 But they're also valuable on the beach. Bob Prescott, who used to run the local Mass Audubon sanctuary, says horseshoe crabs and their eggs play a key role in coastal ecosystems. They're one of the keystone species on the beach. In other words, all those eggs either grow up to be horseshoe crabs and eaten by something else, or the eggs are eaten by birds. So it's all part of the food web, which we basically have destroyed.