Helmets on the Beach
Article and photographs by Pamela Cohen
The horseshoe crab, Limulus polyphemus, a
“living fossil,” miraculously exists as a marvel of evolution
unmodified for over 300 million years. Despite the name,
this unique creature is not a crab, but a marine arthropod
related to arachnids. If you are an amateur rock collector,
perhaps you have a trilobite fossil. Trilobites are relatives
and similar in appearance to horseshoe crabs. A casual
walk on the coast, anywhere from Maine to the Yucatan
Peninsula, yields sightings, seen as hard brown shells
strewn upon the sand. It is a creature that intrigues curious
children, prompting them to ask, “What is that thing? It
looks like a helmet with a tail!” Often mistaken for a dead
specimen is the remnant of a molt.
Do not be apprehensive about being near a live
horseshoe crab on the beach. There is nothing to fear, since
it has no teeth, it cannot bite, nor does it sting. The sharp
pointed tail is not used for defense but as an implement
to aid in turning itself over and for steering. The leathery
looking shell, or carapace, is horseshoe shaped and may
have protrusions which protect young from predators. The
female presents 30% larger than the male. If you carefully
pick up a specimen on Kiawah beach, it can be quite a
learning experience. Feel honored that you are holding
a creature that existed before dinosaurs and
man. There are 10 eyes,
two lateral, located
on the top of
7
the shell, the remaining positioned dorsally and ventrally,
including a photoreceptor on the tail. Although appearing
primitive, its eyes are engineered to function as “high tech”
photoreceptors. A nocturnal chemical response triggers
lateral eyes to become sensitive to light, allowing them
to visualize other horseshoe crabs. They can detect both
UV light and illumination from the moon. This ability
keeps them in sync with the lunar cycle, which is crucial to
spawning. The underside of the shell consists of six pairs
of appendages, all but one with a claw, the majority aiding
in mobility. Two shorter ones, centrally located, help move
food into the mouth. Below those are a series of book gills.
Horseshoe crabs are of vital importance when
considering two facets: their eggs as a food source and
their blood as a contribution to the medical industry. In
Delaware Bay, horseshoe crabs spawn, leaving thousands
of nutrient-rich eggs. Many of these eggs are devoured by
hungry shorebirds, particularly semipalmated sandpipers,
ruddy turnstones, and red knots passing on feeding binges
during migration.