A recent study found that horseshoe crabs have bounced back since population declines in 1990s
ANNAPOLIS – Under the vibrant lights of the Ocean City boardwalk Ferris wheel and Slingshot ride, prehistoric animals scuttled onto the shore of a quiet beach by the thousands.
While the setting has changed, horseshoe crabs and their close relatives have been making this same journey to the world’s coastlines for hundreds of millions of years, through mass extinctions and the shift of continents, in what is considered the oldest wildlife migration on the planet. On spawning days that occur from May to July, these ancient arthropods return to beaches in Maryland and along the East Coast in search of mates.
“The horseshoe crabs are on,” Steve Doctor, a Maryland Department of Natural Resources fisheries biologist, said on a June spawning night. “It’s ‘Jurassic Park’ out there.”
Doctor, who has led the state’s horseshoe crab management program since 2003, brings a team of DNR biologists to Ocean City spawning sites on eight nights of the summer to count the horseshoe crabs. Adding to their otherworldly aspect, horseshoe crabs spawn on nights around the full moon and new moon, when the tides are highest.
This survey work is a part of ongoing monitoring efforts by biologists in Maryland and other coastal states to study the Atlantic horseshoe crab in the Delaware Bay region, where the highest concentration of the species is located. The eggs that horseshoe crabs lay in the sand here every year are a critical food source for the many shorebirds that migrate through the Mid-Atlantic.
The thousands of horseshoe crabs spawning on Maryland beaches are part of a conservation success story. While the population of horseshoe crabs around the Delaware Bay had declined in the 1990s, a Marine and Coastal Fisheries study published in October found that the species had recovered in the region by 2023, after Maryland, Delaware, New Jersey, and Virginia took management action to protect the species.
Ancient roots and modern challenges
With its domed shell and long spike of a tail, a horseshoe crab looks like a living fossil. They aren’t actually crabs or crustaceans, but are more closely related to arachnids. They’re part of an order of arthropods called Xiphosura, which have roamed the world’s oceans for 450 million years.
In that time, horseshoe crabs and their close relatives have changed very little. Fossils found in the Jurassic Period 148 million years ago are nearly indistinguishable from horseshoe crabs on the East Coast today. Scientists have called them a “stabilomorph” for their stability through deep evolutionary history—horseshoe crabs are so well-suited to their ecological niche that haven’t needed to evolve much.
Four species of horseshoe crabs live today: three in Asia and the Atlantic horseshoe crab, which ranges from Nova Scotia in Canada to the Yucatán Peninsula in Mexico. Despite their evolutionary longevity, horseshoe crabs have faced declines with modern threats of habitat loss and overharvesting. These effects can be seen directly in populations—such as the tri-spined horseshoe crab considered critically endangered in Japan—or in other parts of the ecosystem.
In the 1990s, counts of shorebirds stopping over around the Delaware Bay started to decrease. At the same time, the Marine and Coastal Fisheries study noted, there had been an increase in the harvest of horseshoe crabs in the region.
In the U.S., horseshoe crabs are fished primarily as bait for American eel and whelk. Biomedical companies also collect a set number of horseshoe crabs, extract blood from them, and release them back into the water; the unusual blue blood of horseshoe crabs contains a protein that detects bacterial contamination of medical devices, vaccines, and drugs.
An increased demand for eel and whelk led to a nearly 600% increase in coastwide horseshoe crab landings from 1990 to 1997. Those larger harvests appeared to have knock-on effects among shorebirds, which depend on the superabundant horseshoe crab eggs as a fat- and protein-rich food source during their migration. Sightings of red knots dropped from 50,000 birds in the late 1990s to half that number a decade later.
In response, the Atlantic States Marine Fisheries Commission implemented a management plan for horseshoe crabs in 1998. The resulting framework was unique, Doctor said, because it was geared toward multiple species—both red knots and horseshoe crabs.
A recovery, with lingering questions
In the years that followed, New Jersey instituted a moratorium on the commercial harvest of horseshoe crabs, while Maryland, Delaware, and Virginia have set lower harvest quotas for male horseshoe crabs and do not allow harvest of female horseshoe crabs.
With these regulations in place, the horseshoe crab population in the Delaware Bay region has recovered. The latest ASMFC stock assessment estimated 16 million adult females and 40 million adult male horseshoe crabs in the region. While individual state surveys have varied over that time, the Marine and Coastal Fisheries study found that the overall abundance was comparable to levels in 1990, before the higher harvests and the levels of decline.
Using data from horseshoe crab trawl surveys in New Jersey, Delaware, Maryland, and Virginia, the study found that the horseshoe crab population declined from the 1990s to 2005 and remained low until 2010, before increasing through 2023, when the population became comparable to—or possibly exceeded—the amount of horseshoe crabs in 1990.
“When the population did recover, it did at a faster rate than we expected,” Doctor, a co-author on the study, said. “Once it started to recover it just took off.”
Despite the regional recovery of horseshoe crabs, there hasn’t been an accompanying growth of red knots, which have remained stable but haven’t increased. Scientists speculate this could be due to a delayed response to the increase in horseshoe crabs, or it could suggest other factors are affecting the red knot populations.
The ASMFC now considers the horseshoe crab stock to be at a good level in the Delaware Bay, as well as in the southeast. The New England stock is considered stable, while the population in the New York region is poor.
As part of Maryland’s agreement with the ASMFC, state biologists monitor horseshoe crab spawning activity with the annual spawning survey in the Ocean City inlet area. DNR also conducts a seine survey and a trawl survey for horseshoe crabs, which biologists have completed since 1972.
“We try to keep a pretty tight rein on what’s going on with them,” Doctor said.
During the June new moon, when spawning is at its highest in Maryland, DNR biologists visited Skimmer Island in the inlet, a beach on Assateague, and another beach on the southern tip of Ocean City to count crabs. Working with a Maryland Coastal Bays Program scientist and volunteers, the team walked up and down the beaches, setting out transects and counting the jumble of horseshoe crabs within the square to estimate the total crowd.
As biologists stepped around them, the teeming horseshoe crabs remained focused on their purpose. The males followed around the larger females as they sought out a suitable patch of shore. After shifting in the sand, each female crab would lay and bury thousands of eggs to be fertilized by nearby males, once again completing a cycle older than time, bringing about another generation of horseshoe crabs onto the beaches of Ocean City.
By Joe Zimmermann, science writer with the Maryland Department of Natural Resources
