Departures from isotropy: the kinematics of a larval snail in response to food

The swimming behavior of invertebrate larvae can affect their dispersal, survival, and settlement in the ocean. Modelling this behavior accurately poses unique challenges as behavior is controlled both by physiology and environmental cues. Some larvae use cilia to both swim and create feeding currents, resulting in potential trade-offs between the two functions. Food availability is naturally patchy and often occurs in shallow horizontal layers in the ocean. Also, larval swimming motions generally differ in the horizontal and vertical. In order to investigate behavioral response to food by ciliated larvae, we measure their behavioral anisotropy by quantifying deviations from a model based in isotropic diffusion. We hypothesize that larvae will increase horizontal swimming and decrease vertical swimming after encountering food which could lead to aggregation at food layers. We consider Crepidula fornicata larvae which are specifically of interest as they exhibit unsteady and variable swimming behaviors that are difficult to categorize. We tracked the larvae in still water with and without food, with a portion of the larvae starved beforehand. On average, larvae in the presence of food were observed higher in the water column, with higher swimming speeds and higher horizontal swimming velocities when compared to larvae without food. Starved larvae also exhibited higher vertical velocities in food, suggesting no aggregation behavior. While most treatments showed strong anisotropy in larval behavior, we found that starved larvae without food exhibited approximately isotropic kinematics, indicating that behavioral anisotropy can vary with environmental history and conditions to enhance foraging success or mitigate food-poor environments.