MIDDLE TOWNSHIP — The incoming tide gently laps at the shore at Kimbles Beach as moonlight glistens off the Delaware Bay.
The night is quiet but for the eerie, muted sounds of clacking, like hollow rocks tumbling underwater. The noise comes from hundreds of horseshoe crabs bumping together for their annual spawn on Delaware Bay beaches this time of year.
“It’s beautiful out here tonight. Conditions should be just right,” said John Back, a volunteer for the U.S. Fish and Wildlife Service.
Volunteers like Back, wearing hip boots and carrying flashlights, help the Fish and Wildlife Service take surveys of horseshoe crab populations during the height of the spawn.
The work is critical. Horseshoe crab eggs are a vital food source for migrating shore birds such as the federally threatened red knots and are vital to the Delaware Bay’s ecology. Counts like this one are used to compile data on decline or growth in horseshoe crab populations.
The Atlantic horseshoe crab can be found all along the Eastern Seaboard of the U.S., but the heaviest concentrations are in the Delaware Bay, according to Jack Szczepanski, biologist and project manager for the U.S. Fish and Wildlife Service.
Horseshoe crabs have a lineage that dates back more than 400 million years, pre-dating the dinosaurs, Szczepanski said.
Every May and June, the Delaware Bay becomes the largest horseshoe crab spawning ground in the world.
“They are quite remarkable creatures,” said Szczepanski, as he and his volunteers worked the half-mile of shore at Kimbles Beach, counting the number of crabs coming ashore to spawn.
During the spawn, the larger female crabs, with one or more male crabs in tow, dig into the sand at high tide and lay 80,000 to 100,000 eggs in a clutch, where the male will fertilize them.
Horseshoe crabs are not actually crabs, but are more closely related to spiders and scorpions and are gentle by nature, according to Szczepanski.
They don’t have pincers, and their intimidating-looking tails — called telsons — are not dangerous, poisonous or used to sting. Instead, they act as rudders, and they help the horseshoe crabs right themselves when they flip over on their backs.