Nest-Site Selection in the Horseshoe Crab, Limulus polyphemus

Like a number of other species, horseshoe crabs lay their eggs on beaches in the intertidal zone. The elevation of the beach on which they nest differs among populations. We examined two factors that potentially affect egg survival at different beach elevations: erosion and rate of development. We found no evidence that eggs buried at different elevations incur different risks of erosion by wave action. However, the optimal beach elevation for egg development differed between our two study sites, Florida and Delaware, and the difference was related to beach morphology. Rate of development increased with oxygen concentration, redox potential, and temperature, and all three of these factors changed with elevation. The nests in the lower beach failed to develop, especially in Florida, where the beach was fine-grained and poorly drained. The nests in the upper beach were prone to dessication, especially in Delaware, where the beach was course-grained and well-drained. This means that differencces between sites in the optimal location for egg development coincided with horseshoe crab preferences in nestsite selection. We suggest that horseshoe crabs synchronize their nesting with the tides that reach the aerobic sediments on the beach, resulting in nesting patterns that differ with differences in tidal regimes and beach morphology.