The horseshoe crab (Limulus polyphemus) is the American species of a marine animal that is only a distant relation of crustaceans like crabs and lobsters. Horseshoe crabs are more closely related to spiders and scorpions. The crabs have been called "living fossils" because the genus dates back millions of years, and Limulus evolved very little over the years.

Fossils found in British Columbia indicate that the ancestors of horseshoe crabs were in North America about 520 million years ago. During the late twentieth century, the declining horseshoe crab population concerned environmentalists. Horseshoe crabs are a vital food source for dozens of species of birds that migrate from South America to the Arctic Circle. Furthermore, crabs are collected for medical research. After blood is taken from the crabs, they are returned to the ocean.

American horseshoe crabs live along the Atlantic Ocean coastline. Crab habitat extends south from...