KITTS HUMMOCK, Del. — All along the shoreline, for as far as you can see, slick shells of horseshoe crabs glisten in the fading daylight. Listen closely, and you can hear their subtle clacking and the whisper of water over their carapaces.
It’s horseshoe crab spawning season in Delaware Bay. Every May and June, on nights when the moon is new and the tide is high, they crawl onto the beach to mate and bury their eggs.
The ritual goes back 445 million years. Horseshoe crabs are living fossils that have survived four mass extinctions. They are bizarre creatures with 10 eyes that offer insights into how vision evolved. And their blood has saved countless human lives — including yours.
But these creatures, nature’s consummate survivors, are in peril. And to protect them, it’s urgent that biologists understand their life cycles and learn how many there are. That’s why researchers are out in force this night, working quickly to take a census of the crabs before they disappear beneath the waves.
Elle Gilchrist reaches into a pile of crabs. Each is glossy green-brown and shaped like a shallow combat helmet with a six-inch spine sticking out the back. Gilchrist, a 20-year-old intern with the Delaware National Estuarine Research Reserve, expertly flips a crab over to reveal 10 segmented legs and a sheaf of sturdy gills. The males’ limbs end in pincers, which they use to grab onto prospective mates. The insides of the females’ carapaces are lined with thousands of tiny pale green eggs — the reason for tonight’s festivities.
Gilchrist starts to tally the horseshoes in her plot. “One, two, three, four, five males,” she calls to a volunteer taking notes. Then she thrusts her hand among the shells and feels for the huge, smooth carapace of the lady they’re all trying to woo. “One female.” She grins.
Horseshoes crabs are not actually crabs. They’re instead distantly related to spiders and scorpions (though they predate both).
They’ve developed some pretty savvy evolutionary strategies, like sex on the beach. When horseshoes first evolved, land animals didn’t exist yet, which meant no predators could get at eggs laid in the sand.
And so it has. Epochs came and went, oceans rose and fell, the continents converged and drifted apart again, and the horseshoe crabs endured.
“These guys are maybe, debatably, the best-adapted creatures in the world,” Gilchrist says. “The panda? Don’t know how the panda made it. But the horseshoe crab? I’m like, wow, that really has it figured out.”
First of all, there’s their blood. The stuff that runs in horseshoe crabs’ veins is copper-based, and it gleams pale blue when exposed to oxygen. (Iron makes humans’ blood look red.) But the true marvel of horseshoe blood lies with specialized immune cells called amebocytes, which clump up and form a gel on contact with a bacterial invader.
An extract from these cells, limulus amebocyte lysate (LAL), has been used to test for contamination in every vaccine, surgical tool and medical device that’s been inside a human body in the past 47 years, saving untold numbers from dying of infections such as pneumonia and E. coli. No other animal’s blood has such powerful antimicrobial properties, and scientists still aren’t able to reproduce it in a lab.
A vampiric biomedical industry has sprung up to harvest horseshoe blood. Crabbers catch the animals and send them to labs, where about 30 percent of their blood is drawn. The amebocytes are then turned into LAL — $50 million of it every year. And the crabs are dropped back into the ocean, preferably in the same water where they were found.
Horseshoe crabs’ eyes are a study in how one of life’s most complex organs evolved. There are 10 of them, including two that can detect ultraviolet light from the moon and stars. These specialized peepers let the crabs know when it’s the new and full moons — the times when the tide is highest and conditions are right for spawning.
The list of weird and amazing attributes goes on: Their shells repel bacteria. The jagged spines along their sides help the crabs feel their way along the sea floor and contain nerve cells sensitive to minute changes in the water temperature and current. A horseshoe crab chews its food with its legs, then stuffs the food into its mouth, which is situated in the middle of its belly. Their shells are hinged; when a crab finds itself upside down, it will bend at the middle and use its tail to push against the sand and flip itself upright. They can live to be 25 — far longer than most dogs and cats. And their eggs are essential food for migrating shorebirds.
“All of this life, this productivity, is totally lined up and synchronized and linked together for as long as there is coastline,” Tanacredi says. “And it has been for millions of years.”
In a matter of days, the moon will change phase and the horseshoes will vanish beneath the waves for another year. With so few nights left for surveying, it’s worth staying out in such uncomfortable conditions, Gilchrist says. Every crab counts.
