We studied how water motions affect larval behavior and dispersal in two related snail species in the Northeastern Atlantic Ocean. The species are similar but occupy different zones: ?inlet snails? live in turbulent coastal inlets and estuaries, and ?shelf snails? live offshore on the wavy continental shelf.  How do the species stay separated? The snails cannot move far as adults but disperse as tiny larvae that are carried by ocean currents for weeks or months before settling to the sea floor.  Dispersal varies when larvae move vertically (swimming or sinking) in response to cues in their environment, including small-scale motions caused by water waves (shaking) and turbulence (rotating).  In a previous laboratory study, we found that both species of larvae react to strong turbulence by sinking more frequently, but only shelf larvae also react to waves by swimming strongly upward.  These different responses to water motion could enable larvae of closely related species to disperse different distances and directions. We hypothesized that the observed behaviors enable the two species? larvae to remain and settle near their adult habitats and help to keep the adult distributions separate.   

We did numerical experiments to test how larval reactions to water motions affect local retention, dispersal, and settlement patterns in an estuary and on the continental shelf.  The experiments used virtual larvae released and tracked in model simulations of the Northeast Atlantic Ocean, focusing on the area around Delaware Bay. Virtual larvae were given either generic behaviors or responses to turbulence and waves matching those observed in inlet snails and shelf snails.  One experiment focused on dispersal and settlement of larvae released from Delaware Bay. Results showed that the observed turbulence-induced sinking of inlet snails improves their survival by keeping them near release sites, enabling them to grow faster, and helping them to settle sooner. A second experiment focused on larvae released from the continental shelf. Results showed that wave-induced upward swimming, observed in the shelf snails, promotes larval survival on the shelf by helping them to grow quickly and settle before reaching the Gulf Stream.  A third experiment focused on exchange of water and material between the continental shelf and Delaware Bay. Results confirmed that even generic behaviors strongly affect transport direction and retention time in the Bay.

We also studied whether larval dispersal could explain confusing effects of climate change on the distributions of bottom-dwelling species on the continental shelf.  Mobile species such as fish have adapted to a warming ocean by moving their ranges toward the poles, remaining in water temperatures they can tolerate.  Unexpectedly, shelf snails have instead moved to the southwest, into warmer water where they are less likely to survive. We analyzed historical distributions of 50 invertebrates including snails, clams, worms, and sea stars, and found that ? like the shelf snails ? most have unexpectedly moved their ranges to the southwest, into warmer water.  We analyzed historical bottom water temperatures and concluded that the ?wrong-way? range shifts could be explained by larvae being released earlier in spring, exposing them to faster currents to the southwest.  To test this hypothesis, we also did a numerical experiment in which larval release times were set by bottom temperatures in the current, warmer ocean and the past, colder ocean. Preliminary results are consistent with the observed range shifts and suggest that over the last 50 years, ocean warming has reduced or eliminated the supply of larvae to deeper parts of the continental shelf.

Our numerical experiments required us to create a new ocean simulation and to develop new software (ROMSPath), based on an older model (LTRANS), for tracking virtual particles.  The ocean simulation includes a hydrodynamic model (ROMS) and a wave model (SWAN).  The simulation spans 6 years (2009-2015), extends from North Carolina to Nova Scotia, and includes a higher-resolution region of Delaware Bay and the nearby shelf.  The simulation setup files and outputs have been made publicly available for other researchers to use.  The particle tracking software ROMSPath is also freely available and has already been used in other studies of harmful algal blooms, ocean acidification, microplastic transport, and estuarine residence times.

Broader Impacts: This project provided training to a postdoctoral investigator, Jessica Garwood, who has recently begun a faculty position at Oregon State University. Four undergraduate students (3 at Skidmore and 1 at Rutgers) received training in numerical modeling and analysis.  Research results have been shared through peer-reviewed publications, conference presentations, and invited seminars. Products of the ocean model have been distribution online, including model source codes, setup files, and simulation outputs. Particle models have been included in an undergraduate and graduate coursework at Rutgers. Our modeling work supported Rutgers' shared computing cluster, Amarel, which provides researchers with high-powered computing resources at a reasonable cost.

Last Modified: 08/30/2023
Modified by: Heidi L Fuchs