Forget Dinos: Horseshoe Crabs Are Stranger, More Ancient—And Still Alive Today

But now evolution’s ultimate survivors may be in danger

Jesse Lerer, a volunteer, is recording horseshoe crab data on Plumb Beach, Brooklyn. Volunteers record number, sex and other details. Camilla Cerea
Horseshoe Crabs in a quadrant. During the count two teams walk on the beach and put down the quadrant every 17 m counting how many male or female crabs are in the 1 meter square space. Camilla Cerea
Anita Cabrera, on the left, former site coordinator of Plumb Beach, recording the crab while Jacky Lee, volunteer, is pacing. A full or new moon changes the tides and the number of crabs on the beach. In fact, the counts are just two days before, during and two days after full or new moon. Camilla Cerea
To tag a horseshoe crab, volunteers drill a hole into their shell called prosoma, and insert the tag. Tags help scientists to find out about the animal habits. Camilla Cerea
The body of a male horseshoe crab. The hairy center is the mouth. The first two arms have "boxing gloves" to attach to females, while the other arms are used to eat. Camilla Cerea
Phil Cusimano, site coordinator, and Erika Crispo, volunteer, drill a small hole to place the tag on a horseshoe crab in Plumb Beach, Brooklyn. Camilla Cerea
The volunteer team measures the length of a male horseshoe crab. Camilla Cerea
A tagged horseshoe crab. Camilla Cerea
Heather Loebner, volunteer, monitoring on a rainy day at Plumb Beach, Brooklyn. Camilla Cerea
A horseshoe crab on the shore in Plumb Beach, Brooklyn. In the background, volunteers monitor and tag horseshoe crabs. Camilla Cerea
Christine Nealy, site coordinator for Dead Horse Bay, is measuring the animals before tagging. In the background Bridget Klapinsk, volunteer, is collecting the horseshoe crab for the tagging. The program is a collaboration between NYC Audubon and Cornell University. Camilla Cerea
A horseshoe crab. Camilla Cerea
A horseshoe crab. Camilla Cerea
A horseshoe crab. Camilla Cerea
A horseshoe crab. Camilla Cerea
Bridget Klapinsk, volunteer, has done this before. Living in the Rockaways she wants to be part of something to help the bay. Dead Horse Bay, Brooklyn. The program is a collaboration between NYC Audubon and Cornell University. Camilla Cerea

Each summer, guided by the light of the moon, some of the world’s strangest inhabitants ascend the East Coast’s beaches to spawn the next generation. These hard-shelled, many-eyed anomalies remind some of armored aliens or living spaceships. They're actually horseshoe crabs, and they date back 450 million years, having outlived the dinosaurs and survived five mass extinctions—including one that nearly wiped out life on Earth. 

"They look like something you can imagine but never see," says wildlife photographer Camilla Cerea, who has begun documenting the charismatic crab and the people working to monitor it and save it from modern threats. "It's almost like seeing a unicorn." 

Horseshoe crabs—in actuality, marine arthropods that aren’t even distantly related to crabs—aren’t just a curiosity to ogle on the shore. Their bluish, copper-tinged blood is used to test for toxic bacterial contamination, meaning you have them to thank if you’ve ever used contact lenses, had a flu shot or ingested medicinal drugs. Humans bleed 500,000 of the creatures a year to procure this medically valuable substance, before returning the crabs to the waters.

But now, the lethal combination of climate change, habitat loss and over-harvesting means that these living fossils face their biggest existential challenge yet.

Thanks to shoreline development and sea level rise worsened by climate change, horseshoe crabs are steadily losing the beach habitats they rely on for mating and breeding. In addition to extracting their blood, humans harvest the creatures to use as bait for fishing eels and whelk; in some parts of the world humans also eat their eggs or the animals themselves. Last year, the Atlantic horseshoe crab was listed as “vulnerable” on the Red List of the International Union for the Conservation of Nature, with some populations facing even greater risks.

Cerea first heard about horseshoe crabs through her day job as a photographer for the National Audubon Society. The birds that the society is devoted to protecting often feed on their clutches of tasty blue eggs, and as the crabs have declined in some regions, so have birds. When Cerea first looked up the arthropods online, she was captivated. "Honestly, I had never seen something like that in my entire life," she says.

She soon found out she wasn't alone in her appreciation. During their summer breeding season, a devoted corps of volunteers organized by Cornell University and NYC Audubon patrols the beaches of New York City at night to count the horseshoe crabs, and tag them for tracking. "Every volunteer has a different reason to be there," Cerea says. "But everybody has an amazing passion about the horseshoe crabs themselves."

The monitoring in New York is done for this year, but Cerea plans to be back again next year—both as a photographer and a volunteer. "It's such an important and tangible animal, and very few people know it," Cerea says. "They're even older than dinosaurs, but they're real, they're there." Let’s hope we don't end up being the reason that evolution's ultimate survivors aren't here in another 450 million years.

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