Horseshoe crabs are not crabs at all. They are the distant cousins of arachnids such as scorpions and spiders.
Each May and June - usually during a new moon phase - horseshoe crabs make their way into shallow bay areas to spawn. Because so many of these “living fossils” come ashore at once, it is one of nature’s great spectacles. Males select a female and cling onto her back. The female digs a hole in the sand and lays her eggs while the male fertilizes them. The female can lay between 60,000–120,000 eggs in batches of a few thousand at a time. Many shore birds eat the eggs before they hatch.
Unlike humans, horseshoe crabs do not have hemoglobin in their blood, but instead use hemocyanin to carry oxygen. Because of the copper present in hemocyanin, their blood is blue. This blood contains amebocytes, which play a role similar to white blood cells in defending against pathogens. Amebocytes are used for the detection of bacterial endotoxins.