Award: OCE-1060622

Many bottom-dwelling invertebrates (e.g., oysters and snails) are immobile as adults, and in order to exchange genetic material among populations, they must disperse as planktonic larvae that are transported by ocean currents.  Dispersal mechanisms are poorly understood, creating a fundamental barrier to predicting how populations fluctuate and spread. Dispersal patterns can be strongly affected by larvae changing their vertical swimming or sinking behavior in response to cues from the environment.  As larvae move vertically, they encounter and are transported by currents and other physical features with different intensities.  Physical processes such as turbulence and waves produce water motions that influence transport toward or away from adult habitats. One intriguing possibility is that these water motions also act as behavior cues, inducing larvae to move vertically so that they are transported more successfully to suitable habitats.

The goal of this study was to test how turbulence and waves affect larval behavior in species that occupy distinct habitats.  We hypothesized that larvae from turbulent coastal inlets would respond to signals that are largest in turbulence (flow rotation or deformation), whereas larvae from the wavy continental shelf would respond to signals that are largest under surface waves (flow acceleration).  By reacting differently to these signals, closely related species could achieve different patterns of larval dispersal, reinforcing their separate biogeographic distributions.   

Pilot experiments focused on larvae of the Eastern oyster, a reef-building species from shallow habitats.  Oysters form a valuable shellfishery that is endangered by dramatic losses of oyster reef worldwide. Oyster restoration efforts often fail, most notably due to the difficulty in predicting larval recruitment. We described how oyster larvae behave in response to physical cues from turbulence and waves. Their flow-induced behaviors would give larvae better control over their swimming motion in chaotic turbulent flow, raise settlement rates and settler densities on oyster reefs, enhance shoreward transport by waves over the continental shelf, and enable avoidance of predators that capture prey using suction. Overall, the larval reactions to flow would enhance transport to adult habitats, but at a high metabolic cost. Those costs could be offset by the fitness benefits of more successfully reaching adult habitats.

Our main experiments – detailed in a recently submitted manuscript – compared the flow-induced larval behaviors of two closely related mudsnail species from turbulent inlets and the wave-dominated shelf.  These snails reacted similarly to turbulence but reacted differently to waves, with behaviors that would lead to divergent transport patterns in the coastal ocean.  Results suggest that flow-mediated larval behaviors represent fine-tuned adaptations for migration among specific habitats.

This research illuminated the capacity of larval sensory systems to enable larval navigation using cues from turbulence and waves.  Many plankton use water motions to sense predators or prey, but most of them detect flow as bending or stretching (deformation) of sensors on the body surface. In contrast, we were able to pinpoint that oyster larvae detect flow-induced rotation or acceleration using an internal sensory organ like the human inner ear (statocysts). These deceptively simple organs provide larvae with a capacity to sense turbulence and waves as separate processes.  To determine if this sensing ability could enable larvae to navigate in space, we synthesizing the sensory signals produced by turbulence and waves in the surf zone, inlets and estuaries, continental shelf, and open ocean. Results confirmed that ocean regions have distinct ranges of physical sign...