Gerritsen Beach

A prehistoric resident survives in Brooklyn

June 16, 2015 By Benjamin Preston Special to the Brooklyn Daily Eagle
Alan Ascher, a naturalist who has been studying horseshoe crabs for decades, explains different parts of the ancient arthropod to the tour. Eagle photos by Benjamin Preston
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Early each summer, the sands of Plumb Beach, in South Brooklyn, become scattered with lovers. At high tide, you can find them clutching one another in passionate embrace where the gentle waves of Jamaica Bay lap the sand. It may sound like the beach makeout scene in “From Here to Eternity,” but you won’t see anyone who looks even remotely like Burt Lancaster and Deborah Kerr rolling around on the sand. What you will see are horseshoe crabs.

A group of nature lovers assembled at Plumb Beach Sunday evening for a guided tour of the action. With darkness falling and high tide on the way, it wasn’t long before dozens of the dome-shelled, spike-tailed arthropods could be seen dotting the white sand.

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“Plumb Beach is a good place for it because it’s secluded and there’s not much wave action,” Alan Ascher, a naturalist who has been studying horseshoe crabs for decades, said during the tour. He recalled that growing up in the middle of Brooklyn, away from the coastline, he wasn’t aware of them until a high school zoology teacher took his class on a trip to see horseshoe crabs on Gerritsen Beach. “We saw hundreds of them.”

Ascher, who led the tour for NYC H2O, explained that by July, the number of horseshoe crabs laying eggs on the beach would decline until next year’s mating season.

“This is the opportune time,” he said, pointing out a group of researchers tagging and observing some of the crabs. The researchers were conducting a survey of the horseshoe crabs, which Ascher said are a threatened species.

The tour, which began at the Plumb Beach rest stop on the Belt Parkway, included a walk along the marsh just over the dunes from the beach. You may have driven past it hundreds of times without noticing it, but another world exists in that nook. At high tide, the marsh teems with egrets and fish and is an odd sliver of stillness sandwiched between windy Jamaica Bay and the roar of parkway traffic.

“Hey, look, a crane!” one man exclaimed as Ascher led the group through marsh grass and Virginia Creeper past a grove of beach plum and Russian olive trees. A woman standing nearby informed him that it wasn’t a crane, but an egret. A slender-necked white bird tiptoed through the water, occasionally plunging its head into the water to snatch a small fish from the grass.

“Not that one, the one behind it,” he said, grinning, as he pointed at a huge construction crane towering over a development on the other side of the parkway. “Got ya!”

Ascher built suspense as the group snaked its way around the back of the dunes on the way to the Gerritsen Inlet side of Plumb Beach, meting out details about the crabs one parcel at a time.

“They secrete pheromones — it’s sort of like Eau de Horseshoe Crab,” he said when someone asked him how they knew it was time to mate.

The group learned that the horseshoe crab, as a species, is about 450 million years old — older than most other species on earth — and that its blood doesn’t carry iron like ours does. A horseshoe crab blood extract, Ascher explained, is a valuable commodity for healthcare providers, who use it to test patients for bacterial infections. He added that researchers had recently devised a way to replicate the part of the blood used in medicine, thereby reducing the need to bleed horseshoe crabs.

“They don’t kill the crab to take the blood, but there’s about a 10 percent die-off rate from the process,” he said.

Ascher also said, among other things, that horseshoe crabs swim on their backs, that they use their tails as rudders and to flip themselves over and that Delaware is the epicenter of the horseshoe crab population in the United States. There are also, he said, three other species of the crab, which live primarily in Southeast Asia. Uncle Sam’s horseshoe crabs can be found along the Atlantic Coast from Southern Maine to Florida.

As twilight fell, the group emerged from the marsh onto the beach again to see the main attraction. High tide had just peaked. With the new moon only a few days away, the tide swings were more extreme, which Ascher said made for ideal egg-laying conditions for the horseshoe crabs. Most were in pairs, but a few were grouped in larger clusters.

Ascher said that to mate, the female horseshoe crab uses a special set of digging legs to make a hole in the sand just below the high tide mark. The male crab, which is almost always smaller, climbs onto the back of the female’s shell, waiting for her to lay her eggs. As the eggs come out, the male fertilizes them.

According to, it takes a few weeks for the horseshoe crab larvae to emerge. During that time, shorebirds have a feeding frenzy on the eggs, in particular a bird called the red knot. Ascher said red knots like horseshoe crab eggs enough to fly up from South America to get them. But that species, like the horseshoe crab, is also threatened. As the PBS film “Crash: A Tale of Two Species” points out, the two are interdependent.

NYC H2O’s horseshoe crab mating tour is part of a series of tours and lectures the nonprofit runs to help New Yorkers learn more about the city’s water system and natural resources.  In addition to reservoir paddles and bicycle tours, the organization focuses on coastal issues as well. Its next public event will be a beach cleanup in Canarsie, on Saturday, July 11.

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