The Ecological Importance of Horseshoe Crabs in Estuarine and Coastal Communities: A Review and Speculative Summary

Beyond their commercial importance for LAL and bait, and their status as a living fossil, it is often asserted that horseshoe crabs play a vital role in the ecology of estuarine and coastal communities. How would the various ecological relationships involving horseshoe crabs be affected if these animals were no longer abundant? Attempts to understand and generalize the ecological importance of horseshoe crabs are hampered by several constraints. We know relatively little about the ecology of juvenile horseshoe crabs. Most ecological studies involving adult Limulus polyphemus have been conducted at only a few locations, while much less is known about the three Indo-Pacific species. Furthermore, we are attempting to infer the ecological importance of a group of animals whose numbers may have already declined significantly (the so-called "shifting baseline syndrome"). Horseshoe crab shells serve as substrate for a large number of epibionts, such as barnacles and slipper limpets, but the relationships between these epibionts and horseshoe crabs appear to be facultative, rather than obligatory. Horseshoe crabs are dietary generalists, and adult crabs are ecologically important bivalve predators in some locations. The most notable predator-prey relationship involving horseshoe crabs is the migratory shorebird-horseshoe crab egg interaction in Delaware Bay. After hatching, the first and second instars are eaten by surf zone fishes, hermit crabs, and other predators. Virtually nothing is known about predator-prey relationships involving older juveniles, but adult L. polyphemus are important as food for the endangered loggerhead turtle, especially in the mid-Atlantic region.