Like a relic from prehistoric times, the horseshoe crab lives much the same as its ancestors must have over 250 million years ago. Horseshoe crabs are common along the Atlantic coast living in shallow ocean water on sand or mud bottoms. They are an essential part of local ecology, their eggs an important food source for migrating shore birds. More closely related to spiders, these living fossils are true blue bloods, their blood “blue” because oxygen is carried by copper-based hemocyanin, rather than iron that colors human blood red. Female horseshoe crabs migrate to the shore to spawn during the spring high tides on full moonlit nights, a great time to see large numbers on the beach.