Effects of Oyster Aquaculture and Tidal Conditions on the Movement of Horseshoe Crabs, Limulus polyphemus

Horseshoe crabs have a unique role in coastal ecology and human society. During their spawning season, their eggs are a primary food source for many migratory species of shorebirds, including the recently listed rufa red knot. In the biomedical industry their blood is used for maintaining the safety of injectable drugs. Despite their importance, little is known about the effects of oyster aquaculture and tidal conditions on horseshoe crabs’ movement and behavior as they come ashore to spawn. Previous work found that oyster farms did not prevent horseshoe crabs from crossing farms to reach spawning habitats; however, there have been no studies examining movement (speed, direction, and path) as they transit through farm habitat. Likewise, little is known about the effects of tidal conditions (flood vs. ebb) on the movement patterns of horseshoe crabs. This study was designed to test for effects of oyster farms and tides on the movement of horseshoe crabs. Two DIDSON (Dual-frequency IDentification SONar) units were used at an intertidal farm and control site in Delaware Bay during spawning migrations to record horseshoe crab movement during peak spawning (full moon in May 2019). Videos made with sonars were analyzed using Tracker Software to collect horseshoe crab movement data. Specifically, the speed, direction, and path efficiency for individual single male and amplexus pair crabs were measured. Path straightness was affected by oyster aquaculture (ANOVA; p<<0.001) and path direction was affected by tidal conditions (ANOVA; p=0.12), whereas velocity, net distance, and gross distance were not affected by neither oyster farms nor tidal conditions. Although crabs traveling around or through oyster farm racks had a less straight path than those passing through the control, overall movement to and from inshore spawning habitat was not affected. Path direction differed among tidal conditions with horseshoe crabs moving towards the shore during flooding, and during the ebb tide they returned to the bay. In conclusion, oyster farms do not affect the overall movement of horseshoe crabs and crabs follow the tides during their spawning migration.