MAGAZINE

Dazzling photos show horseshoe crabs thriving in protected area

In the Philippines, the tri-spine horseshoe crab has made a home and other species are returning too.

A tri-spine horseshoe crab kicks up sediment along the muddy bottom of the Pangatalan Island Marine Protected Area in the Philippines. After a decade of restoration work to the islet’s bay, its green waters are rich with plankton and ready to welcome back bigger animals.

ByAmy McKeever

Photographs byLaurent Ballesta

July 12, 2022

5 min read

Horseshoe crabs are built to last. With spiky tails, shells shaped like combat helmets, and sharp pincers at the end of eight of their 10 legs, these ancient invertebrates have been scuttling along the ocean floor relatively unchanged for some 450 million years.

They managed to survive the asteroid that killed the dinosaurs. Surviving humans may prove more difficult. Like many marine animals, horseshoe crabs are overfished for food and bait, and coastal development has destroyed spawning sites. But they also are collected en masse for their blue blood, which contains a rare clotting agent critical for the development of safe vaccines. The blood may be lifesaving for humans, but its harvest often kills the animals—particularly in much of Asia, where they are drained of all their blood rather than just a portion of it.

Picture of shrimp hiding amongst a horseshoe crab's pincers. — A horseshoe crab hides an ecosystem within its shell. The hairlike objects along its body are hydroids—tiny, fuzzy invertebrates related to jellyfish—and there are at least eight shrimps clinging to the crab’s pincers. Horseshoe crabs are relatively unstudied; little is known about how they interact with other species.

Picture of a horseshoe crab seen on the sandy floor of a reef. — A tank-like horseshoe crab pushes itself across Pangatalan’s reef, which has benefited from the planting of mangroves and creation of artificial reefs. Members of the class Merostomata—which means “legs attached to the mouth”—horseshoe crabs are more closely related to spiders and scorpions than to crustaceans.

Tri-spine horseshoe crabs have lost more than half their population in the past 60 years. But on the Philippine islet of Pangatalan, the species is an unexpected symbol of resilience. For years the island’s 11 acres were degraded: trees cut down for timber, mangroves burned for charcoal, and coral reefs overfished with dynamite and cyanide. By 2011 these horseshoe crabs, about 15 inches long, were among the biggest creatures left.

Now a marine protected area, Pangatalan is starting to thrive again. Efforts to restore its reefs and plant thousands of trees have led many animals to return, including rare giant groupers that grow to some eight feet long.

Picture of the topside of a horseshoe crab's abdomen. — This isn’t an alien landscape—it’s an extreme close-up of the topside of a horseshoe crab’s abdomen. The gills are on the underside, and the dashes and indentations mark where they attach to the exoskeleton. The dark points beneath are minute spines that may function as whiskers do on a cat.

Picture of a horseshoe crab walking across a sandy sea floor, its legs and underbelly visible. — “Be beautiful or be shocking”—these are typical tactics used to get people to care about nature, says photographer Laurent Ballesta. “I think there’s a third way,” he says. “Try to show the mysteries. When you are in front of something you don’t understand, you forget beauty.”

(For Atlantic horseshoe crabs, love is a battlefield)

Horseshoe crabs may not be as charismatic as elephants or pandas, but perhaps they’ll inspire people to care more about wildlife. Appreciation for horseshoe crabs has grown thanks to their role in COVID-19 vaccine development. Conservationists hope that regard will translate to stronger habitat protections and wider adoption of a synthetic alternative to crab blood—saving horseshoe crabs just as they’ve helped save us.

Picture of three golden-yellow and black striped fish swimming above a horseshoe crab as it stirs up sand in its path. — Golden trevallies swim above a horseshoe crab, hoping to catch leftovers as it digs in the mud for clams and other prey. As bigger fish slowly return to the reef, horseshoe crabs may no longer rule the ecosystem. But they remain symbols of its resilience.

Amy McKeever is a senior staff writer. Laurent Ballesta, named Wildlife Photographer of the Year in 2021, is also a marine biologist.

This story appears in the August 2022 issue of National Geographic magazine.