Called "living fossils," the horseshoe crab has inhabited the earth's oceans for about 200 million years with little change in morphology. As with spiders and scorpions, it is said that these crabs are descendants of the trilobite, which inhabited the earth during the Paleozoic era beginning around 600 million years ago. Consequently, the horseshoe crab is considered to be more closely related to spiders and scorpions rather than to true crabs.

The idea that horseshoe crabs have not changed in appearance for over 200 million years comes from the fact that the horseshoe crabs of today are virtually identical in shape to MesoLimulus, a type of horseshoe crab that was discovered in the Jurassic strata famous for its association with the age of the dinosaurs. There may have even been horseshoe crabs in the movie "Jurassic Park!" Because MesoLimulus emerged 400 million years ago, some would argue that the modern-day horseshoe crab actually has a history spanning 300 to 400 million years.

Four species live in the present world: the Atlantic horseshoe crab, the Japanese horseshoe crab, the southern horseshoe crab and the mangrove horseshoe crab. Although their fossils have been found in Europe, their current habitats are mainly confined to the eastern coastal zones of the continents. While the Atlantic horseshoe crab inhabits the east coast of North America, the other three species are found in the southeast coast of Asia.

Among these species, the Atlantic (Limulus polyphemus) and Japanese (Tachypleus tridentatus) horseshoe crabs are used for commercial LAL production. The Limulus reagent (LAL) is named after the scientific name for the Atlantic horseshoe crab. Our Limulus reagent is manufactured from the Atlantic horseshoe crab.

To obtain the blood needed for LAL manufacturing, a needle is inserted directly into the heart of the crab and blood is collected until the flow ceases naturally. Most crabs are known to survive after they are released back into the sea after blood collection, likely because they have an open circulatory system that ensures an ample supply of blood (hemolymph) remains in the body even after almost all the blood from the heart is drawn. Horseshoe crabs are marvelous organisms and it can be said that LAL depends on their blood donations.

The horseshoe crab's blood is white while circulating in the body but turns blue when exposed to air. As with spiders and true crabs, a copper-based protein called hemocyanin carries oxygen throughout the body. Most of their blood cells are creamy-colored cells called amebocytes, unlike the red blood cells found in human beings.

The horseshoe crabs of Japan and China are usually known to live in pairs, with the male hitched behind the female. In the case of the Atlantic horseshoe crab, however, a large number of males will cluster around a female who is laying eggs. Despite their behavioral differences, they are similar in that the amebocyte lysate formulated from both species reacts with both endotoxins and (1→3)-β-D-glucan.

References

1. Sekiguchi, K. (1984). Biology of horseshoe crabs. Tokyo: Science House.
2. Sekiguchi, K. (1991). The wonders of the horseshoe crab. Tokyo: Iwanami Shoten.