Spring fling: Prehistoric horseshoe crabs spawn on moonlit beach

Spring fling: Prehistoric horseshoe crabs spawn on moonlit beach


By J. Freedom du Lac, Published: May 11

TED HARVEY CONSERVATION AREA, Del. — The arthropod orgy was well underway when Breanne Preisen trudged over the low dune onto the narrow beach, where tens of thousands of horseshoe crabs were getting down to the age-old business of reproduction.

“There’s a female,” Preisen said, pointing at one of the peculiar sea-things crawling along an undeveloped stretch of the Delaware Bay shoreline. “She has a male attached to her.”
Preisen inspected the cluster of horseshoe crabs more closely, then corrected herself: “Two males!” She smiled.

“It’s basically a mad dash to see who can get their boys to her girls first. That’s why they always come, just to mate. That’s it. I think it’s pretty cool.”

In one of nature’s longest-running — and oddest-looking — spring flings, spawning horseshoe crabs have been coming ashore here every May and June since prehistoric times. The shoreline is the breeding epicenter of a keystone species that’s being monitored closely by environmentalists and scientists after suffering a sharp population decline.

The ancient reproductive ritual is a surreal spectacle, with the bay’s beaches becoming crowded at high tide with harmless creatures that look like flattened Army helmets on top of hairy red spider legs. The long, bayonet-like spikes on the backs of their shells only add to the otherworldly allure of the living fossils, which leave millions of fertilized eggs in the Delaware sand each year.

Of course, there’s more than mere voyeuristic value to the Limulus polyphemus species: Horseshoe crabs and their eggs are food staples for endangered sea turtles as well as migrating shorebirds, and their unique blue blood contains an agent used to test drugs and prosthetic devices for infectious bacteria and to diagnose spinal meningitis.

“Fascinating animals,” said Preisen, 33, who grew up on these beaches and has long been enthralled by the invertebrates, which have been around for hundreds of millions of years. “I love coming out to see them.”

One recent night, Preisen and a group of volunteers stormed the beach at Ted Harvey, not far from the southeastern edge of Dover Air Force Base. Their mission: take a sample count of the horseshoe crabs that were mating just before the rise of the full moon, as the spawning cycle heats up.

When the tide reached its high point — just around 8 — the citizen scientists began heading south, split into two groups.

Eric Sawicki, a seventh-grader from nearby Magnolia, dropped a one-by-one-meter square made of PVC pipe at the waterline, crouched down, touched each of the crabs, then called out a set of numbers: “17 and 3” — 17 males, three females, which are significantly larger than the males.

Sawicki’s mother, Teresah, scribbled the figures on a census form. “That’s a pretty good count,” she said of the numbers, which would eventually be entered into a computer model to help with a broader population estimate.

Eric moved 20 meters down the beach and explained why he was out counting crabs for the third consecutive year: “I love doing anything outdoors,” he shrugged. Horseshoe crabs, he said, are bizarre but amazing.
“I went around the world with the Air Force, and I’d never seen anything like them before I came here,” said his mother.

Eric called out another count: “4 and 0.”
Teresah shook her head. The beam from the flashlight strapped around her forehead zigged back and forth, too.

“They say the zero is just as important,” she said.

The crab counts have been taking place around the Delaware Bay for two decades, since about the time marine biologists, birders and others started noticing serious population shrinkage because of climate change or overharvesting by fishermen, who sold the horseshoe crabs as eel, conch and welk bait. Or maybe both.

The ecological domino effect has hit the endangered loggerhead sea turtle, whose diet includes horseshoe crabs, and migratory shorebirds that make long pit stops along the Delaware Bay — most notably the ­­red knot, which has traditionally come here to gorge on horseshoe crab eggs midway through its arctic migration from the southern tip of South America.

The red knot population has fallen so sharply that some environmentalists have petitioned the federal government to declare the tiny rust-colored birds as endangered.

“Horseshoe crabs are so important because they serve as a really important forage item for a whole host of species, including those migratory shorebirds,” said Stewart Michels, fisheries scientist with the Delaware Division of Fish and Wildlife.

The census information gathered on the bay’s beaches is used to make real fishery-management decisions, including harvesting limits. In Delaware, the annual horseshoe crab quota is 100,000 males, and only after June 8, when the migratory shorebirds have left for the year. No females may be harvested at any time.

The limits could change in a couple of years, Michels said, if the population trends warrant it. For now, he said, the horseshoe crab population in Delaware Bay appears to be stabilizing.

It’s certainly active at the moment: For some reason, Michels said — probably higher-than-usual water temperatures — this year’s spawn seem to be peaking weeks ahead of schedule. He’d been out the previous night for one of the spawning season’s first counts, he said, “and we had tremendous counts.”

Back on the beach at Ted Harvey, Preisen said she was encouraged by the volume of horseshoe crabs clustered along the shoreline.

“But if you look at old pictures of what the beaches used to look like a long time ago, with horseshoe crabs piled high,” she said, “this is kind of sad.”

Preisen, a state-park naturalist whose work does not involve marine biology, is a veteran volunteer counter, having done them several times each spring for more than a decade.

Horseshoe crabs used to be considered a nuisance, with locals grinding them up to be used as nitrogen-rich fertilizer, she said. They are strange and smelly — especially the dead ones that flip over on the beach and get baked by the sun and picked over by the gulls.

Her history with the creatures keeps her coming back, she said. “I have that connection with them from my childhood.”

Nearby, Glenda Cellini was counting crabs with her 9-year-old daughter and 13-year-old niece. They’d come from Cochranville, Pa., near the mushroom farms, because they’d seen horseshoe crabs on one of the nature channels on cable.

So weird!

So of course they just had to see them up close, Cellini said: “They’re really amazing.”

Her daughter, Mia, shrieked as she tried to turn over a male crab that had flipped onto its back. Once upright, it scurried into the surf.

A C-17 buzzed through the darkened sky, to Dover. Jokes flew, too.

“It smells like Ron Jeremy,” somebody observed, referring to the old porn star.

“It’s external reproduction,” said Kelly Valencik, the Delaware National Estuarine Research Reserve coordinator who was leading the census.

Not really sex on the beach?

“It’s not what you’d think,” she said. She blushed.

The females burrow in the sand and lay the eggs.

The males, who attach themselves to females or otherwise follow them around, do the fertilizing.

Repeat. A lot.

A female lays 88,000 eggs in a season, or maybe more.

And then what?

“They’re done,” Breanne Preisen said. “That’s it.”

She laughed. “There’s no afterparty.”