With spring now well underway, our beaches become prime destinations for a host of newly active marine life, from nesting loggerhead turtles to migrating red knots. Often overlooked, though, are the animals that have lived on earth longer than almost any other species - horseshoe crabs (Limulus polyphemus), which begin coming ashore in great numbers during the late spring and early summer to mate and lay eggs.
They aren't crabs, of course, but arthropods related to spiders and scorpions, which first appeared in the fossil records of 400-million years ago, surviving shifting continents, ice ages and dinosaurs to earn the nickname "living fossils." Despite their formidable appearance, especially the long whip-like tail, horseshoe crabs are harmless and stingless. The tail is used to right themselves if they become flipped over by surf.
Red knots and other migrating birds depend on the crabs' fat and protein-rich eggs to bulk up before continuing on to summer nesting sites; loggerheads, particularly egg-bearing females, prey on them for their calcium-rich exoskeltons. And humans have harvested them as fertilizer and cattle feed, but
especially prize them nowadays for their copper-laden blue blood, which contains a chemical used as a component of vaccines and intravenous medicines to detect toxic impurities. If you've received any vaccination or infusion, you have horseshoe crabs to thank for the drugs' safety.
Our Atlantic horseshoe crabs belong to one of four species found throughout the world. Along with their useful blood, another unique feature is a set of ten eyes - two compound eyes on the top of the shell, with more photoreceptive cells than any other animal, and eight simpler eyes around the shell's perimeter which are sensitive to the length of daylight and to ultraviolet light, giving horseshoe crabs exceptional night vision and allowing them to minimize predation by being most active at night.
In May and June, especially on full-moon nights, males and females come ashore in great numbers to mate. Each female lays about 4,000 eggs in each of multiple nests she digs in the sand between high and low tide marks. Two to four weeks later, the hatchlings claw their way out of the sand at high tide, to be carried out to deeper water where they'll live on the seabed during multiple moltings as they grow to adult size.
Although horseshoe crabs are not considered endangered, harvesting them for their blood is somewhat controversial, even though only a third of the blood is removed from each animal before it's returned to the ocean, where the lost blood is regenerated in about a week. But disrupting populations on beaches and tidal marshes, however temporarily, can affect the food supply upon which other migrating and nesting species depend. South Carolina's DNR and federal wildlife officials are considering denying permits for harvesting at the Cape Romaine Wildlife Refuge to the north, and permanent bans are in place on several islands that form part of the ACE Basin to the south. With spawning season just around the corner, harvesting's future continues under negotiation.