Anyone who has spent much time along the Rhode Island coast is familiar with one of the most ancient creatures on Earth, the horseshoe crab. More closely related to spiders and scorpions than to crabs, they evolved more than 300 million years ago, long before the dinosaurs, and they have remained mostly unchanged ever since. With 10 eyes, blue blood, and six pairs of appendages, they can look somewhat frightening. Just don’t believe claims that their tail – called a telson – is poisonous or can sting. Not true.
What is true about these remarkable creatures is that horseshoe crabs are far less common at their spawning grounds than they used to be. The days of seeing hundreds or thousands of horseshoe crabs scurrying about in local coves are mostly over. And their decline has implications for a whole host of other marine life. Some sea turtles, for instance, feed on horseshoe crabs, and many shorebirds, including the very rare red knot, consume horseshoe crab eggs to fuel their migration northward. Humans, too, depend on these surprising animals, because their unusual copper-based blood has properties that are used by the biomedical industry to ensure that our medical devices, vaccines and intravenous solutions are free of harmful bacteria.
The cause of their decline is somewhat uncertain, though large numbers are harvested for use as bait in the eel and conch fisheries. They also get stranded on beaches and accidentally flipped upside down by waves, which gives gulls and other predators easy access to their sensitive parts. And some die during the process of draining a portion of their blood for biomedical testing.
In Rhode Island, assessments of horseshoe crab populations by state and federal agencies have found them at “low levels of abundance” ever since a management plan was put in place in 1999. Scott Olszewski, a marine fisheries biologist at the Rhode Island Department of Environmental Management, who conducts spawning surveys at several area beaches, said that despite efforts to manage the population, their numbers have not rebounded. About 30 to 40 fishermen obtain permits each year to harvest horseshoe crabs in the state, and they are allowed to take up to 14,500 for use as bait and another 34,000 for the biomedical industry (though those in the latter category are released back where they were captured after some of their blood has been collected).
“When horseshoe crabs decide it’s time to spawn, they come up on the beach in massive numbers, which makes harvesting them pretty easy,” said Olszewski. That’s why the fishing season often closes just a few days or weeks after spawning begins.
Other scientists are now paying close attention to horseshoe crabs as well. Researchers at Sacred Heart University are overseeing teams of volunteers who are monitoring them at multiple locations in Long Island Sound, including at Napatree Point in Westerly. And others are doing the same on Cape Cod and in the Mid-Atlantic States.
At Napatree, volunteers led by the Watch Hill Conservancy go out at high tide on full moon nights from May through July to count and tag the animals. It’s an enjoyable few hours working by moonlight to the sound of the lapping waves, and it helps provide insight into the natural history of an ecological oddity that will likely outlive us all.
To learn more about becoming a horseshoe crab volunteer at Napatree, contact the Watch Hill Conservancy at firstname.lastname@example.org. And if you see large numbers of spawning horseshoe crabs at other coastal locations, report them to Olszewski at email@example.com.
This article first appeared in The Independent on May 19, 2016.