The ancient horseshoe crabs have been around in their present form for about 250 million years, predating most other life on the planet. They are found in the western Atlantic from the Gulf of Maine to the Yucatan. Delaware Bay is home to the largest population in the Atlantic. Though commonly called crabs, they are really more closely related to scorpions, ticks and land spiders. They have hard, curved top shells which make them difficult for predators to overturn and they can go for up to a year without food. When they do eat, horseshoe crabs prefer a modest, readily available diet of worms and mollusks. An adult grows to a shell size of more than two feet, and they have long, barbed tails which act as rudders for moving through mud.