# Atlantic Horseshoe Crab (*Limulus polyphemus*)

> The Atlantic horseshoe crab (Limulus polyphemus) is an ancient marine arthropod, more closely related to spiders and scorpions than to true crabs. It has a domed horseshoe-shaped shell, a long spike-like tail, and reaches about 60 cm (24 in) including the tail. Found in shallow coastal waters and sandy beaches, it crawls along the bottom feeding on worms and molluscs, often living more than 20 years. It is listed as Vulnerable.

- **Category:** Invertebrate
- **Conservation status (IUCN):** Vulnerable
- **Range:** Shallow coastal waters and beaches of the western Atlantic, along the eastern United States and the Gulf of Mexico.
- **Canonical page:** https://globalanimalguide.org/animals/horseshoe-crab/
- **Last reviewed:** 2026-07-16

## Scientific classification

- **Kingdom:** Animalia
- **Phylum:** Arthropoda
- **Class:** Merostomata
- **Order:** Xiphosura
- **Family:** Limulidae
- **Genus:** Limulus

## Facts at a glance

| Attribute | Value |
| --- | --- |
| Scientific name | Limulus polyphemus |
| Diet | Omnivore (worms, mollusks, algae, carrion) |
| Habitat | Shallow coastal waters and sandy beaches |
| Lifespan | Often 20+ years |
| Size | Up to about 60 cm (24 in) including tail |
| Top speed | Slow; crawls and plows through sand |
| Conservation status | Vulnerable (IUCN) |

## A living fossil

Horseshoe crabs belong to an ancient lineage whose body plan has changed remarkably little for hundreds of millions of years, earning them the nickname "living fossils". Despite the name, they are not true crabs but chelicerates, more closely related to spiders and scorpions. The Atlantic species, Limulus polyphemus, is one of only four horseshoe crab species alive today. Their long evolutionary history makes them a favourite subject for studies of animal design and survival.

## Anatomy and blue blood

The horseshoe crab has a hard, domed carapace shaped like a horseshoe, a smaller abdominal section, and a long, pointed tail called a telson. It grows to about 60 cm (24 in) including the tail and weighs around 2 kg, with several pairs of legs and multiple eyes, including large compound eyes on the shell. Its blood is copper-based and appears blue, and it contains a clotting agent that reacts to bacterial contamination. That property makes horseshoe crab blood valuable for testing the safety of medical products.

## Habitat and movement

Atlantic horseshoe crabs live in shallow coastal waters and estuaries along the western Atlantic, moving onto sandy beaches to breed. They are slow-moving bottom-dwellers that crawl and plough through the sand rather than swim quickly, with a top speed of only around 1 km/h. The tail is used mainly as a lever to right themselves if a wave flips them over, not as a weapon. They tend to stay in soft-bottomed areas where food is easy to find.

## Diet and feeding

Horseshoe crabs are omnivores that feed on marine worms, small molluscs, crustaceans, algae, and carrion. Lacking jaws, they crush food between the bristly bases of their legs and push it toward the mouth as they move. They forage mainly along the seabed, ploughing through sand and mud in search of buried prey. Their scavenging helps keep the shallow-water bottom clear of dead material.

## Breeding and conservation

In spring and early summer horseshoe crabs gather on sandy beaches in large numbers to spawn, with females laying thousands of eggs in the sand and males fertilising them. These eggs are a vital food source for migrating shorebirds, so the crabs' breeding success affects other species too. The Atlantic horseshoe crab is listed as Vulnerable by the IUCN, threatened by habitat loss, harvesting for bait, and collection for the biomedical industry. Long-lived individuals, often surviving more than 20 years, take about a decade to reach breeding age.

## Frequently asked questions

### Is a horseshoe crab really a crab?

No. Despite its name, the horseshoe crab is not a true crab but a chelicerate, more closely related to spiders and scorpions. It belongs to an ancient group that has existed for hundreds of millions of years.

### Why is horseshoe crab blood blue and valuable?

Their blood is copper-based, which makes it appear blue, and it contains a clotting agent that reacts to bacterial contamination. This property is used to test that medicines and medical devices are free of dangerous bacteria.

### Are horseshoe crabs dangerous?

Horseshoe crabs are harmless to people. They have no claws or stinger, and their long tail is used only to flip themselves upright if a wave turns them over, not to attack.

### What do horseshoe crabs eat?

They are omnivores that eat marine worms, small molluscs, crustaceans, algae, and carrion. Lacking jaws, they crush food between the bases of their legs as they crawl along the seabed.

### How long do horseshoe crabs live?

Atlantic horseshoe crabs often live more than 20 years. They grow slowly and take around a decade to reach breeding age, which makes their populations sensitive to overharvesting.

### Are horseshoe crabs endangered?

The Atlantic horseshoe crab is listed as Vulnerable by the IUCN. Habitat loss, harvesting for bait, and collection for the biomedical industry have reduced their numbers, and their eggs are also crucial food for migrating shorebirds.

## Sources

- [IUCN Red List — Limulus polyphemus.](https://www.iucnredlist.org/search?query=Limulus%20polyphemus&searchType=species)
- [Animal Diversity Web — Limulus polyphemus. University of Michigan Museum of Zoology.](https://animaldiversity.org/accounts/Limulus_polyphemus/)
- [Wikipedia — Atlantic Horseshoe Crab.](https://en.wikipedia.org/wiki/Atlantic_horseshoe_crab)

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Source: Global Animal Guide (https://globalanimalguide.org/animals/horseshoe-crab/). Licensed CC BY-NC 4.0 — free for non-commercial reuse with attribution.