Time for Horseshoe Crabs and the Shorebirds That Love Them

By DAVE TAFT

Humans may be quick to judge, but only one horseshoe crab can ultimately gauge the loveliness of another. When these crabs appear on a beach — suddenly and in large numbers — they can seem fearsome and even malevolent, and that’s before you glimpse beneath their intimidating, pointy-tailed carapace at a mouth set in the center of 12 madly churning legs. Beauty, in this case, is in the compound eyes of the beholder: Horseshoe crabs each have a total of 10 eyes, through which they obviously find one another appealing. These trilobite relatives have been attracting mates successfully for well over 400 million years.

In and around New York City, the crab (Limulus polyphemus) returns to shallow bays in spring, after a winter spent in deeper water. At the full and new moons of May and June, females migrate toward shorelines and release powerful pheromones. As each makes her way to shore, dozens of males jockey for position, but only one latches directly onto her back, using specifically modified front claws that efficiently hook onto the rear half of the shell. These claws, which resemble boxing gloves, are a surefire identifier of a crab’s sex. (Size is another: Adult males are far smaller than females.)

Though only one male is on the female’s back, a half-dozen or more in thrall to her pheromones swarm nearby. As the female and her mate partially bury themselves in the sand to lay eggs, other males surround them, in hopes that their sperm might also fertilize some of the eggs.

Following the extreme high tides allows the crabs to lay eggs where aquatic predators will not go: the damp sand covered by water only once or twice a month. Still, some eggs wash out of nests, becoming valuable forage for an array of animals, and the basis of a critical and far-reaching food chain. Teeming masses of shorebirds fuel their northward migration along the Atlantic Flyway by feeding on crab eggs. Timing is critical, as is the abundance of eggs. A poor year for crab reproduction can mean a poor year for birds in places thousands of miles to the north, with unfortunate ecological implications. It is possible that climate change has already begun to disrupt this delicate system. Overharvesting may also be playing a role in declining crab numbers.

While an adult female may lay up to 80,000 eggs in a season, few of the resulting hatchlings will live to adulthood.

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