Limulus polyphemus, commonly known as the horseshoe crab, is a marine arthropod found along the Atlantic coast of North America. It has a hard, rounded shell and long, spiky tail spine. Horseshoe crabs are not true crabs; they belong to a group called chelicerates, which also includes spiders and scorpions. These creatures are vital to the ecosystem and have existed for over 450 million years. They play a crucial role in the food web, serving as prey for birds and other animals. Additionally, their blue blood contains a substance called limulus amebocyte lysate, which is used in medical testing to detect bacterial contamination.