Horseshoe crabs are marine arthropods that have existed for over 450 million years, making them one of the oldest living species. They are not true crabs but are more closely related to spiders and scorpions. Horseshoe crabs have a hard, rounded shell and a long, spiked tail called a telson. They primarily inhabit shallow coastal waters and are often found on sandy or muddy bottoms. These creatures play a crucial role in the ecosystem and are known for their blue blood, which contains a unique substance called Limulus Amebocyte Lysate (LAL). This substance is used in medical testing to detect bacterial contamination in vaccines and medical equipment, highlighting the importance of horseshoe crabs in both nature and human health.