Horseshoe crabs are marine arthropods that have existed for over 450 million years. They are not true crabs but belong to a group called chelicerates, which also includes 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 feed on small invertebrates. These creatures play a crucial role in the ecosystem and are known for their blue blood, which contains a unique substance used in medical testing to detect bacterial contamination. Horseshoe crabs are also important for shorebird migration, as their eggs provide a vital food source.