For Atlantic horseshoe crabs, love is a battlefield

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

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.

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