BROOKSVILLE — They’re not as cute as puffins nor as graceful as osprey, but for the people of the Blue Hill peninsula, the annual return of the ancient horseshoe crab – like these other wildlife species elsewhere – marks the arrival of spring. The horseshoe crabs come back to the coast from deeper waters; it’s one of the few places in the state where they breed.

”They’re so weird and prehistoric. There are very few things today like that. (The horseshoe crabs) make everyone feel more connected to the water around us,” said Chrissy Beardsley Allen, development director at Blue Hill Heritage Trust.

Horseshoe crabs have existed for 450 million years – a good 200 million years before the dinosaurs. They are not plentiful in Maine because the state is at the northern end of the species’ range in the Atlantic Ocean. The exact size of Maine’s horseshoe crab population is unknown, but some biologists worry that they face a grave threat: loss of the beach habitat where they breed due to rising sea levels and shoreline development.

For the last three years, the Blue Hill Heritage Trust has lead a horseshoe crab walk during the breeding season to raise awareness of the species and of the need for conservation along Maine’s coast. This year’s walk is scheduled for Saturday.

Chrissy Allen of Blue Hill Heritage Trust holds a partial shell of a horseshoe crab. The organization leads an annual walk to highlight threats to the species. Shawn Ouellette/Staff Photographer

Despite its name, the horseshoe crab is not a crab. It is an arthropod, more closely related to spiders, with 10 legs and a hard outer shell, or exoskeleton. It has nine eyes, several light receptors and a long, pointy tail, which it uses to flip itself over when it ends up on its back (not to sting or attack, a common misconception). Females can grow to 19 inches, males just 15 inches. Both can live up to 20 years, dining on crustaceans, small clams, worms and algae.

During the winter, horseshoe crabs migrate to continental shelves in the deeper ocean, then they come inshore to breed in the spring. The females make nests on sandy areas or beaches to lay eggs, which the males fertilize by releasing sperm. Over several nights, a female may lay as many as 100,000 eggs (but few survive; most are eaten by other wildlife).

Win Watson, a professor of marine science at the University of New Hampshire who studies the species, said that in Maine and New Hampshire the crabs spawn when the water warms up to 50 to 60 degrees, generally the last two weeks of May. They require high tidal water with a certain salinity, which is why they breed only in particular bays and rivers. In states where they are abundant – such as Delaware and Florida – hundreds of thousands of the horseshoe crabs gather onshore to breed, piling on top of each other.

On the Blue Hill peninsula, the horseshoe crabs breed in the brackish section of the Bagaduce River. The oldest fossils thought to be in the same genus in the area date back 148 million years, said Sarah O’Malley, an instructor of biology and ocean science at Maine Maritime Academy and a member of the Heritage Trust board. They’ve been spawning here ever since ice sheets retreated along the Maine coast after the last ice age, she added.

“The genus we have goes back hundreds of millions of years,” O’Malley said. “It’s pre-T-Rex for sure.”

Atlantic horseshoe crabs in the mood for love congregate on the shore in Delaware. Photo courtesy of Encyclopedia of Life

THREAT ASSESSMENT

These crabs belong to the Gulf of Maine horseshoe crab population, which is genetically different from the three horseshoe crab populations to the south, O’Malley said. The four breeding grounds in Maine – Casco Bay, Damariscotta, the Bagaduce River and Taunton Bay – collectively are dwarfed by populations farther down the East Coast, especially those in Delaware Bay, Chesapeake Bay and the Gulf of Mexico.

For years, fishermen in Maine used the horseshoe crabs for bait. Each year, they’d collect every last crab from some of the state’s four breeding areas, often leaving none to beget the next generation. In 2004, the state tightened its fishing regulations, closing the horseshoe crab season from May 1 to Oct. 30, a period that includes the mid-May to mid-June breeding season.

“At that point, the harvest of horseshoe crabs virtually ended,” said biologist Pete Thayer, of the Maine Department of Inland Fisheries and Wildlife.

In the years following the new regulations, the state’s horseshoe crab populations rebounded, so in 2011, the department stopped counting the arthropods. Given the very minimal harvest now allowed, the surveys were no longer needed, he said.

But other marine agencies are concerned that the populations here are small, and could one day vanish. According to a report in 2016 in the scholarly Fish Biology and Fisheries journal, “exploitation (for bait) and habitat loss threaten the species, and the risk of extirpation (local as opposed to global extinction) is greatest in the Gulf of Maine region…”

The Atlantic States Marine Fisheries Commission stock assessment in 2013 reported that the population was “decreasing in abundance” in the New York and New England regions. The most recent stock assessment, which was released on May 1, reported that in the Northeast region the populations are now in “neutral condition,” or stable, but in New York they are considered in “poor condition.”

Watson said the population in New Hampshire appears stable.

But O’Malley said rising sea levels resulting from climate change are a clear threat to horseshoe crabs because they mean the loss of beach habitat, a threat exacerbated by development. Sandy beaches that the horseshoe crabs need to breed won’t just migrate inland, they’ll vanish.

Such threats are what make the stretch of Bagaduce River protected by the Blue Hill Heritage Trust so special, O’Malley continued.

“There’s not a lot of gene flow between the four populations along the East Coast,” O’Malley said, “so if you want to protect diversity, you have to protect the population in Maine.”

SPECTATOR SPORT

O’Malley, left, and Allen walk along the Bagaduce River in the Snow Natural Area in Brooksville. Horseshoe crabs have bred for millennium in the brackish waters here. Shawn Ouellette/Staff Photographer

Along the Bagaduce, the crabs spawn beside the 40-acre Snow Natural Area that has 2,550 feet of river frontage under conservation, making the breeding season a true spectator event. The land trust has been holding the horseshoe crab walk ever since it acquired the Snow property in 2014, when it merged with The Conservation Trust of Brooksville, Penobscot and Castine.

Locals have been coming to the Bagaduce for generations to see the crabs breed in springtime.

There’s an inlet where the popular Bagaduce Lunch stand sits on Route 175 where you can get take out, sit on the lawn or at a picnic table, and watch the crabs breeding on the beach below. The seasonal eatery has been in the same family for more than 50 years, and Allen said horseshoe crab watching has been a summer activity here for at least as long.

“I can remember going down the boat landing as a kid and watching them,” Allen said.

“We get 30 to 40 people of all ages who come to the (Blue Hill Heritage Trust horseshoe crab) program, both kids and adults.” she added. “A lot of adults don’t know they’re here, or that they’re prehistoric. For us, it marks the opening of summer when the horseshoe crabs arrive.”