of turbulence in the laboratory using particle image velocimetry (PIV).  The PIV system uses a high-power laser to illuminate small scattering particles in a 130 L tank, where turbulence is generated by oscillation of two parallel grids.  The movement of these particles is recorded on video, and pictures of the turbulent flow are produced by comparing two consecutive video frames.  The pictures give a visual indication of the scales and intensity of turbulence, and the velocity vectors are used to calculate various quantitative descriptors, including dissipation rate.

The paths of larvae are also being recorded at the same time as turbulence measurements.  Larvae are distinguished from the scattering particles by size, and are easily detected by eye in the laser light sheet.  Video records are analyzed to track larval motions and determine how larvae behave when they encounter specific features of turbulence.

In a series of preliminary experiments, using a range of turbulent intensities that are relevant to the flows in coastal environments, we tracked larvae and particles to document mean larval vertical motions relative to the fluid (i.e., their behavioral velocities). We found that behavioral velocities became downward at turbulent dissipation rates above 0.06 cm2/s3. As turbulence intensity increased above this threshold, a larger proportion of the larval population exhibited sinking behavior.  These results suggest that oyster larvae use turbulence as a cue to drop to the seafloor in areas where the adults will have enhanced access to particulate food.

Our initial attempts to use PIV simultaneously with larval tracking showed that oyster larvae have a distinct response to the green laser typically used in PIV.  To solve this problem, we are now using a near-IR laser and high-speed Photron camera to investigate responses of individual larvae to specific aspects of turbulent flow. We are also developing a three-dimensional particle tracking system (using three Hitachi KPF120 near-IR sensitive cameras and strobed near-IR LED lighting) that will provide valuable complementary information on larval motion.