Horseshoe crab

Horseshoe crabs are marine arthropods in the order Xiphosura, not true crabs but relatives of spiders and scorpions.

Horseshoe crabs are marine arthropods in the order Xiphosura, not true crabs but relatives of spiders and scorpions. Four living species are recognized today, found on the Atlantic coast of North America and in coastal waters of East and Southeast Asia. They are often called "living fossils" because their body plan has changed little over hundreds of millions of years.

Classification and evolution

Horseshoe crabs belong to the order Xiphosura within the chelicerates. Fossil evidence shows a very ancient lineage, with relatives appearing in the Early Ordovician and the group often described as having changed little over time.

Anatomy

The body is divided into a cephalothorax or prosoma, an abdomen or opisthosoma, and a long tail spine called the telson. The telson helps the animal right itself if it is overturned.

Distribution and habitat

Living horseshoe crabs occur along the eastern coasts of Asia and the Atlantic coast of North America. They are most often found in shallow marine waters, estuaries, and on sandy beaches during spawning.

Ecology and human use

Horseshoe crabs feed on worms and other small marine animals, and their eggs support shorebird populations. Their blood is used in biomedical applications, especially tests for bacterial contamination.