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Horseshoe Crab

The horseshoe crabs that congregate along Delaware Bay beaches each spring belong to an ancient and unique lineage of animals. Their ancestors go back some 500 million years, to a time when the land was barren and life existed only in the primeval oceans. Across the vast expanses of time since, horseshoe crabs have been survivors of remarkable distinction, withstanding the rigors of climatic extremes, adjusting to changing seas and shorelines as continents drifted great distances, and enduring long after multitudes of other animals, including the dinosaurs, appeared, flourished and passed to extinction.

There are only four species of horseshoe crabs worldwide. All are similar in terms of biology, life history and ecology, but the Atlantic horseshoe crab (Limulus polyphemus) is by far the most numerous.

The abundance of horseshoe crab eggs available to shorebirds is probably the most important indicator of the Delaware Bay stopover. Selected beaches are sampled for eggs once each week through May and early June. This not only indicates how horseshoe crabs are using the beaches, but it also reflects the availability of food for shorebirds.

By sampling eggs along bay beaches, resource managers are able to understand how beach management activities affect horseshoe crab egg densities, how the number of eggs differs between beaches, and how many horseshoe crab eggs are available as food to migratory shorebirds.