Clawing Back

All it took was opening the valves. An inlet that is said to have once sheltered the Mayflower had been closed off since 1868 to pave the way for train tracks, and later a road, to the tip of Cape Cod. East Harbor became Pilgrim Lake, and the clams, oysters, quahogs and horseshoe crabs that had called the lagoon home disappeared as freshwater gradually replaced saltwater. Over time, some new denizens arrived and prospered — to near-biblical proportions.

By the beginning of this century, large clouds of midge flies in spring and summer blanketed windows and windshields and reportedly triggered allergies in some of the human residents who inadvertently gobbled up the tiny insects. “There were so many of them that they would cover people’s houses and make the insides dark,” said Sophia Fox, the aquatic ecologist at Cape Cod National Seashore.

The water quality deteriorated further, and in 2001, a lack of oxygen caused a massive kill of alewives, which had been introduced in the lake in hopes that they’d prey on midge larvae. The dead fish decomposed in the summer sun, producing a noxious odor. “Rotting fish and blackened houses and all that stuff is really not tolerable for people,” Fox said.

The town of Truro and the park quickly agreed on a possible — and relatively low-cost — fix using the existing one-way culvert under the road that had been releasing excess lake water into Cape Cod Bay. Town and park officials hoped that allowing seawater to flow once again into the closed inlet would make it more hospitable for its aquatic critters — and less so for the midges. So Truro opened the culvert’s clapper valves, and soon, one by one, marine species began coming back, and the saltmarsh lagoon started to look like its former, healthy self. In 2008, the body of water got its East Harbor name back. Also, people could again enjoy the view from their windows.

“This is what we call our happy story in the world of doom and gloom that we live in,” Fox said.

A big part of that happy story is the return to East Harbor of the Atlantic horseshoe crab, a critically important species that has been in decline. Around 2010, park staff noticed horseshoe crab molts in the lagoon, but they thought those had washed in through the culvert. The 4-foot-wide culvert was 700 feet long, so it seemed improbable the animals themselves would embark on such a long trek in the dark. Still, park scientists and volunteers set about looking for live crabs across the 720-acre East Harbor. It didn’t take long to find a few intrepid critters that had indeed gone through the culvert, and every year since, there seemed to be more of them. After a nearly 150-year-long absence, horseshoe crabs had made East Harbor theirs again.

Horseshoe crabs are not actual crabs but are more closely related to spiders and scorpions. They’re not exactly pretty, but they’re easy to approach and even handle, said Lawrence Niles, who co-founded the Horseshoe Crab Recovery Coalition. “They look like trouble, but they’re not,” Niles said.

The animals play an outsized ecological role. Their eggs feed a host of shorebirds, including the threatened red knot, which counts on the eggs to fuel its long-distance migration. Crabs, whelks and fish also feast on horseshoe crab eggs, and turtles and sharks prey on the adults.

Horseshoe crabs have survived at least four mass extinctions over the past 445 million years, but they’ve been up against it of late. In the 1950s and ’60s, several Massachusetts towns paid a bounty to fishers for each horseshoe crab they killed because of misplaced concerns that the species threatened the clam harvest. These days, crabs are used as bait by commercial eel and conch fisheries, but Niles said the most serious threat to their existence comes from pharmaceutical companies. An extract from the crabs’ bright-blue blood is used in the detection of toxins in drugs, vaccines and medical devices, and labs draw the blood of countless crabs captured for this purpose. Some of the animals are then released, while others are sold as bait. In 2021, for example, the Atlantic States Marine Fisheries Commission reported that hundreds of thousands of horseshoe crabs died to meet the demand for bait and blood, but Niles said the annual toll could be much higher. The situation is so dire that in February, a group of conservation organizations petitioned the federal government to list horseshoe crabs under the Endangered Species Act.

The tide may be turning, however. In July, experts at the U.S. Pharmacopeia approved the use of a synthetic product to replace the blood extract for toxin testing, and Niles said a couple of major companies have already made the switch. If the demand for blood drops and the population starts rebounding, it will be even more crucial to restore the animal’s habitat, Niles said.

For the past couple of decades, Rachel Thiet, a soil and wetland ecologist at Antioch University in New Hampshire, and her students have worked with park staff to monitor the restoration of East Harbor. Every time Thiet and her team surveyed East Harbor, the assemblage of species was different, with shellfish, fish, invertebrate predators and then mammals such as raccoons and otters coming in. Thiet observed that recovery initially happened in boom-and-bust cycles, where one species’ abundance might spike before falling back down, as was the case with soft-shell clams. “It kind of re-equilibrated over time,” she said. Whether that self-regulation will continue is an open question. The amount of ocean water coming through the culvert remains limited, and the unnaturally shallow lagoon can overheat. That happened one recent summer, with most seagrasses bleaching and dying off. The culvert was widened a tiny bit during a repair in 2022, but increasing the tidal flow through another opening, which is not currently under consideration, would be a much bigger undertaking.

Meanwhile, scientists have continued to study how horseshoe crabs are using East Harbor. Thanks to radio tags, they learned that some crabs leave the lagoon through the culvert. (One female swam against the tidal current for two hours to make it to Cape Cod Bay.) In another sign of recovery, Fox said that East Harbor recently witnessed its first horseshoe crab “spawning aggregation,” an orgy-style mating behavior where several males huddle around a female to fertilize her eggs. Over an area of some 200 square feet, “we saw about 1,000 crabs paired up in chains of four or five,” she said.

Many questions remain, including whether microscopic larvae and tiny juveniles might also be floating out from East Harbor, perhaps helping to reestablish a depleted horseshoe crab population in the bay. Funding for the horseshoe crab study is running out, but one thing it has already shown is that even though horseshoe crabs typically spend much of their non-spawning time in open waters, many are wintering at East Harbor. It takes about nine to 12 years for a horseshoe crab to reach sexual maturity, so it’s quite possible that some of the animals breeding now have spent their entire lives in the lagoon, making it what Fox calls “a true habitat.”

“So maybe that culvert is a little bit of a deterrent to some animals,” she said, “or they’ve decided that it’s safe and cushy there.”