Horseshoe crabs are among the world’s oldest and most fascinating creatures. The earliest horseshoe crab species had already inhabited Earth at least 200 million years before the dinosaurs arrived or about 400 million years ago. Even though horseshoe crabs have a hard shell and numerous appendages with claws, it’s not really a crab. Horseshoe crabs belong to the phylum Arthropoda, along with crabs, insects, and other invertebrates with jointed legs, but their closest living relatives are spiders and scorpions. A horseshoe crab’s tail, while menacing, is not a weapon. Instead, the tail is used to plow the crab through the sand and muck, to act as a rudder, and to right the crab when it accidentally tips over.
They can be found in the Atlantic Ocean along the North American coastline. They can be seen along the East and Gulf coasts of the United States and Mexico. Horseshoe crabs utilize different habitat depending on the stage in their development. The eggs are laid on coastal beaches in late spring and summer. After hatching, the juvenile horseshoe crabs can be found offshore on the sandy ocean floor of tidal flats. Adult horseshoe crabs feed deeper in the ocean until they return to the beach to spawn.
The horseshoe crab spends most of its time rooting on the bottom looking for food. Because they lack jaws, the horseshoe crab uses the spiny bases of their legs to crush and grind their food, then put it into their mouths. They eat mostly worms and mollusks such as razor clams, and soft shelled clams.
Each spring during the high tides of the new and full moons thousands of horseshoe crabs gather along the beaches. Males, two-thirds the size of their mates, cluster along the water’s edge as the females arrive. With glove-like claws on its first pair of legs, the male hangs on to the female’s shell and is pulled up the beach to the high tide line.
The female pauses every few feet to dig a hole and deposit as many as 20,000 pearly green, birdshot-sized eggs. The male then fertilizes the eggs as he is pulled over the nest. After the spawning is complete, the crabs leave and the waves wash sand over the nest.
Several types of shore birds eat horseshoe crab eggs, various fish, invertebrates and sea turtles feed on the larvae, and humans catch adult horseshoe crabs to use as bait and for medical research.