The horseshoe crab, scientifically known as Limulus polyphemus, is a unique marine animal that has been around for more than 450 million years. Despite its ancient origins, the horseshoe crab plays a crucial role in modern biomedical research.

The horseshoe crab has unique blue blood that contains a substance called Limulus amebocyte lysate (LAL). LAL is used to test for bacterial contamination in medical equipment, vaccines, and injectable drugs. The horseshoe crab’s blood contains a clotting agent that reacts to the presence of bacterial endotoxins. This reaction is used to determine if a product is contaminated with bacteria, making LAL an essential tool in the fight against bacterial infections.

The horseshoe crab’s blood is also used to study the immune system. The amebocytes, or white blood cells, of the horseshoe crab have been found to have similar properties to the white blood cells of vertebrates. This has led to the study of the horseshoe crab’s immune system as a model for understanding the human immune system.

Despite their name and appearance, horseshoe crabs aren’t crabs. In fact, they aren’t crustaceans at all and are actually arthropods in the subphylum Chelicerata. This means they are more closely related to sea spiders, scorpions, and other arachnids than to the crustaceans whose name they share.

Along with numerous eyes, horseshoe crabs also have several legs numbering 10 in total. Five of the pairs, or pedipalps, are used for locomotion. In the males, the pedipalps change over time and eventually become glove-shaped so as to help them better hold on to females during mating. Like other arthropods, the front two legs, or chelicerae, or smaller and are specially designed to help the crabs move food into the mouth. Although they appear rather clumsy on land, horseshoe crabs can move rather freely in the water. They possess a flap-like structure next to the abdomen known as book gills that allow them to breathe underwater. However, these gills also provide another benefit: the ability to swim upside-down. These underwater acrobatics demonstrate that there’s more to horseshoe crabs than meets the eye.

Also the Pokemon, Kabuto, is based on the horseshoe crab: "Kabuto is said to still exist in the Pokémon world’s present day, suggesting that it's partially inspired by another group of arthropods called the limulids, or horseshoe crabs, which is called kabutogani in Japanese. "