Weird creatures facts: Horseshoe crab

The horseshoe crab can grow up to 60 centimetres (24 in) in length (including tail); the female is typically 25 to 30 percent larger than the male.

The Horseshoe Crab has blue blood, as it uses copper rather than iron as the base of its system.

Before becoming mature around age 9, they have to shed their shells some 17 times. They can live for as long as 20 years

Horseshoe crabs are stem group chelicerates, thus distant relatives of spiders.

Horseshoe crabs are often referred to as living fossils, as they have changed little in the last 445 million years

Horseshoe crabs possess the rare ability to regrow lost limbs, in a manner similar to sea stars

Enzymes from horseshoe crab blood are used by astronauts in the International Space Station to test surfaces for unwanted bacteria and fungi

Horseshoe crabs are also used in finding remedies for diseases that have developed resistances to penicillin and other drugs.

General info
The horseshoe crab or Atlantic horseshoe crab (Limulus polyphemus) is a marine chelicerate arthropod. Despite its name, it is more closely related to spiders, ticks, and scorpions than to crabs. Horseshoe crabs are most commonly found in the Gulf of Mexico and along the northern Atlantic coast of North America. A main area of annual migration is Delaware Bay, although stray individuals are occasionally found in Europe.

The other three species in the family Limulidae are also called horseshoe crabs. The Japanese horseshoe crab (Tachypleus tridentatus) is found in the Seto Inland Sea, and is considered an endangered species because of loss of habitat. Two other species occur along the east coast of India: Tachypleus gigas and Carcinoscorpius rotundicauda. All four are quite similar in form and behavior.

The extinct diminutive horseshoe crab, Lunataspis aurora, 4 centimetres (1.6 in) from head to tail-tip, has been identified in 445-million-year-old Ordovician strata in Manitoba.