BASIC INSTINCTS (^) | EXPLORE
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 fertilized eggs. Yet after
450 million years, L. polyphemus is still here. —DINA FINE MARON
WHEN SHE ATTRACTS HIM,
AND HIM, AND HIM, AND ...
HABITAT/REGION
Atlantic horseshoe crabs
are found on the U.S. coast-
line 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 mollusks.
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 navi-
gate and right themselves
when overturned. If you
see a hapless horseshoe
crab on its back, biologist
Jordan Zimmerman asks
that you flip it over.
On the New Jersey side
of Delaware Bay, Atlantic
horseshoe crabs mate on
the shallow sand coast.
PHOTO: HELMUT CORNELI, ALAMY STOCK PHOTO
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