Horseshoe crabs belong to the phylum Arthropoda and are actually more closely related to spiders than other marine invertebrates. They are one of the most primitive groups of animals, surviving relatively unchanged for hundreds of millions of years, and their natural history is one fascinating chapter in the story of evolution.
These ancient creatures possess ten eyes which are especially useful during the full moon when they move onto the beach by the thousands to mate. Their sensory perception is the focus of much current research, as are several unique physiological qualities which make them a valuable subject to biomedical science. Until recently they were gathered en masse and used as fertilizer. These and other pressures have been detrimental to horseshoe crab populations. Numbers are dwindling rapidly and, if the decline continues, they will soon be listed as endangered.