For Atlantic horseshoe crabs, love is a battlefield

Trysts often involve one female and a heap of ardent males, all vying to fertilise her eggs.

Published 28 Apr 2022, 18:17 BST
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On the New Jersey side of Delaware Bay, Atlantic horseshoe crabs mate on the shallow sand coast.

It’s high tide on the Delaware Bay, and Atlantic horseshoe crabs (Limulus polyphemus) are clambering ashore to mate. For this shield-shaped arthropod, assignations are typically a group affair: one female, one male, and a lot of male hangers-on.

The trysts sometimes begin underwater. A male angling for fatherhood uses his boxing glove–like front legs to clasp onto a female’s abdomen and hitch a ride directly behind her through the surf. So situated, he stands ready to contribute sperm the moment she begins laying her eggs on the sand. However, “there’s a lot more competition on the beach,” says Jordan Zimmerman, a horseshoe crab biologist at Delaware’s natural resources agency. For reasons still unknown to science, some females are so appealing that, even if they’re already otherwise engaged, more males seek them out.

With spare studs in a polyamorous heap around her, the female releases her eggs. The attached mate deposits his sperm, and the third wheel—and fourth, fifth, and sixth—“pounces” to deposit his also, Zimmerman says. This waiting-in-the-wings technique can be surprisingly effective: Paternity tests have shown that satellite males sometimes father as many of the female’s brood as the attached male.

Yes, mating is a battle. And after the animals spawn, ravenous shorebirds compete for the clutches of fertilised eggs. Yet after 450 million years, L. polyphemus is still here.

Habitat/region

Atlantic horseshoe crabs are found on the U.S. coastline from New England to Florida and in Mexico. They’re more closely related to scorpions and spiders than to crabs, despite their name, and eat mostly worms and molluscs.

Other facts

The biomedical industry catches about 500,000 U.S. horseshoe crabs annually and takes a portion of their toxin-sensitive blue blood to use in lab safety tests, including for COVID-19 vaccines. Most of them are returned to the water, but an estimated 15 to 30 percent don’t survive.

Despite appearances, these animals don’t sting. Their tails help them navigate and right themselves when overturned. If you see a hapless horseshoe crab on its back, biologist Jordan Zimmerman asks that you flip it over.

This story appears in the June 2022 issue of National Geographic magazine.
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