Limulus polyphemus

Limulus polyphemus, commonly known as the Atlantic horseshoe crab, is a marine arthropod with a hard, horseshoe-shaped carapace. Despite its name, it’s more related to arachnids than true crabs. It has a long, pointed tail called a telson, used for righting itself. Horseshoe crabs are vital to ecosystems, especially as a source of blue blood containing a unique substance used in medical testing for bacterial contamination. They inhabit shallow coastal waters along North America’s Atlantic coast and have remained relatively unchanged for millions of years, making them living fossils.