A Closer Look at Horseshoe Crabs, an Ancient and Valuable Animal

National Geographic Horseshoe Crabs
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. (Photo by Laurent Ballesta)

Horseshoe crabs are ancient invertebrates that have existed along the ocean floor unchanged for some 450 million years. They survived the asteroid that killed the dinosaurs, but surviving humans is proving more challenging.

The August issue of National Geographic takes a closer look at these prehistoric creatures with photos, captured by Laurent Ballesta, that show them in their habitats in a way that many have likely never seen them.

Horseshoe crabs are built like tanks. Their bodies are covered in shells, they have sharp and spikey tails, and eight of their 10 legs feature sharp pincers. They have been able to withstand multiple changes to the planet over the course of the last 450 million years, but overfishing and coastal developments that have destroyed spawning sites has reduced the population of horseshoe crabs by half over the last 60 years.

National Geographic Horseshoe Crabs
A tanklike 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. (Photo by Laurent Ballesta)

These ancient creatures are hunted for food and for use as bait, but perhaps the most valuable aspect of the horseshoe crab is its blue blood: it contains a rare clotting agent that is critical for the development of safe vaccines.

National Geographic Horseshoe Crabs
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. (Photo by Laurent Ballesta)

In recent years, one of the crabs’ habitats in the Philippine islet of Pangatalan has been categorized as a marine protected area, and after years of severe degradation due to deforestation and overfishing that caused mass damage to the local reefs and fish species, the area is bouncing back.

National Geographic Horseshoe Crabs
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. (Photo by Laurent Ballesta)

This has been especially important in the last couple of years due to the COVID-19 pandemic and the need for vaccine development. While the horseshoe crab is not as cute or charismatic as an elephant or a panda, they are in no less need of protection. National Geographic says that conservationists hope that the recognition of their need will turn into stronger pushes to protect their habitats and result in wider adoption of a synthetic alternative to their blood — saving horseshoe crabs the same way they have saved humanity.

National Geographic Horseshoe Crabs

For more on this story, visit National Geographic’s website or find it in the latest issue of the magazine titled Stonehenge Revealed.

Image credits: Photos by Laurent Ballesta, courtesy of National Geographic.