Atlantic Horseshoe Crab (Limulus polyphemus): Facts

What does an Atlantic horseshoe crab look like?

The body has three parts. At the front is a broad, rounded shield called the prosoma, shaped like a horse’s hoof, which gives the animal its name. Behind it, joined by a hinge, is a smaller section called the opisthosoma, edged with short movable spines. At the back is a long, stiff tail spine called the telson.

The smooth shell is greenish gray to dark brown. Older adults often carry hitchhikers such as barnacles, slipper shells, worms and algae, and scientists have nicknamed them “walking museums” for the number of species that can live on one shell.

Underneath, the front section carries six pairs of limbs. The first pair are tiny pincers called chelicerae beside the mouth. Behind them come five pairs of legs; in adult males the first pair of legs ends in a rounded clasper, sometimes compared to a boxing glove, which they use to hold onto females. The last pair has leaf-like tips for pushing through sand. Under the rear section are five pairs of flat book gills, each with up to about 200 thin, page-like folds where oxygen passes into the blood.

A horseshoe crab has two large compound eyes on the sides of its shell, several smaller simple eyes on the top and underside, and light-sensitive cells along its tail. Why the animal is called a crab at all is explained in Is a horseshoe crab a crab?

How big do horseshoe crabs get?

Females are clearly larger than males, typically about 25–30% longer and wider and sometimes more than twice as heavy. The largest females reach about 60 cm (24 in) long including the tail and weigh up to about 4.8 kg (11 lb).

Size also changes along the coast. Crabs are biggest in the middle of the range, between Georgia and Cape Cod, and become smaller to the north and south. In Delaware Bay, adult females average about 25.5 cm (10 in) across the shell and males about 20 cm (8 in), while in Florida females average about 21.5 cm (8.5 in) and males about 16 cm (6.3 in). Some populations on Mexico’s Yucatán Peninsula are smaller still.

Like all arthropods, a horseshoe crab grows by molting. The old shell splits along its front edge and the crab walks forward out of it, leaving an empty shell that looks like a whole animal. Many “dead crabs” on beaches are really these cast-off shells.

Habitat and range

The Atlantic horseshoe crab is the only living horseshoe crab in the Americas; the other three species live in Asia. It is found along the Atlantic coast of the United States from Maine to Florida and around the Gulf coast from Florida to Louisiana. A separate breeding population lives on the coasts of Mexico’s Yucatán Peninsula. Stray individuals have turned up as far away as Nova Scotia, the Bahamas and Cuba.

The largest spawning gathering in the world is in Delaware Bay, between New Jersey and Delaware. Other important areas include Chesapeake Bay, Cape Cod and the coasts of Florida.

Horseshoe crabs live on soft, sandy or muddy bottoms, from shallow lagoons, estuaries and tidal flats to depths of more than 200 m (660 ft) up to about 56 km (35 mi) offshore, although most stay shallower than about 30 m (98 ft). They cope with water ranging from nearly fresh to almost twice as salty as the sea. In the north they spend winter on the continental shelf and become active as water warms in spring.

What do horseshoe crabs eat?

Horseshoe crabs are bottom feeders that forage mainly at night. They eat small clams and mussels, marine worms and other small invertebrates buried in the sand, along with scraps of dead fish and some algae.

They have no jaws. Instead, the spiny bases of their legs, called gnathobases, crush and shred food as the legs move, so a horseshoe crab in effect chews while it walks. The small pincers then push the food into the mouth, and a muscular gizzard lined with tough ridges grinds it further.

Behavior, senses and swimming

Adults usually walk slowly along the bottom, plowing through the top layer of sand in search of food. Young crabs and adults heading to spawning beaches also swim, but they do it upside down, shell facing the bottom, beating their legs and gill flaps and using the tail as a rudder. When waves flip a crab onto its back, it digs its tail into the sand and levers itself over.

Their activity follows the tides. Laboratory studies show that horseshoe crabs have internal tidal clocks that control when they move, as well as a daily clock that changes how their eyes work.

Those eyes have made the species famous in science. Each compound eye has about 1,000 light-sensing units with large, easy-to-study nerve fibers, and research on them helped explain how eyes sharpen contrast at edges. The American physiologist H. Keffer Hartline shared the 1967 Nobel Prize in Physiology or Medicine partly for this work. At night the eyes become far more sensitive to light, which helps males find females by sight on dark spawning beaches.

Spawning on Delaware Bay beaches

In the mid-Atlantic, spawning begins in spring as the water warms and peaks in May and June. Crabs come ashore at high tide, especially around the new and full moons when tides are highest, and often at night. In southern Florida and the Yucatán, spawning can happen all year.

A female crawls up the beach with a male clinging to the back of her shell, and extra “satellite” males often crowd around the pair. She digs a shallow nest about 15–20 cm (6–8 in) deep near the high-tide line and lays a cluster of small greenish eggs, which the males fertilize outside her body. Over a season a female makes many nests and can lay tens of thousands of eggs in total.

The eggs hatch after roughly two to four weeks, depending on temperature. Waves and other digging crabs bring many eggs to the surface, where they become a vital food for migrating birds.

Life cycle and lifespan

The newly hatched young are called trilobite larvae because they look a little like the extinct trilobites. They are about 1 cm (0.4 in) long, with only a stub of a tail, and they swim for about a week before settling on nearby tidal flats.

Juveniles stay in shallow water near the spawning beaches for their first two years or so, then move into deeper water as they grow. They molt about 16 or 17 times, growing by roughly a quarter to a third each time, and reach adulthood at about 9 to 11 years old. Females take longer and go through one more molt than males. Adults no longer molt, and they are thought to live about 20 years or more, although their exact age is hard to measure.

Eggs for shorebirds: horseshoe crabs and red knots

Each May, hundreds of thousands of migrating shorebirds stop on the beaches of Delaware Bay to feast on horseshoe crab eggs. They include ruddy turnstones, sanderlings, semipalmated sandpipers and red knots.

The red knot depends on this food more than most. The subspecies that stops in Delaware Bay flies between southern South America and the Canadian Arctic, one of the longest bird migrations on Earth, and each bird can nearly double its weight in about two weeks of feeding on eggs. When crab numbers in the bay fell in the 1990s, red knot numbers fell sharply too, and this population was listed as threatened under the US Endangered Species Act in 2014.

Adult horseshoe crabs have few natural predators. The loggerhead sea turtle is one of the main ones, while gulls take eggs and stranded crabs and many fish eat eggs and larvae.

Blue blood and medical testing

Horseshoe crab blood carries oxygen with hemocyanin, a copper-based protein that turns blue in air. It also contains cells called amebocytes that clot when they meet toxins from certain bacteria. An extract of these cells, known as Limulus amebocyte lysate, or LAL, has been used since the 1970s to check injectable drugs, vaccines and medical devices for bacterial contamination.

Crabs are collected, part of their blood is drawn, and they are returned to the sea, but a share of them die afterward and survivors may spawn less. A synthetic replacement made without crab blood is now officially recognized in Europe and the United States. The full story is in Why is horseshoe crab blood blue?

Myths and facts

Myth: the tail is a stinger. The telson has no venom and cannot sting. It is used to steer and to flip the crab upright, and lifting a crab by its tail can injure it.

Myth: horseshoe crabs bite or pinch. They have no jaws, and their small claws are too weak to hurt people.

Myth: this exact species is hundreds of millions of years old. Horseshoe crabs as a group are very ancient, and the oldest known fossil, from Manitoba in Canada, is about 445 million years old. But the Atlantic species itself has no fossil record, and the earliest fossils of its genus come from the Late Cretaceous. “Living fossil” describes a body plan that has changed little, an idea also discussed in Why is the coelacanth a living fossil?

Conservation status and threats

The IUCN Red List assessed the Atlantic horseshoe crab as Vulnerable in 2016, with an overall decreasing trend, mainly because of overharvesting and habitat loss. Trends differ from place to place: some populations along the US coast are stable or increasing, while others have declined, and the Yucatán population has fallen since the 1960s.

In the late 1800s and early 1900s, millions of crabs were taken from Delaware Bay and ground into fertilizer and livestock feed. Today the main pressures are harvest for bait in whelk and eel fisheries, collection for the medical industry, and the loss of spawning beaches to seawalls, bulkheads and erosion. Many crabs also die each year after being stranded upside down on beaches.

Management has tightened since the 1990s. The Atlantic States Marine Fisheries Commission adopted a coastwide management plan in 1998, and since 2012 the Delaware Bay harvest has been set using a model that also takes red knots into account. New Jersey has banned the harvest since 2008, and Mexico has protected its Yucatán population as endangered since 1994. Beach volunteers also count spawning crabs and gently turn stranded ones back over by the edge of the shell.