U.S. Fish & Wildlife Service
The Horseshoe Crab
Limulus polyphemus
A Living Fossil
A Primitive Ancient Creature
Horseshoe crabs are evolutionary
survivors that have remained relatively
unchanged in appearance for 350 million
years. The horseshoe crab is not actually
a true crab, but a member of an ancient
group of arthropods, closely related to
spiders and scorpions. There are four
species of horseshoe crabs around the
world and only one in North America.
The species in North America is the
most abundant in the world and ranges
on the Atlantic coast from Maine to the
Yucatan Peninsula.
Amazing Spectacle
In the late spring and early summer,
horseshoe crabs arrive on the beaches en
masse to lay their eggs. The peak of
spawning on the Atlantic coast occurs in
Delaware Bay where thousands of crabs
will arrive on the sandy beaches in May
and June. Delaware Bay provides an
excellent spawning area for crabs
because the sandy beaches are protected
from harsh wave action. The beaches’
sand and pebble mixture is perfect for
incubating horseshoe crab eggs.
Crabs arrive on the spawning beaches
during the high tides of full and new
moons when the water rises highest on
the beach.
When the female is ready to lay her
eggs, she crawls up to the high water
line on the beach with a male attached to
her. The male clasps onto the female’s
shell with a modified pair of claws. The
female drags him around during the
spawning process. In addition to the
attached male, several other males may
also attempt to fertilize the female’s eggs
by arranging themselves on and around
the spawning couple during the egg-
laying process. A female may have five or
more males attempting to mate with her
in a single egg-laying episode.
During spawning, the female crab
partially buries herself in the sand while
she deposits a cluster of about 4,000 tiny
green eggs. In an evening of egg laying,
a female crab can lay several egg
clusters, and she may spawn repeatedly
over several nights to lay 100,000 or
more eggs.
New Life Beginning
The eggs will