Horseshoe crabs: Strange 10-eyed sea creatures that chew with their legs facing extinction

Nature’s consummate survivors may now be in peril

A horseshoe crab’s mouth is in the middle of its belly, surrounded by its legs (Ricky Carioti/The Washington Post)

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, who sports a long blond ponytail, galoshes and silver horseshoe-crab-shaped earrings, 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 mass of shells and feels for the huge, smooth carapace of the lady they're all trying to woo. “One female.” She grins.

Some people might be squeamish at the sight of hundreds of many-legged creatures congregating for what amounts to a giant arthropod orgy. But not Gilchrist. She describes the animals as “precious” and the mating ritual as “awe-inspiring.”

Horseshoe people are prone to gushing. Maggie Pletta, DNERR's education coordinator, likes to shock volunteers by literally kissing a crab, pressing her face against the mouth hidden amid its many legs. Stew Michels, a scientist with Delaware's Division of Fish and Wildlife, goes into raptures over the annual spawning: “I don't know, man... It's just tremendous.”

Conservation ecologist John Tanacredi, director of the Center for Environmental Research and Coastal Oceans Monitoring at Molloy College in New York, is so enamored he wants the United Nations to create a new UNESCO designation for them: “World Heritage Species.”

“These animals walked below the legs of brontosaurus at some point in time,” Tanacredi says. “They really should be the paradigm for survival and sustainability.”

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. At the end of spawning season, adult crabs could return to the water feeling relatively secure that their DNA would live on.

And so it has. Epochs came and went, oceans rose and fell, the continents converged and drifted apart again, and the horseshoe crabs endured. They outlasted their marine cousins the trilobites. They witnessed the rise and fall of the dinosaurs. They persevered through four mass extinctions, including one that wiped out 90 percent of all life on Earth.

“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 specialised 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 like 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 specialised 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. Other pairs help the crabs find mates and see through the sea floor murk. Even the spear-like tail has an eye, which keeps the brain synchronized with the daily cycles of light and darkness.

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, including the endangered red knot, which flies north at the same time the horseshoes are spawning.

“All of this life, this productivity, is totally lined up and synchronised and linked together for as long as there is coastline,” Tanacredi says. “And it has been for millions of years.”

Yet, despite decades of study, horseshoes still hold mysteries. Scientists don't quite know what the crabs get up to in the years between birth and their first spawning season, at age 10. They moult so often (18 times in their first decade of life) they're difficult to tag and track, though scientists are beginning to study the creatures' microbes as a possible marker of where they've been. They're incredibly difficult to breed in captivity, perhaps in part because they seem to prefer to lay their eggs on sand from particular beaches. As for what they sense in the sand, no one knows. Scientists aren't even sure how many horseshoe crabs live in the ocean - estimates range from 4 million to about 12 million for the lone Atlantic species. Three more species live in the Pacific.

These unanswered questions have become all the more urgent in recent years, Tanacredi says. In 2016, the International Union for Conservation of Nature changed the status of Limulus polyphemus from “lower risk” to “vulnerable.” The assessors noted that the overall Atlantic population had declined by at least 30 percent in the past 40 years.

The causes for this decline are complex, but humans undoubtedly have a major role. There's the LAL industry, which bleeds a half a million horseshoes a year. The process doesn't kill the crabs, but some horseshoes do die accidentally. And after being abducted, bled and transported hundreds of miles, many more are disoriented and debilitated. The final mortality rate could be as high as 29 percent, according to research published in 2015.

Still more crabs are harvested as bait for eel and conch fishing. Some, particularly in southeast Asia, are caught for human consumption. The shoreline habitat horseshoes depend on for spawning is being lost to development. And it's not known how much of an effect global climate change, and the accompanying ocean acidification and rising sea levels, may on have on the crabs.

Horseshoe crabs withstood a devastating asteroid strike that killed most other creatures on this planet. But will they survive us?

That's up to humans, Tanacredi says, especially ones like Gilchrist. When a storm rolls in about half an hour into the survey, Gilchrist simply pulls up the hood of her jacket and keeps trudging down the beach in the driving rain.

“Eleven males, two females,” she shouts over the din. The two volunteers who are working with her, a mother and daughter, look slightly daunted by the weather but neither complains. Gilchrist tries to keep them motivated: “Whoo, gotta love fieldwork!” she says with a laugh.

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.

The Washington Post