These fascinating creatures (Limulus polyphemus) are found commonly along our beachfront sitting within the intertidal flats. The horseshoe crab is one of the oldest living species. Fossils of horseshoe crabs have been dated at 360 million years old. They evolved in the shallow seas of the Paleozoic Era (540-248 million years ago) with other primitive arthropods called trilobites, a long extinct close relative of the horseshoe crab. Among today’s species, they are more closely related to spiders than crabs.
Horseshoe crabs lay eggs in the intertidal flats during the spring months. These spawning events are heaviest during the full moon and can often result in hundreds of crabs coming ashore to spawn in one night. The female crab lays over 4,000 eggs per nest about 6-10 inches deep in the sand. Each egg is roughly the size of a pinhead. Once fertilized by the male, the light green eggs begin to develop. The larval stage of the egg is a staple food source for at least 20 species of migrating shorebirds, such as red knots, providing an essential source of energy (fat) during their long flight to breeding grounds. The young, which are sand-colored to blend perfectly with their background, remain near shore, and gradually move into deeper water as they age and become darker. The blood of horseshoe crabs has also become valuable to the medical industry, which uses it to test for bacterial contamination in injectible medicines.
The horseshoe crab is harmless, but utilizes its shell as protective armor to fend off predators. Should you see crabs buried down deep in the sand you should leave them in place as they may still be in the process of egg laying. If you see them struggling on their back in the sun, please flip them over and help them to the ocean, as the heat of a summer day can cause death.
SC Department of Natural Resources: www.dnr.sc.gov/cwcs/pdf/Horseshoecrab.pdf