Close ties
In an everchanging coastal habitat, horseshoe crab eggs prove critical to the survival of species like the rufa red knot
The shore glows white in the light of a full moon. The night slowly shrugs off the warmth of another beautiful day in May. As the surf crashes on the beach, you spot a prehistoric creature crawling from beneath the foam.
But wait, there’s a second, this one slightly smaller, clinging to the first. Then there’s another pair. Soon, hundreds pile atop one another on the sand. The rays of your headlamp catch their shimmering exoskeletons, as the water rolls from their backs.
Strength in numbers
It might sound like the start of a science fiction novel, but for staff and volunteers at Prime Hook National Wildlife Refuge, it’s just another night of horseshoe crab surveys. Every year, during the high tide of a full or new moon, they gather after midnight, on weekends, and in the pouring rain to count crabs as the creatures crowd the shores of Delaware Bay.
Each year, thousands of horseshoe crabs visit the Delaware Bay in the single largest spawning event in the world for these remarkable animals. Females bury themselves at several locations along the beach, with males aboard their backs ready to fertilize newly laid eggs.
For horseshoe crabs, abundance is key. To bolster the odds of survival, females can lay more than 80,000 eggs over several nights. This trait helps ensure successful reproduction. Other species, including shorebirds, sea turtles, and fish, have evolved to take advantage of the concentration of nutrients that comes with an influx of hundreds of thousands of eggs.
A vital layover
Some of the beneficiaries of this feast soar above the beaches and wade in the surf. Between 425,000 and one million shorebirds visit Delaware Bay annually to gorge themselves on recently spawned horseshoe crab eggs.
For one species, this all-you-can-eat dining option is not only convenient; it’s crucial.
The rufa red knot is one of the most impressive migrants in the animal kingdom. The robin-sized shorebird spends the summer breeding season in the Canadian Arctic and winters as far south as the tip of South America, a roundtrip of up to 18,600 miles each year!
Migrating rufa red knots complete these expeditions in long stretches, flying up to 1,500 miles at a time without rest. When they finally arrive at Delaware Bay beaches, they are often emaciated, having depleted all of their fat storage.
A stopover like no other, the abundance of horseshoe crab eggs provides enough nutrients for the shorebirds to nearly double their weight in less than two weeks of feasting.
“There are few places like the bay, where these birds can amass the kind of fat they need to survive the Arctic,” said Wildlife Restoration Partnerships’ wildlife biologist Larry Niles. “Without it, the odds of survival and the ability to produce young drop drastically.”
A fading feast
An abundance of horseshoe crabs in the bay, however, has never been guaranteed. In the late 1800s and early 1900s, millions of horseshoe crabs were harvested as feed for livestock and fertilizer for crops.
After a period of recovery and relative abundance in the second half of the 20th century, horseshoe crab numbers again dwindled in the 1990s, as they were increasingly harvested for bait in the American eel and conch fisheries. Harvest numbers increased from around 100,000 crabs in the early 1990s to more than 2.5 million by 1999, before dropping again. Smaller-but-growing numbers are now captured for biomedical uses then released back to the wild.
Niles has led red knot surveys in Delaware Bay since 1986. He remarked, “Because of the longevity of our project, we have proved that the loss of Delaware Bay horseshoe crab eggs causes devastating fatalities of shorebirds going to the Arctic.”
Taking action before it’s too late
After witnessing and studying the damage caused by over-harvest, in 1998, the Atlantic States Marine Fisheries Commission, representing 15 states from Maine to Florida, developed a horseshoe crab management plan. In the Delaware Bay region, it greatly restricts the harvest of male horseshoe crabs and prohibits the harvest of females entirely. The collaborative efforts of this organization and partners have effectively stabilized populations.
Past struggles of horseshoe crab populations have led scientists to a deeper understanding of how essential a single species is to the health and well-being of both wildlife and people. The horseshoe crab carries on its back an entire ecosystem of wildlife, the economy of a fishing community, and the technology to produce life-saving vaccinations.
Conservation organizations are now prioritizing horseshoe crabs as they face an entirely new emergency. As climate change continues to reshape our shores, sea-level rise threatens to wash away the sandy bay beaches where the crabs lay their eggs.
“We are going to face climate change,” said Niles. “But we don’t have to sit back. We can’t change water temperature; we can’t change the winds in the south; but what we can do is restore the crab population to full productivity.”
Shoring up spawning habitat
Two of the most effective ways to increase wildlife populations in the face of climate change are creating and restoring key habitats. Through funding allocated for Hurricane Sandy restoration, national wildlife refuges like Prime Hook had the opportunity to increase horseshoe crab productivity by developing ideal conditions for spawning.
Horseshoe crabs need beaches that are protected from the harsh waves of the open ocean. Through collaborative projects involving federal, state, and non-governmental agencies, more of these important habitats are being established along the Delaware coast.
Restoration efforts in locations like Fowlers Beach at Prime Hook National Wildlife Refuge have transformed eroded shoreline into prime habitat for horseshoe crab spawning events. Early evidence suggests these efforts were not in vain.
“Before the restoration, this specific area wasn’t really a horseshoe crab beach,” said supervisory wildlife biologist Susan Guiteras. “Despite what we anticipated, they began spawning on the beach right away.”
Additionally, in 2018 and 2019, Niles and his team noted that not only did horseshoe crab egg density increase on restored beaches, but between 90 and 95 percent of rufa red knots in the bay were occupying those locations.
Biologists, maintenance professionals, and engineers have collaborated to create habitat that supports a variety of species and is built specifically to endure — and in some circumstances lessen — the effects of sea-level rise.
“Seeing the restored beach withstand the flooding puts it all together,” said Guiteras. “These are the conditions we are trying to work with.”
Restoration efforts continue along the Atlantic coast, but there is much to be done in little time. Now, more than ever, it’s crucial to study the relationship between rufa red knot and horseshoe crab populations so we may give these and other connected species the best chance at survival.
