Horseshoe crabs are frequently seen on the beaches of Cumberland Island. These intriguing animals are usually about 18 inches long and look like an old metal helmet with a tail. They stay primarily on the ocean floor, feeding on mollusks, worms and seaweed. They use their 10 eyes (!!) to find mates and sense light as they navigate along the ocean floor.
While they might have a startling appearance, they are one of the most harmless creatures on the seashore. They cannot bite or sting, and their barbed tail is used to move through sediment, as a rudder and to right itself if it tips over.
But did you know that most horseshoe crabs do not even make it to the larval stage before being eaten? Some migratory shorebirds, including the red knot, rely on horseshoe crab eggs to survive. Fish and sea turtles are also known to feast on the eggs of the horseshoe crab.
If an egg does survive, it will spend the next 10 years or so molting and growing. You can tell a molt from a dead crab by how hard the shell is. A molt that has been left behind will be flexible and can crack if pressed too hard.
If you look carefully at the rounded front end of the molt, you’ll see where the top and the bottom have separated to allow the horseshoe crab to crawl out of the old exoskeleton, leaving it behind.
Finally, it will be able to reproduce, and during late spring and early summer, adult horseshoe crabs travel from deep ocean waters to beaches along the East and Gulf coasts to breed. Males, usually about two thirds the size of their mates, will cluster along the water’s edge and await the arrival of the females on the shore.
Males have ‘glove-like’ claws on the first pair of their legs which they use to hang onto the females’ shells. Females make their way to the high tide line with the males hanging onto their backs. Once the female reaches the high tide line, she pauses every few feet to dig a hole and deposit as many as 20,000 pearly green birdshot-sized eggs. The male on the female’s back fertilizes the eggs as he is pulled over the nest. After the spawning is complete, the crabs leave and the eggs are covered with sand as the waves wash over them.
Learn more about these animals, which are most closely related to spiders and predate the dinosaurs.
We’re often contacted by visitors who have witnessed damage to the island’s ecosystem, or blatant disregard of policy protections. We encourage you to report these to the National Park Service and also complete this form.
Thank you for your respectful stewardship of Cumberland Island.