On a tranquil Saturday evening in late May, a dozen volunteers formed a semicircle around a Toyota Prius in the Plumb Beach parking lot just off the Belt Parkway, awaiting their instructions.

They were there to count horseshoe crabs. Just over the dunes that abutted the parking lot, thousands of arthropods were emerging from the brackish waves of Jamaica Bay to mount each other in the twilight in New York's longest continuously running orgy.

"There's something about the horseshoe crab that is so different and interesting," Ann Seligman, a New York City Audubon Society site coordinator, said. "It's a really spectacular sight when you see a bunch of horseshoe crabs along a beach line and think about their history and their place within their planet."

For the past 450 million years, long before Robert Moses made plans to carve a circumferential parkway along the city's coastline, before revelers first climbed into the Coney Island Cyclone's cab and plunged down its wood-and-steel track, and before Roll-N-Roaster served its first thinly sliced roast beef and "cheez" sandwich, this swath of tidal beach was the domain of gentle sea scorpions.

Somehow, the Atlantic horseshoe crabs know to return to the same estuaries each year in late spring to procreate. They even time their amorous activities with the evening's high tides whenever a full or new moon arises. That made Memorial Day weekend one of the best opportunities to observe mounds of male crabs piling on top of sand-encrusted females as the surf washes over them, like Burt Lancaster and Deborah Kerr in "From Here to Eternity."

The horseshoe crabs were already getting busy when the crowd of volunteers trudged onto the beach. The group consisted of a mix of environmentally conscious young parents, who brought their kids for an educational evening during their holiday break; millennials; and older couples. They wore windbreakers, rubber boots, or Tevas, and hats with headlights draped around them so they could spot critters well past nightfall. Some carried thin PVC pipes arranged in a square shape, which would be used to estimate the crabs' population. Others hauled bags with tarps, calipers, and cordless drills. (The Audubon Society requested volunteers watch a training video before joining one of the monitoring sessions, which run from May 14 to the end of June.)

Gina Duclayan, a fourth grade teacher at PS 38 in Brooklyn, has been visiting the shore for 15 years with her kids but this was their first year tracking the crabs' population.

"It's so important to be aware of the real nature that exists in the city and be in touch with it. That helps kids and adults care about their urban environment," Duclayan said. "We have so much wildlife in our city that people aren't aware of."

Scores of horseshoe pairs, triads, and quartets glistened in the twilight. Two volunteers walked the length of the beach holding a string while counting their steps, consistent counting measures that help them estimate at how many crabs are on the entire beach. Then they placed a plastic square, called a quadrat, in front of them and recorded how many crabs fit inside before adding up the tally.

Once the count was done, it was time to measure and tag the critters. The sex of each crab is pretty easy to identify. Females are larger than males and their first pair of appendages are pincers, while males' front legs look like boxing gloves, which help them hook up to a female during mating. Males will clasp a female in the surf, with the female then dragging her primary suitor onto the beach before digging a hole in the wet sand to lay thousands of peppercorn-sized, chartreuse-colored eggs. The male fertilizes the clumps of eggs with his sperm—all while other males surround the female and secrete their sperm, too, in an attempt to join the action. Two to four weeks later, the eggs will hatch, and baby crabs will crawl into the ocean, if they're not eaten by nearby, hungry shorebirds first.  

"Males will climb onto almost anything—boots, boats, whatever's in the water, you'll see them on it," Seligman said. "They take advantage of whatever opportunity they can get. Males that are around the edges also have pretty decent reproductive success."

Before the group split off in opposite directions along the seashore, Seligman warned the volunteers not to disturb any females submerged in the silt or pick up any crabs by their spindly tail, known as a telson, which helps them right themselves in the surf. Copulating couples that were not firmly planted in the sand, as well as nearby males or females looking for mates, were fair game for observation.

On a section of the beach past a small rock jetty, a dark-shelled straggler lurked outside a party of five jaunty crabs scrambling over each other in a frenzy. Seligman snatched him up, exclaiming he was "very old"; his shell was a little worn but he was otherwise in good health. (Crabs can live to their early 40s in captivity, but their exact ages are difficult to determine.)

One volunteer recorded the old creature's sex and how much of its surface area was covered with barnacles, while another measured the width of his shell with a caliper. Then the volunteer's child reluctantly took the cordless drill and punctured the left side of its shell. A trickle of milky-blue blood the color of a Dairy Queen Misty Freeze slushy dribbled down the tarp. The child snapped a white numbered plastic tag in the hole and dropped the crab on the edge of the surf.

Once all the data is compiled, Seligman sends the records to the Audubon Society, which shares them with the U.S. Fish and Wildlife Service to determine the stability of the population.  

This reporter felt a wave of sympathy for these ancient creatures, especially the aged loners looking for love on a romantic Brooklyn evening, only to be lifted onto a tarp by a family from Park Slope, flashed by several headlamps, and have a hole drilled into their shell.

The group of volunteers moved further east along the beach and picked up a pair of crabs. The male was clasping a cherrystone clam while he mounted the female. (The volunteers nicknamed him George Costanza.)

More crabs sauntered onto the shore looking for love as night fell. A strong wave washed over one pair, temporarily inverting them before a volunteer flipped them over and left them to engage in sandy intimacy.

Lee Bronsnick, whose family came from the Lower East Side to see the horseshoe crabs, marveled at how hands-on the monitoring session was. Both his children repeatedly handled and tagged animals as his wife filled in data.

"We had a great time. It was really fun to be outside at night in a natural area so close to where we live in the city," Bronsnick said. "It was sort of weird to be off the Belt Parkway but feel so remote."