Atlantic horseshoe crab

Horseshoe crabs live in the sea along the coast.

They live near North America, from cold Maine to warm Mexico. In spring they come up onto sandy beaches.

Atlantic horseshoe crabs live along the Atlantic coast of North America, from Maine down to Florida, along the Gulf of Mexico coast to Louisiana, and around Mexico's Yucatán Peninsula.

They live in bays and shallow coastal waters. Young crabs stay on sandy and muddy flats near the beaches where they hatched, while older crabs move out to deeper water. In winter, some adults stay in estuaries and others travel out to the continental shelf.

The species ranges from the Gulf of Maine to Florida and along the Gulf of Mexico coast from Florida to Louisiana, with a separate population on Mexico's Yucatán Peninsula; it is considered absent from Texas to Tabasco, Mexico. Genetic studies divide it into regional population units: Gulf of Maine, Mid-Atlantic, Southeast, Florida Atlantic, northeastern Gulf and Yucatán.

Habitat needs change through life, from intertidal beaches and tidal flats for eggs and larvae to the edge of the continental shelf for adults. Adults overwinter in estuaries or on the shelf and return to beaches and estuaries in spring. USFWS calls the species an ecological generalist that tolerates a wide range of conditions.

Horseshoe crabs eat small animals in the sand.

They like little clams and worms. They have no teeth, so they crush their food with their legs!

Horseshoe crabs are not picky eaters. They hunt small animals living in the sand and mud, such as little clams, crustaceans and worms, and they will also eat algae.

They have no jaws or teeth. Instead, they crush hard food between their legs before passing it to their mouth. Like birds, they have a gizzard, a muscly part of the gut that grinds food before it reaches the stomach.

Young and adult crabs spend much of their time on sandy tidal flats, feeding on invertebrates. Adults are important predators of small invertebrates living in the sediment, taking small clams, crustaceans and worms, plus other animals and algae.

Lacking mandibles, horseshoe crabs crush hard food between their legs before passing it to the mouth, and a gizzard grinds it further before it reaches the stomach, a feature FWC compares to birds.

Horseshoe crab eggs are a feast for birds.

Hungry shorebirds stop on beaches to eat the eggs. Then they fly north. You can help a crab that is stuck upside down.

When horseshoe crabs spawn, their eggs become a feast. Hundreds of thousands of migrating shorebirds, including the red knot, stop at beaches like those of Delaware Bay to eat the eggs and refuel before flying north. FWC compares the beaches to a gas station for birds.

Waves sometimes flip horseshoe crabs onto their backs, and some cannot turn over. With a grown-up, you can help by gently picking a crab up by both sides of its shell, never by the tail, and placing it back in the water.

Delaware Bay hosts the world's largest horseshoe crab spawning population and is the largest shorebird staging area on the Atlantic Flyway: an estimated 425,000 to one million migrating shorebirds stop there to refuel. Shorebirds such as the federally threatened red knot time their migrations to peak spawning.

Eggs also feed many fish, and adults are eaten by sea turtles, alligators, horse conchs and sharks. Many crabs die after being overturned by waves during spawning; FWC advises lifting them by both sides of the shell, never by the tail, which can injure them. Molted shells left on beaches are often mistaken for dead crabs.

Horseshoe crabs have blue blood.

Doctors and scientists use a little of it to check that medicines are clean. That keeps people safe.

Horseshoe crab blood is copper-based and turns blue when it is exposed to lots of oxygen. It contains special cells, and a substance made from them, called LAL, clumps up when it touches tiny amounts of harmful bacterial toxins. Companies use LAL to check that vaccines, medicines and medical equipment are clean.

To get it, workers collect adult crabs, take some of their blood and return most of them alive to the water. Not all survive: some crabs die after the procedure, so the numbers are counted and managed.

Horseshoe crab blood contains large cells called amebocytes, from which Limulus amebocyte lysate (LAL) is made. LAL clots in the presence of bacterial toxins, so pharmaceutical companies use it to test the sterility of vaccines, drugs, prosthetics and other medical devices. FWC states that up to one third of a crab's blood can be removed without killing it.

Under the interstate management plan, most crabs bled by the biomedical industry must be released alive where they were caught, but a portion die. The Atlantic States Marine Fisheries Commission reports that in 2024, 1,073,329 horseshoe crabs were collected coast-wide solely for biomedical use, with an estimated 184,693 deaths. Biomedical use has increased since reporting began in 2004.

People catch horseshoe crabs for many reasons.

Now there are rules about how many can be caught. Some places are safe zones where they cannot be caught at all.

The IUCN Red List lists the Atlantic horseshoe crab as Vulnerable. People catch horseshoe crabs for fishing bait and for medicine, and many years ago they caught millions to use as fertilizer.

Since 1998, states along the Atlantic coast have worked together to manage them, with limits on how many can be caught. A large sanctuary at the mouth of Delaware Bay protects them, and in recent years only male crabs from Delaware Bay may be caught for bait.

The IUCN Red List lists Limulus polyphemus as Vulnerable. From the 1850s to the 1920s, 1.5 to 2 million were harvested each year for fertilizer and livestock feed; bait demand for the whelk fishery drove a harvest peak of nearly six million pounds in 1997.

The Atlantic States Marine Fisheries Commission's 1998 management plan and its addenda set state-by-state bait quotas and created the 1,500-square-mile Carl N. Shuster Jr. Horseshoe Crab Sanctuary off Delaware Bay. For Delaware Bay-origin crabs, an adaptive framework that accounts for red knots sets harvest; the limit for 2026 and 2027 is 500,000 males and zero females. The 2024 assessment update rated the coast-wide and Delaware Bay stocks good, but the New York region poor.