01-06-2025
For 400 million years, one local sea creature has been calling the coast home.
The horseshoe crab was about to make the same journey his species had made since before the dinosaurs roamed the earth. But as the young male horseshoe crab scuttled through Edisto Beach's murky water towards the sand, a hand reached down, scooped him up and momentarily delayed his search for a mate.
Daniel Sasson, a crustacean researcher with the South Carolina Department of Natural Resources, turned to the group of volunteers gathered around him on the beach. Some were local residents and seasoned DNR volunteers with the merch to prove it. Others were uninitiated environmental enthusiasts from across the Lowcountry.
The group of about 30 had arrived at the beach just as the sun was about to slip below the horizon. They shared one purpose: to document the spawning of American horseshoe crabs, a natural event that has occurred for over 400 million years.
Horseshoe crabs have witnessed numerous mass extinctions and the shifting of continents. While its numbers are largely good in the state, issues like coastal erosion and sea level rise still threaten the living fossils. Documenting horseshoe crabs surveys like the one on Edisto Beach help scientists better understand the species' range and abundance. This information helps researchers make better decisions about managing the species, whose eggs provide energy to a threatened bird as it makes one of the longest migrations on the planet. Pharmaceutical companies also take horseshoe crab blood for use in an important medical test, though the practice has been the source of controversy in recent years.
While colloquially called crabs, horseshoe crabs are not closely related to the fiddler crabs that were darting across the sand on Edisto Beach. American horseshoe crabs are part of a group that includes ticks, spiders and scorpions.
Every year, horseshoe crabs crawl from deep in the Atlantic Ocean onto the beaches of the eastern United States to spawn, or deposit eggs, in the sand. Most of the spawning activity occurs during the months of May and June, often under a full moon at high tide. In South Carolina, they lay eggs from the Charleston area down to Hilton Head's Calibogue Sound.
As all seven pairs of horseshoe crab legs continued to shuffle under a dome-shaped shell, Sasson demonstrated the correct way to hold the horseshoe crab. The long, stiff tail, known as a telson, is off limits when picking up a crab, he said. Instead, Sasson instructed the volunteers to grip under the shell with their thumbs and support the body with the rest of their hands.
'This way you don't want to put your fingers in here where they can bend and sort of pinch your fingers,' Sasson said. 'If they actually pinch you with their claws, it doesn't actually hurt.'
As he held the flailing crab, Sasson explained to the group that males search for the much larger females and then attach themselves with a modified pair of claws. Females then lay eggs in the sand before retreating back to the ocean. When walking down the beach for the survey, volunteers were instructed to call out the number of horseshoe crabs they saw so DNR staff could record the data on a spreadsheet.
As the lone males searched for mates, they'll approach anything that may resemble a female, Sassoon said. He warned the group that, despite five sets of specialized eyes, males sometimes mistake a human foot for a mate and try to latch on.
Sasson carried the horseshoe crab up the beach, measured him and recorded his estimated age, sex and size on a spreadsheet. He then inserted a tag into a small hole he made in the shell, and a volunteer timidly brought the newly dubbed 518516 back to the ocean. He moved back into the water, once again in search of a mate.
Horseshoe crab eggs are an important source of food for rufa red knots, a threatened species of bird. Rufa red knots make one of the longest migrations in the animal kingdom when they fly from South America to their breeding grounds in the Arctic. Along the way, they stop on the East Coast and gorge themselves on horseshoe crab eggs before completing their journey.
Companies also harvest the animals' blue blood, which pharmaceutical companies use to ensure drugs and devices are sterile. The practice of draining blood from horseshoe crabs in South Carolina has been the source of controversy and legal battles between environmental groups and pharmaceutical company Charles River Laboratories and the DNR in recent years.
While scientists and volunteers are trained to handle the crabs for research purposes, it's best for the average person walking on the beach to leave horseshoe crabs alone. But if a horseshoe crab is flipped over on its back, Sasson said it's helpful for beachgoers to gently flip them over and allow the horseshoe crab to go on their way.
The tags attached to the horseshoe crabs are small, white plastic circles with a number written on them. When the horseshoe crabs return to the beaches in the following years, anyone can report tagged horseshoe crab sightings to the U.S. Fish and Wildlife Service.
The sun had set at this point and volunteers strapped headlamps on — turned to a red light setting so they didn't damage the horseshoe's sensitive eyes — and spread across the beach. The small beams of light were trained on the water, and soon, horseshoe crabs began to emerge.
Morgan Corecki, a kayak guide based near Charleston, carried one crab back to the tagging station.
'I think it's fascinating to see how hands-on the community can be in citizen science,' she said while waiting in line, 'It's one of my favorite things to see in action — empowering community members to be a part of science.'
The data from the tags helps scientists understand how far the crabs can move. While most return to the same general area each year to spawn, one crab tagged in South Carolina was found in Delaware, Sasson said.
The data on returning horseshoe crabs helps scientists better understand how many survive from year to year. While some live up to 25 years, horseshoe crabs molt as they grow, meaning scientists can only tag adults. Some have recorded crabs that reappeared 10 years after they were originally tagged.
While researchers tag many horseshoe crabs across the east coast, it's rare to receive reports of tagged horseshoe crabs returning to the beach. In a given year, 30 to 50 reports of horseshoe crabs that were previously tagged in the state come back to researchers, Sasson said. Researchers generally need over a thousand of these reports before they can use the information in statistical models that help understand the status of the species.
These spawning surveys and tagging regularly occur on the beach, but researchers at the DNR recently discovered that horseshoe crabs spawn regularly in salt marshes. For over 50 years, conventional wisdom in the field suggested that horseshoe crabs only spawn successfully on beaches.
After years of comments from locals regarding the presence of the horseshoe crabs in the marsh, the DNR and other states took a closer look. They found that horseshoe crab spawning is widespread in the salt marsh, a finding that has important implications for species management in South Carolina, the state with the largest amount of salt marsh on the East Coast.
This year, researchers are trying to better understand what characteristics make a salt marsh attractive to horseshoe crabs for spawning. They're headed to Turtle Island, a barrier island in Jasper County, to investigate further.
But on Edisto Beach, the survey and tagging finish around 10 p.m. The same hands that scooped up the horseshoe crabs return them to the water under the light of a headlamp.
Volunteers and DNR staff gingerly walk across the dark beach inundated by the high tide, guided only by headlamps. The animals once again begin to move along the shoreline, where they'll mate and return to the deep and the cycle repeats as it has for millions of years.
