The horseshoe crab is a marine arthropod found along the Atlantic coast of North America and in parts of Asia. Despite its name, it is not a true crab but belongs to a group called chelicerates, which also includes spiders and scorpions. Horseshoe crabs have a hard, rounded shell and long, spiky tails, and they play a crucial role in coastal ecosystems. These creatures are known for their blue blood, which contains a unique substance called limulus amebocyte lysate (LAL). LAL is used in medical testing to detect bacterial contamination in vaccines and medical devices. Horseshoe crabs have existed for over 450 million years, making them one of the oldest living species on Earth.