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Marine Animal Encyclopedia

American Horseshoe Crab Limulus polyphemus

Despite its name, the American horseshoe crab is more closely related to spiders than to crabs. It is mainly active at night and scavenges anything it can find, including small worms, bivalves, and algae.

Its horseshoe-shaped, greenish-brown outer shell, or carapace, is for protection, and adults have few predators. It has six pairs of appendages: the first pair is used for feeding; the other five are for walking. The American horseshoe crab’s five platelike “book” gills each contain many membranes, like the pages of a book, and are situated toward the tail.

As well as being used for respiration, the gills are also used for propulsion when swimming. They can also absorb water, which helps the American horseshoe crab to fill its new shell after it has shed the old one. Its long, rigid tail is used for steering and for righting itself.

The reproductive cycle is closely linked to the spring and fall high tides (especially the spring tides in the northern part of the range) and the lunar cycle. At full moon, the adults gather in large numbers on sandy beaches to breed.

Female American horseshoe crabs lay up to 20,000 eggs in a nest near the high-tide mark, providing a vital food source for birds and other marine creatures. After hatching, the young remain hidden in the sand for safety. They emerge some weeks later at high tide and take to the water until they molt for the first time, after which they look like small adults and start to live on the sea floor.

American Horseshoe Crabs and Medical Research

If the American horseshoe crab is injured, some of its blood cells form a clot, which kills harmful bacteria that are also dangerous to humans. In order to exploit this property for human benefit, crabs are collected from shallow waters on the Atlantic coast of North America during the summer months.

Researchers then remove about 20 percent of the blood from each crab. From this they extract a protein that is used to detect bacterial contamination in drugs, vaccines, and other medical products that are given intravenously. Bleeding the crabs is not fatal to them, and afterward they are returned to the sea to recover.

American Horseshoe Crabzoom image
  • Subphylum Chelicerata
  • Length Up to 24 in (60 cm)
  • Weight Up to 11 lb (5 kg)
  • Habitat Sandy or muddy bays to 100 ft (30 m)
  • Distribution Western Atlantic and Gulf Coast from southern Maine to the Yucatán Peninsula
American Horseshoe Crab habitat mapzoom image