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Horseshoe crabs’ hard-shell truth

Horseshoe crabs have been around for 450 million

Horseshoe crabs have been around for 450 million years. Credit: Newsday / Michael Dobie

She’s right at the water’s edge, a dark lump moving slowly over the light-colored rocks.

John Tanacredi has been scouting around Mount Sinai Harbor for more than an hour on a recent Saturday morning, and this is only the second horseshoe crab he’s seen. That’s not great, but it’s not atypical. You can go to a known breeding spot one day and find nothing, he says, come back a day later and see a thousand.

“As much as we know about horseshoe crabs,” Tanacredi says, “is as much as we don’t know.”

It’s an interesting juxtaposition, this sense of mystery about a creature that’s been here nearly forever.

Here’s another: The horseshoe crab is the natural world’s Rodney Dangerfield. It gets no respect, a judgment so off the mark it’s laughable.

Tanacredi gets fired up about that. He’s executive director of Molloy College’s Center for Environmental Research and Coastal Oceans Monitoring. Horseshoe crabs are a passion for him and other scientists alarmed at their rapid depletion in other parts of the world. They’re working hard to make sure that doesn’t happen to the North American species.

When you study up a little, you begin to understand what we’d be missing if they disappeared.

Horseshoe crabs have been around for 450 million years, which means they had notched 200 million years before they had to start learning how to dodge the feet of dinosaurs. They’ve survived five mass extinction events, and the way we humans are treating our home they might have an opportunity to go for six.

Their eggs — a female lays up to 120,000 tiny green ones per season, thousands at a time in each divot she digs on the shoreline — are a source of food for a variety of birds. Like red knots, who feast on their incredible migration from the southern tip of South America to their breeding grounds in the arctic. The decline in red knots and other species has been linked to the decline in horseshoe crabs.

Perhaps most important, the blue blood of horseshoe crabs has a chemical that detects even tiny traces of bacteria. The federal government requires that the blood be used to test medical equipment and drugs given intravenously. It’s literally a lifesaver.

“Put it all together, these animals are perfect,” Tanacredi says, twinkling. “They’re cool.”

They’re also stressed. Horseshoe crabs return to the same breeding sites year after year, but that fidelity is not being rewarded. Habitat destruction in Japan and other parts of Asia has caused big population declines. And crabs are now an exotic food. In Singapore alone, Tanacredi says, 10,000 adults per day during breeding season are harvested and bled out, and their meat shipped to Vietnam, Thailand and elsewhere.

Here, problems include coastal development, bulkheads, boats, and petroleum and sewage discharge. And New York State allows 150,000 crabs yearly to be harvested commercially, mostly as bait for conch and eel.

Tanacredi’s team monitors 111 sites around Long Island. State officials do their own counts. Tanacredi says he’s seen a 1 percent decline per year in crabs, but an 8 percent drop in suitable habitat. There are variables in what’s suitable to a female crab — the slope of the beach, the size of the grains of sand, wind conditions, the presence of people. Only when it’s all right do they lay eggs — in the spring, at high tide, under a full moon.

As Tanacredi talks, another crab moves parallel to the shore, 15 feet away. It zips along, aided by the incoming tide, a dark oval skimming through the light green water.

Tanacredi takes in the whole of Mount Sinai Harbor, from the slopes covered with slipper shells to the fishermen and the boats and the jetty. And the crab.

“It’s not the ideal beach,” he shrugs, “but they’re here.”

For how much longer?

Michael Dobie is a member of Newsday’s editorial board.