The Nature Conservancy - Jurassic Beach

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Fine Dining: Crab eggs that wash up onto shore are snapped up by the countless migrating birds—including red knots and piping plovers—that stop at the Delaware Bay for a feast.

Go Deeper

Delaware Bayshores
More than a million migratory shorebirds depend on the horseshoe crabs converging on Delaware beaches each year.

Sidebar: Delaware's Dinosaur

The American horseshoe crab is an ancient species whose closest relative—the trilobite—died out more than 250 million years ago. The horseshoe predates the dinosaurs by more than 100 million years and survived their extinction unphased. “It has remained basically unchanged for 245 million years,” says Wendy Scott, who has been volunteering at crab counts for seven years. “It hasn’t needed to evolve, because it’s basically perfect already.”

But horseshoes aren’t actually true crabs. They are not even crustaceans, but are more closely related to arachnids, such as spiders and scorpions. There are four distinct species of horseshoe crab. However, only the American horseshoe crab can be found in the Atlantic. The other three species live in the Indian and Pacific Oceans.

The scientific name of the American horseshoe crab is Limulus polyphemus. In Latin, limulus means “sideways” or “odd.” Polyphemus is the name of the one-eyed giant, Cyclops, from Greek mythology—which the horseshoe, with one pair of eyes set close together in the middle of its shell, is said to resemble. The horseshoe crab has 10 eyes in all, including a number of small eyes and photoreceptors on its shell and tail. The crab has been studied by vision researchers for more than 70 years because its eyes are similar to those of vertebrates.

In order to chew its prey of clams and marine worms, the crab must be walking. It has no jaws, but instead uses its walking legs to help crush food. Its front appendages help move food into its mouth, which is located at the center of its body.

While horseshoes can be found along the coastline from Maine to the Yucatán Peninsula, most of the population is concentrated between Virginia and New Jersey. They spend winters in deeper waters and in springtime head to shallow waters and beaches to spawn.

The crab’s epicenter, the Del-aware Bay, offers miles of ideal spawning habitat: sandy beaches protected from heavy wind and waves. On spring nights with a full or new moon, the New Jersey and Delaware shores host the world’s largest concentration of spawning horseshoe crabs.

At high tide, hundreds of thousands of horseshoes leave the water and scoot onto the beaches for their annual spawning aggregation. As the much-larger females arrive, they dig nests in the sand and lay eggs. The males crowd around, jostling to be the first to clasp the back of the female’s shell with clawed appendages called pedipalps. Each female lays as many as 20,000 eggs in the sand, while the males, still attached to the females, are dragged around as they try to fertilize the eggs.

The crabs’ annual spawning ritual in turn attracts a host of other species, most of them looking to get a free meal out of the event. Shorebirds mainly eat the eggs that have been brought to the surface by the waves or the movements of the crabs—eggs that wouldn’t have hatched anyway.

At least 11 species of migratory birds, including semipalmated sandpipers, sanderlings and red knots, depend on horseshoe eggs to help fuel their migrations. The dove-sized red knots more than double their weight during their two-week stopover in the Delaware Bay. “That’s the highest weight gain for any vertebrate in the world,” says Kevin Kalasz, a scientist with the Delaware fish and wildlife agency’s Endangered Species Program.

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