Population Genetics and Movement Show Metapopulation Dynamics of Mid-Atlantic Region Horseshoe Crabs

Management of the American horseshoe crab Limulus polyphemus in the Delaware Bay region is aimed at promoting the sustainability of fished populations and providing abundant eggs to support the foraging of migrating shorebirds. To advance understanding of regional horseshoe crab population structure and frame recommendations for management, we conducted population genetics and mark-recapture studies to characterize metapopulation dynamics of horseshoe crabs in the mid-Atlantic region, USA. We used new microsatellite DNA data on seven regional-fished populations and an existing dataset on 25 spawning assemblages to estimate contributions to fisheries. We found that about 90% of the crabs landed originated from the mid-Atlantic management unit, but could not estimate contributions from its subpopulations with precision. A mark-recapture study supported the inference of high rates of migration and metapopulation structure among regional spawning assemblages. Horseshoe crabs spawning in Tom’s Cove on Assateague Island, Virginia, during new and full moons in May–June 2008 included 39 individuals that had already been tagged in Delaware Bay and off of Ocean City, Maryland. Of the 8574 crabs that we marked in this study, 1019 were recaptured in the local commercial fishery the same summer. After spawning in Tom’s Cove, horseshoe crabs dispersed and within 6 months, 58 were collected in offshore biomedical and research trawls. After 10 years, resightings of marked crabs were made as far away as South Carolina and Massachusetts, but with a strong directional bias towards the northeast. The population genetics and movement studies illustrated how mid-Atlantic coastal populations are linked genetically and demographically to the Delaware Bay population, a finding with implications for management of fisheries and recovery of the threatened red knot Calidris canutus rufa.