Tuesday offers last chance to see this summer’s horseshoe crab migration in Ocean City

Hundreds of round, mud-caked carcasses bobbed lifelessly between the boat slips in a townhome-lined canal in north Ocean City, smelling like a fish market during a heat wave.

These dead Atlantic horseshoe crabs followed ancient instincts last week to spawn on a sandy shoreline — a reproductive ritual that predates the dinosaurs. But instead these armored arthropods found a dead-end canal at 94th Street, with nowhere to go. Although scientists don’t know why they died, the crabs couldn’t surmount the bulkhead overnight and baked in the next day’s brutal heat.

The die-off is one sign of the species’ annual spawning migration, now on display at the highest tides in Ocean City. Thousands teem across beaches to lay eggs above the high-tide line during full and new moons around the summer solstice. It can be a spectacle, and your last best chance to see it will be around the new moon in Tuesday’s early hours.

But the die-off also highlights one of the growing threats to the prehistoric-looking population, which scientists have found to be in decline.

After 450 million years on Earth, the amount of horseshoe crabs has been diminishing in recent years — and nobody’s sure why, said Eric May, a longtime professor of fisheries at the University of Maryland Eastern Shore.

“People are concerned about the fact there is an issue with the species,” he said.

It’s gotten bad enough that the Center for Biological Diversity, an environmental nonprofit organization, is suing the Trump administration for denying endangered species protections to the Atlantic horseshoe crab.

Theories abound, from climate change to disease. But a more concrete reason is habitat loss caused by urbanization of bay waters.

“We’re looking at riprap, hard shoreline development, rather than natural shoreline development,” May said.

Another theory says it’s the harvest for biomedical purposes.

A horseshoe crab’s famously blue-tinted blood is harvested and used to make a compound that can ascertain with uncanny accuracy whether vaccines, drugs and devices have been contaminated with dangerous bacteria.

That’s transformed them into a commodity. Harvesters capture live horseshoe crabs and drain nearly a third of their blood, then release them. A gallon of their blood is worth about $60,000.

“It puts them under significant stress,” May said. “We return them back to the water with the expectation that most of them will survive, but there are suggestions that there is up to a 30% mortality.”

It all focuses on the horseshoe crab’s annual spawning migration, for which Ocean City and the nearby Delaware Bay are a global hot spot, said Steve Doctor, a fisheries biologist with the Maryland Department of Natural Resources.

Adult crabs live in the deeper waters of the Atlantic’s continental shelf, but from late May through early July the crabs’ migrate to the Eastern Shore’s coastal bays to make more of themselves. Their spawning is linked precisely to moon phases, tides and water temperature.

“I like how they have an alignment to the phases of the moon. It’s demonstratable,” Doctor said. “The moon powers a lot of things, but with the horseshoe crabs you can actually measure it.”

Crabs spawn on the full and new moons because that’s when the tide is highest. The male crabs show up first and wait for females.

Clambering over each other in the bay shallows, the males latch on to the female’s hard outer shell, or carapace. Then the ladies drag their mates up past the high-tide line to dig a shallow nest. After the female deposits her eggs, the attached male fertilizes them.

Scientists say females can lay as many as 20,000 eggs, pale green and the size of a poppy seed, at once.

“For the next two weeks after that,” Doctor said, “the tide goes down. It’s really warm, so it’ll incubate those eggs. They’re cold-blooded, so they’re completely dependent on temperature. When the tide’s high again, they’re able to hatch and go on their way.”

A newborn crab under magnification is identical to an adult, Doctor said — just smaller.

“At 3 millimeters,” he said, “they look like a horseshoe crab.”

There are a few misconceptions about the species, starting with the name. They’re not crabs at all but are more closely related to spiders.

Their spiky tail, or telson, is not a stinger. Instead, it serves as a rudder that guides them while swimming. The tail also helps crabs to right themselves when flipped on their armored backs.

Overturned crabs that can’t get back on their feet — they have six pairs of them — likely bake in the sun and end up as a snack for birds that have learned to time their migratory flights to coincide with the crabs’ mating period.

There are some signs the crabs may be rebounding. According to one 2023 study, horseshoe crab populations have steadily rebuilt in the Delaware Bay since those harvest restrictions, returning to near-1990 levels.

Because horseshoe crabs are considered an interstate resource, they’re managed cooperatively by the Atlantic States Marine Fisheries Commission.

Although prized for their blood, horseshoes aren’t good for eating. What little meat they offer is tough and rubbery, and it can make people sick if not prepared properly.

There are signs the crabs may be rebounding. One 2023 study found that the Delaware Bay’s horseshoe crab populations have rebuilt to near-1990 levels since harvest restrictions were imposed.

Part of Maryland’s crab management plan involves a population survey. The state Department of Natural Resources took a team to Ocean City at peak spawn the night of June 30, when Doctor said as many as 10,000 crabs were spotted.

The group visited Ocean City’s Inlet Beach, the north shore of Assateague Island and Skimmer Island, a spit of sand flanking the resort’s Route 50 bridge, said Joe Zimmerman, a science writer and media liaison with the agency. (Hint: These may be good places to see them Tuesday.)

He said he witnessed the beach blanketed with crabs, crawling over and around his feet in the darkness as he stepped through the sand. At the shore break, the lights from downtown Ocean City lit the night sky.

“It was a pretty powerful experience,” Zimmerman said. “I think the sheer magnitude and number of these horseshoe crabs is pretty spectacular.

“They’ve been doing this longer than we’ve been here. Longer than animals have been here. Longer than the continents,” he added.

Despite the human pressures on the population, May, the UMES scientist, thinks the spectacle of the horseshoe migration will continue for eons to come. After all, as one of the oldest species on the planet, they’ve survived ice ages, prehistoric global warming and more.

“They are a remarkable species. There’s more to the story than we know. There’s a lot left to be done to fully understand these crabs, how they operate. They have outlived everything.

“I think,” he said, “they will outlive human activity.”

5 spots to see horseshoe crabs

Here are five spots in Ocean City where you might see horseshoe crabs during Tuesday’s new moon.

• Homer Gudelsky Park off Old Bridge Road in West Ocean City (an area locals refer to as “Stinky Beach”)
• Skimmer Island as viewed from the Route 50 bridge
• Sunset Park, downtown Ocean City off Philadelphia Avenue
• Old Landing Road, near 100th Street bayside
• Northside Park, at 125th Street, along the bayside pier