Ancient creatures with 12 legs, 10 eyes, and blue blood were once so prevalent on southern New England beaches that people, including children, were paid to kill them.

Their helmet-like bodies can still be seen along the region’s coastline and around its salt marshes, but in a fraction of the numbers witnessed seven decades ago. There are many reasons why.

In the 1950s coastal New England paid fishermen and others bounties to kill the up to 2-feet-long arachnids — horseshoe crabs are more closely related to spiders, scorpions, and ticks than to crabs — because they interfered with human enjoyment of the shore and were viewed as shellfish predators.

People, not just fishermen, were reportedly encouraged to toss horseshoe crabs above the high-tide line, so they would dry out and die. They were labeled “pests” and ground up for fertilizer. Beachfront property owners were apparently concerned the creature’s presence and their decaying death would impact real estate values.

Those ignorant days may be over, but horseshoe crabs are facing other threats to their existence.

The Center for Biological Diversity, an Arizona-based nonprofit, and 22 partner organizations recently petitioned NOAA Fisheries to list the Atlantic horseshoe crab as an endangered species under the Endangered Species Act. Horseshoe crab populations have crashed in recent decades because of overharvesting and habitat loss, according to the petitioners.

“Horseshoe crabs are imminently threatened by habitat loss, overexploitation, inadequacy of existing regulatory mechanisms, and other natural and manmade factors, including climate change,” according to the 142-page petition. “They are in danger of extinction across a significant portion of their range, and threats are likely to persist and worsen in the foreseeable future.”

The body-armored arthropods — also known as the American horseshoe crab, because they are the only living species of horseshoe crab native to the Americas — are used by the biomedical industry, which takes the animal’s copper-based blood for tests to ensure that medical devices, vaccines, and intravenous solutions are free of harmful bacteria.

Horseshoe crab blood harvests have doubled since 2017, with nearly a million horseshoe crabs harvested for their blood in 2022, according to the Center for Biological Diversity. The petition noted synthetic alternatives to horseshoe crab blood tests are already being used in Europe, but companies in the United States have been slow to adopt the alternatives.

The biomedical industry has claimed most of those captured for their blood are returned to the sea. The petitioners don’t trust that assertion, as it has been estimated that the mortality rate of horseshoe crabs harvested for their blood is about 30%.

The petition noted horseshoe crab harvests for blood are increasing, “with little oversight, transparency, or regulation, which further depletes dangerously low horseshoe crab populations.”

Horseshoe crabs are also harvested for use as bait by the commercial sea snail and eel fisheries and by recreational anglers. Even though horseshoe crab populations have fallen to historic lows, regulators have increased horseshoe crab harvest quotas, according to the petition.

“Despite the declines in horseshoe crab populations and habitat, the commission regulating horseshoe crab harvests, the Atlantic States Marine Fisheries Commission (ASMFC), is proposing significant increases to horseshoe crab harvest quotas for bait, including recommendations to increase total horseshoe crab bait harvest quotas in four Mid-Atlantic states and to resume the harvest of female horseshoe crabs for bait,” the petition states.

Among the 22 partner organizations that co-signed the petition was the Southeastern Massachusetts Pine Barrens Alliance, an all-volunteer, Plymouth-based nonprofit that was created in 2013. The organization’s volunteers have been conducting local horseshoe crab surveys since 2017.

“We see them being harvested by the boatloads, and and the ones that aren’t harvested, the males, which are smaller than the females, so not as desirable, we see them being wrenched off, pulled by their telsons [tails] and thrown back into the water,” Sharl Heller, the organization’s co-founder and current president, recently told ecoRI News. “And if you know anything about horseshoe crabs, you know you can kill horseshoe crabs by pulling their telsons in the wrong direction.”

Frank Mand, the organization’s vice president, noted the alliance initially got involved with the surveys to educate people about the “beauty and wonder” of the natural world.

“When we began surveying, we did so in a positive sense that we were going to contribute to the greater awareness of their numbers and their habits and their habitats,” he said. “But what we saw right off the bat was so few of them, and that began to concern us.”

Mand said Massachusetts’ policies and practices are “essentially indifferent to their plight.”

“They were once ubiquitous in nature … there were millions and millions and that’s been knocked down to a critical point,” he said. “So we began to become instead of just surveyors but advocates for their protection.”

Last March the Southeastern Massachusetts Pine Barrens Alliance filed a petition to have the Atlantic horseshoe crab state-listed as endangered. No public hearing has yet been scheduled.

The alliance’s Massachusetts petition, among other things, calls for the immediate end of harvesting horseshoe crabs for bait, strict quotas on the biomedical take, and funding the restoration of horseshoe crab habitat.

The best available scientific information demonstrates that listing the horseshoe crab as threatened or endangered throughout all or a significant portion of its range may be warranted, according to the Center for Biological Diversity-led petition.

“The American horseshoe crab is threatened by habitat loss, sea level rise, and climate change across the entirety of its range, and overharvest threatens the American horseshoe crab across a significant portion of its range,” the petition states. “Populations have declined across the entirety of the horseshoe crab’s range, and their numbers continue to decline or remain at historically low levels across nearly all of their range.”

As horseshoe crabs have declined, so have other species, such as endangered sea turtles, fish, and birds. Most notably, the rufa red knot, a shorebird species that feeds on horseshoe crab eggs during its 19,000-mile migration from South America to the Arctic, was listed as threatened in 2015 under the Endangered Species Act. The listing decision cited the horseshoe crab harvest as one of the contributing factors to the red knot’s decline.

“Horseshoe crab eggs are incredibly nutrient dense, sustaining the federally threatened red knot on their long migratory journey,” said Steve Holmer, vice president of policy at the American Bird Conservancy. “Greater protection of the horseshoe crab is needed to fully recover the red knot, as well as conserve other shorebird species, such as the ruddy turnstone and semipalmated sandpiper.”

Nearly twice as old as dinosaurs, horseshoe crabs have been crawling ashore underneath the light of a full moon for some 450 million years to lay thousands of eggs in the sand. In the past three decades, however, horseshoe crab populations have declined by two-thirds in Delaware Bay, their largest population stronghold, according to the petitioners.

Late last year the ASMFC said it would prohibit the harvesting of female horseshoe crabs that originate in Delaware Bay during the 2024 fishing season. Despite that decision, the chair of the ASMFC horseshoe crab management board told The Associated Press in November that the Delaware Bay horseshoe crab population has been increasing over the past two decades.

“Despite this positive finding, the board elected to implement zero female horseshoe crab harvest for the 2024 season as a conservative measure, considering continued public concern about the status of the red knot population in the Delaware Bay,” John Clark said.

Mand debunked the board’s Delaware Bay claim.

“It’s a very simple thing to deflate … if you had a million of one kind of species and they went up to 1.1 million, you’d say, well, that’s a nice increase,” he said. “If you have 100,000 of what used to be a million and they went up by 10 percent to 110,000, you may have been able to stabilize what’s left, but what’s left is such a pale version of what was there.”

The management board also ruled to allow more harvest of male horseshoe crabs in the mid-Atlantic to help make up for the lost harvest of females.

Each spring along the Atlantic and Gulf coasts, horseshoe crabs lay their eggs in massive beach spawning events. But this fragile landscape continues to be destroyed by development, shoreline hardening, and rising waters.

Horseshoe crabs rely on estuaries for food resources, places to spawn, and for larvae and juveniles to develop and grow. Many of these ecosystems have become increasingly developed, dominated by human activity, and polluted.

The hard-shelled creatures are a keystone species, making them vital to the marine environment. Besides their eggs being a critical food source for other species, including many migratory shorebirds, their walks along the ocean floor rototill the bottom, helping to keep the seafloor healthy.

The docile and slow-moving animals also play host to other life. Barnacles, blue mussels, eastern oysters, ghost anemones, red beard sponges, scuds, skeleton shrimps, snail furs, and other creatures make the horseshoe crabs’ carapace their home.

The points on their shells protect them from some predators such as gulls, but not from others.

Limulus polyphemus were hunted heavily by humans from the 1800s to the early 1900s, with catches often topping 5 million crabs in year, according to the ASMFC. Millions more were likely harvested unreported. At the time, many fishers and fishery regulators didn’t see the value of horseshoe crabs to the greater ecosystem.

Over time, their importance was learned and catch numbers dwindled. By 2001, state-by-state harvest quotas were required by the ASMFC.

Beginning last year and continuing this year, Massachusetts’ long-standing commercial quota for horseshoe crabs harvested for bait purposes was reduced from 165,000 to 140,000 and the state’s first commercial quota for horseshoe crabs harvested for biomedical purposes was set at 200,000.

“Capping total horseshoe crab harvest and mortality is likely the single most important conservation measure the state can take this year,” Dan McKiernan, director of the Massachusetts Department of Fish and Game’s Division of Marine Fisheries, is quoted in a July press release announcing the quota change. “This eliminates the potential for uncontrolled growth in the biomedical fishery, which could negatively impact the resource moving forward. Capping harvest and mortality at near recent levels is appropriate, as the horseshoe crab population has steadily increased since 2010.”

Mass Audubon, which recently launched a campaign to push for stronger protections for horseshoe crabs, has noted that, “To make matters worse, 140,000 of these crabs are used annually as bait for whelk — a type of shellfish classified as depleted and overfished in Massachusetts.”

Last year in Rhode Island, the Department of Environmental Management issued 61 horseshoe crab harvest permits, according to DEM spokesperson Evan LaCross.

He noted the Ocean State’s annual quota for bait harvest is 14,655 crabs. “2023 bait landings were substantially under the annual quota, but are confidential due to the number of participants,” LaCross wrote in an email.

The annual horseshoe crab bait quota allocated by the ASMFC for Rhode Island is 26,053. Recreational anglers can’t possess more than five horseshoe crabs in a single day. In 2017, DEM set the daily possession limit at 60 for commercial fishers, according to Katherine Rodrigue, the agency’s lead horseshoe crab scientist.

Rhode Island’s annual quota for biomedical harvest is 34,194 crabs. “The number of crabs harvested in 2023 is confidential but was below the annual quota,” according to LaCross.

The Ocean State’s commercial bait harvest is closed from May 1-31. The state’s biomedical harvest is closed during the period 48 hours preceding and 48 hours following the month’s new and full moons. These closures are to protect horseshoe crabs during their peak spawning period, according to Rodrigue.

She said Rhode Island’s horseshoe crab population is in decline, but the species isn’t state listed as endangered.

Connecticut last year banned the harvesting of horseshoe crabs in state waters — a result of mounting concerns about the population of horseshoe crabs, which has reportedly seen a dramatic decline in Long Island Sound in recent years.

In recent years environmental groups, including many of those that signed the Feb. 12 petition, have called for greater protection of horseshoe crabs, and have scored some victories. For instance, in August the federal government announced it was shutting down the harvest of the prehistoric crabs in the Cape Romain National Wildlife Refuge in South Carolina during the spawning season.

When the moon waxes full from March to July, the southern New England coastline calls once again to these primordial creatures. Males, which are about 20% smaller than females, use a specially developed appendage to latch themselves onto the back of females during breeding season.

Since they can be easily knocked over, horseshoe crabs favor a calm shore, often choosing marshes or other sheltered areas to deposit their eggs. When horseshoe crabs attempt to come ashore, wave action can knock them on their backs. The telson is the only hope they have of righting themselves, by sticking the appendage into the sand and flopping back onto their legs.

Each female lays between 80,000 to 100,000 eggs over the course of several nights. In less than a month, the juvenile horseshoe crabs emerge and make their way back to the sea, but only a tiny percentage survive the 10 years it takes to reach sexual maturity. Most fall prey to birds, fish, and reptiles. Horseshoe crabs can live up to 25 years.

By the end of July, horseshoe crabs move into deeper water, vanishing until the next breeding season.

“Towns paid a bounty for kids or anybody to get rid of them, to destroy them, to kill them, to pull them off the beach so people could get down to the water without stepping on any,” Mand said. “And now, if somebody sees a horseshoe crab, it’s on Facebook, because they’ve never seen them before. ‘Wow, what is this creature’ — a creature that was so liberating across the area that it was considered a nuisance. And now it’s an oddity, a rarity, a strange beast. Everybody who goes to the beach should be familiar with these, but they’re not.”

A sister species to the Atlantic horseshoe crab — the tri-spine horseshoe crab in Asia — faces similar threats and is nearly extinct.