Horseshoe Crab

(Limulus polyphemus)




Facts about this animal

The Horseshoe Crab can reach a length of 60 cm and a weight of 1.8 kg, the male being smaller. The brown to grey body is horseshoe-shaped and separated in a carapace (prosoma) and an abdomen (opisthosoma) – both being unspanned above – , the later carrying a long, movable caudal spine or tail (telson). The carapace is strongly armoured and bears two prominent compound eyes and 7 simple eyes (one pair behind the compound eyes, 3 at the front and one ventral pair near the mouth). On the underside of the shell there are 5 pairs of walking legs, the anterior 4 pairs ending in forceps-like claws. In front of them there is a pair of chelicerae to feed. The mouth is located between the hips of the walking legs which have projections to help feeding. Behind them there is a pair of chilaria forming a kind of lower lip. The abdomen has 6 ventral pairs of rowing legs of which the posterior 5 pairs carry book gills with about 150 lamellae each. The first pair covers them and bears the genital apertures. Hemocyanin makes the blood of the Horseshoe crab blue.

The Horseshoe Crab is nocturnal and is usually buried in the ground by day. By night it feeds on bivalves and other mollusks, worms, other benthic invertebrates and pieces of fish which it finds digging into the ground by means of chemo receptors to the hips of the walking legs. Because there are no jaws there is a gizzard containing sand and gravel. Nevertheless, indigestible parts are expelled. It can swim belly up but rarely does. As defending tactic it can curl itself up or hide buried in the ground by means of the horseshoe-like front of the carapace.

In late spring the Horseshoe Crabs come to the shore for spawning in the tidal zone but protected from surf. Males clasp on the back of their females by means of specially formed forelegs which have bulbous claws. Depending on body size the females lay their 15’000-90’000 and 2-3 mm large eggs divided in 20 clutches into 15-20 cm deep and 30 cm wide self-made pits forming rows which are 4 m long. Each clutch is inseminated and covered with sand. The first instar larva is called trilobite larva and has just 9 pairs of legs and no anal spine. It is a free swimmer for about 5-7 days. The second instar, called presterichia, already looks like the adult and lives near the shore. Maturity is reach after about 16-18 moults at the age of 9-12 years. Moulted skins wash up on beaches. Horseshoe Crabs can live for as long as 31 years. Lost limbs are regenerated after some moults.

The blood of the Horseshoe Crab contains cells called amebocytes which release a clotting substance if they scent bacterial endotoxins. This is used in the Limulus Amebocyte Lysate (LAL) test to detect contamination of pharmaceuticals with bacterial endotoxins and to test for several bacterial diseases. “Milked” Horseshoe Crabs are subsequently released where they were captured and recover within a few weeks.

Populations of the Horseshoe Crabs have decrease since they are used as bait in whelk and conch trapping. Consequently, some predators of the Horseshoe Crabs also became rare like the Red Knot and the Atlantic Loggerhead Turtle.

Did you know?
that, despite its name, the Horseshoe Crab is more closely related to spiders than to crabs? that only 4 very similar species of this 400 million year old Arthropod class are still living viz. L. polyphemus, Tachypleus tridentatus, T. gigas and Carcinoscorpius rotundicauda? that Horseshoe Crab is of commercial importance because its blue blood is used in the pharmaceutical industry? that the Horseshoe Crab has been extensively used in research into the physiology of vision?


Name (Scientific) Limulus polyphemus
Name (English) Horseshoe Crab
Name (French) Limule
Name (German) Pfeilschwanzkrebs, Königskrabbe
Name (Spanish) Cangrejo herradura, Ostrorep, Cangrejo cacerola
Local names Español (add.): Cacerolita del mar, Cangrejo bayoneta, Tanquecito panishde mar, Cucaracha marina, Límulo
CITES Status Not listed
CMS Status Not listed



Photo Copyright by
Gerald Dick



Range Found on the east coast of North America from Nova Scotia (Canada) over the United States south to the Yucatan Peninsula (Mexico). Stray individuals are occasionally found in Europe.
Habitat Shallow sandy coasts down to 30 m depth (rarely 200 m), ground dweller. Intertidal sand flat areas are needed for spawning and early stages.
Wild population Several tens of millions but counts or estimates, if done at all, are inconsistent. Studies have shown that 10 to 15 percent of the individuals bled for LAL die.
Zoo population 175 reported to ISIS (2008)

In the Zoo

Horseshoe Crab


Find this animal on ZooLex


Photo Copyright by
Gerald Dick

Why do zoos keep this animal

Horseshoe Crabs are long-lived and – despite their size – harmless dwellers of shallow waters which can be displayed in touch pools. They can be used in education either to show a living fossil having hardly changed during the last 400 million years surviving all global catastrophes since or to point out its importance for modern medicine using its blood to test for contamination of pharmaceuticals.