NEWS Horseshoe Crabs Have Been Hiding Out in the Marsh

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Hakai Magazine

Every spring, since before dinosaurs roamed the Earth, American horseshoe crabs have been crawling out of the ocean under a full or new moon to lay their eggs on the shoreline. And for all that time, scientists have been overlooking a key hotspot for horseshoe crab reproduction.

American horseshoe crabs—roughly 60-centimeter-long arthropods with hard domed shells—are hardy generalists with an impressive ability to survive in a wide range of conditions. Even so, scientists have long thought that beaches are the only suitable spot for these crabs to lay their eggs. But new research from across the eastern United States now shows that horseshoe crabs spawn in salt marshes at roughly the same rate—a discovery with important implications for these economically important invertebrates and the wider marsh ecosystem.

For at least two decades, researchers at the South Carolina Department of Natural Resources (SCDNR) had heard anecdotal reports of horseshoe crabs spawning in salt marshes. At first, the experts wrote off the observed behaviors as maladaptive flukes. But as the stories built up, “we asked ourselves some questions and dug around some of the salt marshes in South Carolina,” says Michael Kendrick, a marine scientist with the SCDNR. “We began to find not just freshly laid eggs, but developing embryos” in numbers remarkably similar to those the agency had observed on beaches.

Expanding their scope, the scientists and their collaborators went looking for marsh-spawning horseshoe crabs in New Hampshire and Connecticut, ultimately finding the animals laying and fertilizing eggs on beaches and in salt marshes at similar numbers across the three states. Given that much of the coastline in these states is salt marsh, the work suggests that scientists’ past spawning surveys have missed a huge proportion of horseshoe crabs.

The work, says Kendrick, “demonstrates that there are still really important discoveries to make, even for species or systems that are otherwise well studied. It’s fascinating and serves as an important source of inspiration that hopefully can inspire others to continue to ask questions and challenge some of our assumptions.”

When horseshoe crab ecologist Mark Botton first saw the results, they reminded him of the times he saw the animals spawning near tidal creeks during his own expeditions in New York’s Jamaica Bay. But Botton, a professor emeritus at Fordham University in New York who was not involved in the research, says he never thought to look for eggs farther into the marsh creeks.

“[The authors] showed quite nicely that spawning in the salt marsh estuarine creeks is not just some fluke,” Botton says. “It seems like it is a habitual thing that [horseshoe crabs] can do.”

Figuring out where horseshoe crabs put their eggs is more than an idle curiosity.

On beaches, horseshoe crabs disturb the sand with their crawling and shuffling, which brings buried nutrients and minerals to the surface, ultimately helping to increase biodiversity in the area. It’s possible that their scuttling has a similar effect in salt marshes, but more research is needed to confirm this.

But with new understanding of horseshoe crabs’ fondness for salt marshes, Daniel Sasson, a marine scientist at the SCDNR and lead author of the paper, says conserving these ecosystems may offer a new way to protect the species—especially in places where beach habitat is scarce or at risk of disappearing to the rising sea. That’s particularly important because, in addition to their ecological role, horseshoe crabs are vital to the biomedical industry, which uses their blue blood to test vaccines, drugs, and medical devices for certain toxins fatal to humans.

These findings don’t just affect American horseshoe crabs. A single horseshoe crab may lay up to 100,000 eggs in a season, which feed all sorts of animals, including a shorebird known as the rufa red knot. These distinctively auburn birds routinely gorge themselves on horseshoe crab eggs along the Eastern Seaboard of the United States during their annual migration from South America to the Arctic. No one really knows if these seabirds also forage for eggs in salt marshes. But if they do, the United States’s management plan for this threatened species might need to change, says Sasson. Other animals, too, might be connected to horseshoe crabs in ways scientists don’t yet understand.

Even with a 450-million-year-old species, there’s still a lot left for us to learn.