The strangest animal one is likely to find on Georgia beaches is the Atlantic horseshoe crab, Limulus polyphemus. Not a true crustacean, this creature is more closely related to spiders (arachnids) than to hermits or fiddlers. The outline of the dark brown chitinous carapace, up to a foot or more across, does somewhat resemble the shape of a horseshoe. The long spiky tail jutting behind, plus the five pairs of jointed legs dangling over leathery book gills underneath the body make the horseshoe crab truly bizarre.
Photo of an Atlantic horseshoe crab on a beach. Author: Pos Robert, U.S. Fish and Wildlife Service Horseshoe crab close up photography of arthropod public domain image
In fact, horseshoe crabs are living fossils; remains of arthropods essentially identical in form to modern species have been found in rocks dating as far back as the Ordovician period, 450 million years ago. Read more here:
Today, four species of horseshoe crab still roam the shallow waters of sea coasts just as their ancestors did before the era of the dinosaurs: the Atlantic horseshoe along the North American Atlantic and Gulf Coasts and three other species in the western Pacific Ocean off the coasts of Japan, China, and Indonesia.
These curious-looking crabs scuttle about on subtidal sand and mud bottoms, feeding on small animals in the sediment. In spring, adult horseshoes migrate in hordes onto beaches to mate and lay enormous amounts of eggs in the moist sand of the surf zone. There the developing larvae will be sheltered from deeper-water predators until the baby horseshoes hatch and return to the sea. The annual egg deposition of horseshoe crabs is a welcome feast for migrating red knots, ruddy turnstones and other shore birds, which stop on Atlantic beaches to fuel up on the lipid-rich ova before their final push to their own breeding grounds on arctic tundra.
Photo of "Horseshoe crab mating" by Asturnut (\Licensed under CC BY-SA 3.0 via Wikipedia - http://en.wikipedia.org/wiki/File:Horseshoe_crab_mating.
The Georgia Outdoors video on Little St. Simons Island has an interesting segment about horseshoe crabs mating on the island's beach.
Often breeding horseshoe crabs become disoriented on the beach and can't find their way back to the sea. Their dead bodies provide a feast for ghost crabs. Sometimes vultures, or even raccoons, will stop by for a Limulus snack. http://www.georgialifetraces.com/tag/horseshoe-crab/
Horseshoe crabs are true blue-bloods. The hemoglobin in their blood contains copper rather than iron like the hemoglobin of our own red blood. Their blood also contains bacteria-gobbling cells called amebocytes, which defend the crabs against invading pathogens. Amebocytes from the blood of Atlantic horseshoe crabs are harvested to make a product widely used in medicine to detect the presence of disease-causing bacteria. The blood is extracted from the base of the tails of captured horseshoe crabs, and then the animals are released back into the ocean. Read more about this in a CNN report: