There's a tie that binds two creatures often seen during spring in the Lowcountry, and its red knot is quickly coming undone.
The red knot is a bird that feeds on the eggs of horseshoe crabs to sustain its annual 10,000-mile migration, one of the longest of any bird, according to the Cornell Lab of Ornithology's Web site, All About Birds.
The tie has loosened drastically over the years, as the red knot's favorite food source has dwindled, mostly in northern states where fishermen harvest horseshoe crabs for bait.
Barry Lowes of the Hilton Head Audubon Society says he recalls seeing thousands of red knots flock to Hilton Head Island's beaches during the 1980s in time for the horseshoe crabs' mating season.
But through the years, he said he has seen the number of red knots decline drastically. Now, only a handful of the birds stops at Lowcountry beaches on the way to the Arctic, he said.
"Like lemmings going over a cliff, they just crashed," Lowes said. "In a few years, they were gone. We do see them on the beach, but just a handful, and that's the disquieting thing."
The red knot is about the size of a robin, about 5 ounces. It also has a reddish breast in breeding plumage and is gray the rest of the year.
Their journey begins in Tierra del Fuego in South America, where they fatten up on mussels. They flock together in thousands as they make their way to Arctic breeding grounds. They stop along the Eastern Seaboard, timing their arrivals to the ancient mating movements of the horseshoe crab.
The horseshoe crab is not really a crab. It's a marine arthropod, more akin to the spider and scorpion. When it's time to mate, they gather in large numbers in coastal waters, awaiting the full and new moon tides to emerge on the beach to mate. One female horseshoe crab can lay as many as 75,000 eggs during the spring ritual.
The world's largest gathering place for the horseshoe crab is Delaware Bay. It was the decline in the numbers of red knots there that first alerted scientists to the horseshoe crabs' decline in the bay, according to "Crash: A Tale of Two Species," a PBS documentary that premiered in February 2008.
The documentary cited a 70-percent decline in horseshoe crabs in the bay. It partly blamed an increase in harvesting the crab in the 1990s for bait, primarily for the American eel and conch fisheries. The birds were unable to obtain enough fat to fuel the rest of their two-day straight journey to their breeding grounds. The red knot's population in Tierra del Fuego has dropped 50 percent from the mid-1980s to 2003, according to the All About Birds Web site.
The horseshoe crab population in the Lowcountry, however, appears to be stable. In April, May and June, the scary looking but harmless prehistoric creatures continue their ancient spawning ritual.
South Carolina law protects the crabs, prohibiting their harvest for bait, according to the S.C. Department of Natural Resources Web site. The horseshoe can be harvested for the medical industry but the state requires a license to do so. The crabs must be returned to the waters after one-third of their blood is extracted. It has been estimated about 10 percent to 20 percent of them don't survive the ordeal, the site says.
The horseshoe's blue blood is used to detect bacteria in intravenous fluids and injectable drugs and is used for other medical purposes, according to the DNR Web site. The horseshoe crab is not listed as endangered or threatened in South Carolina.
Their gelatinous eggs about the size of BBs attract about 20 different migratory birds, but Lowes said it appears the red knots have had the most difficulty in adjusting to the reduction in horseshoe crab eggs.
"A lot of other shorebirds feed on those eggs too, but the red knots seem to be the biggest," he said.