# Atlantic Horseshoe Crab — Facts, Threats & Conservation

*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.

**IUCN status:** Vulnerable  ·  **WARN range:** Oceans, North America

## Quick facts
| Fact | 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) |

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

## Conservation status
- **Status:** Vulnerable (IUCN Red List category)
- **Population:** Global population not compiled for this guide
- **Trend:** See the IUCN Red List assessment
- **Assessed:** See the IUCN Red List assessment
- **CITES:** See CITES appendices
- Conservation figures are drawn from the IUCN Red List and CITES; this guide links to the authoritative sources rather than restating population counts.

## Key facts: Atlantic Horseshoe Crab
- 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".
- The horseshoe crab has a hard, domed carapace shaped like a horseshoe, a smaller abdominal section, and a long, pointed tail called a telson.
- Atlantic horseshoe crabs live in shallow coastal waters and estuaries along the western Atlantic, moving onto sandy beaches to breed.

## 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.

## What WARN does
WARN does not run a dedicated atlantic horseshoe crab programme — this guide is part of the World Animal Rescue Network's free educational library. The atlantic horseshoe crab faces documented conservation pressure in the wild, which is why clear, sourced public information about it matters. Building public understanding of species like the atlantic horseshoe crab supports the broader conservation awareness that underpins WARN's partner-led rescue work across its network.

If clear, science-based wildlife guides like this help you understand and value animals, please consider supporting the World Animal Rescue Network's educational and rescue work so we can keep producing them.

## Frequently asked questions: Atlantic Horseshoe Crab
### 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.

### Where do atlantic horseshoe crabs live?
Atlantic Horseshoe Crabs live in Shallow coastal waters and sandy beaches. 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.

### How big is an atlantic horseshoe crab?
An Atlantic Horseshoe Crab is about Often 20+ years. The Atlantic horseshoe crab (Limulus polyphemus) is an ancient marine arthropod, more closely related to spiders and scorpions than to true crabs. Conservation status: Vulnerable (IUCN).

## Sources
- [IUCN Red List — Limulus polyphemus](https://www.iucnredlist.org/search?query=Limulus%20polyphemus&searchType=species)
- [Wikipedia — Atlantic Horseshoe Crab](https://en.wikipedia.org/wiki/Atlantic_horseshoe_crab)
- [Wikidata — Atlantic Horseshoe Crab](https://www.wikidata.org/wiki/Q1133152)

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Full guide: https://worldanimalrescuenetwork.org/wildlife-guides/horseshoe-crab