Abstract:
The goals of this thesis were to quantify the behavior of gastropod larvae (mud snails
Ryanassa obsoleta) in turbulence, and to investigate how that behavior affects larval supply
in a turbulent coastal inlet. Gastropod larvae retract their velums and sink rapidly in
strong turbulence. Turbulence-induced sinking would be an adaptive behavior if it resulted
in increased larval supply and enhanced settlement in suitable coastal habitats.
In laboratory experiments, mud snail larvae were found to have three behavioral modes:
swimming, hovering, and sinking. The proportion of sinking larvae increased exponentially
with the turbulence dissipation rate over a range comparable to turbulence in a tidal inlet,
and the mean larval vertical velocity shifted from upward to downward in turbulence
resembling energetic nearshore areas.
The larval response to turbulence was incorporated in a vertical advection-diffusion
model to characterize the effects of this behavior on larval supply and settlement in a tidal
channel. Compared to passive larvae, larvae that sink in turbulence have higher near-bed
concentrations throughout flood and ebb tides. This high larval supply enables behaving
larvae to settle more successfully than passive larvae in strong currents characteristic of
turbulent tidal inlets.
A study was conducted at Barnstable Harbor, MA to estimate the responses of larvae
to turbulence in the field. Gastropod larvae from different coastal environments had
genus-specific responses to turbulence, suggesting that larvae use turbulence for large-scale
habitat selection. On ebb tides, mud snail larvae had a similar response to turbulence as
in the laboratory, with greater sinking velocities in strong turbulence. Behavior estimates
differed for flood and ebb tides, indicating that additional physical cues influence behavior.
Turbulence-induced sinking behavior would enhance retention and promote settlement of
mud snail larvae in habitats like Barnstable Harbor.
