Horseshoe crabs drawn to gentle tides of Great South Bay

Fire Island Horseshoe crabs mating cluster on beach

Delaware Bay is known as the epicenter of horseshoe crabs on the east coast, but biologists are just beginning to realize how popular the the south shore of Long Island is with the primitive arthropods, says Kim McKown, the New York State Department of Conservation Crustacean Leader.

But no one is sure exactly how many are there. Vaguely 6 million horseshoe crabs (Limulus polyphemus) live, or at least mate, off the shores of NJ and DE. But nobody has a good count on the New York region–or has a handle on whether global warming might be pushing them north.

In general the numbers on either side of Long Island were at lows in the 80s, recovered in the 90s and then plateaued or declined. CT’s trawling survey shows a steady population, but in NY, “last year was the lowest values we’ve ever seen,” she says.

“We’re still in the development phase of our program, now up to 13 sites,” McKown says. They know that some beaches have really high counts–but that an individual beach’s popularity with the crabs varies wildly from year to year. The larvae are so teeny they can’t possibly be tagged, so no one is sure if they are drawn to their birth beach. But tagged adults are found around the same beaches repeatedly.

Biologists are still figuring out what makes a beach attractive to horseshoe crabs. Horseshoe crabs lay eggs in the biggest crowds around the nighttime high tides of the full and new moons in May and June. If the crabs were rating beaches on YELP, they would probably have star categories for the force of waves, the quality of the sand, human disturbance. And poaching–an ongoing problem because fishermen use the crabs as bait–would be a huge turn-off. Then there’s the whole issue of when to lay your eggs. Horseshoe crabs want low light and a high tide in the middle of the night. If the waves are too high or it’s still light out for the high tide, forget it.

That said, the horseshoe crabs on the north shore of Long Island break all the rules. The shore has no barrier island, so the water is much rougher. That throws off all the etiquette of egg-laying. These arthropods spread out their mating, so you may only see a few hundred at once. They’ll do it in the daytime and at more gentle tides when they can lay eggs in sand instead of the rocks that the big tides expose. They also do more laying submerged in water, so you may not see them.

If you’re looking for horseshoe crabs in the area, McKown has some suggestions. “It is really spectacular to see so many crabs coming up to the shore,” she says. Although the real peak season just passed, you can still learn about crabs and how to submit data at nyhorseshoecrab.org.

  • Plum Beach on Jamaica Bay — The highest consistent site in the area, with thousands of crabs possible.
  • Pikes Beach, West Hampton Dunes — Several hundred to 1,000 crabs
  • West Meadow on the North Shore of Long Island — Probably the most active site on the north shore. Some daytime spawning.
horseshoecrab Where to SEE HORSESHOE CRABS
NY, NJ, MD, MA, ME, NH, VT, CT, RI, PA SEE ANIMALS IN THE NORTHEAST (NY, NJ, MD, MA, ME, NH, VT, CT, RI, PA)

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