The American horseshoe crab (Limulus polyphemus) spawns regularly in salt marshes

While considered ecological generalists in most aspects of their life history, horseshoe crabs (Limulus polyphemus) have traditionally been regarded as reproductive habitat specialists, given that spawning is thought to occur primarily on beaches where conditions for embryonic development are considered optimal. Observations of horseshoe crabs spawning in other habitats were deemed isolated and the behavior non-adaptive. Here, we used spawning and egg surveys to compare the use of beach and salt marsh habitats for spawning by the horseshoe crab in three US states along the Atlantic coast. We found similar spawning and egg densities in both habitats but were more likely to find eggs in marsh habitats, indicating that spawning in marsh habitats is common and geographically widespread. These results suggest that the conservation of salt marshes may be critical for the protection of this species and that management strategies should be revised to incorporate this generalist behavior.