prominent are the pair on either side of the shell. Collections
of those organs form crude images like the eyes of spiders,
to which horseshoe crabs are more closely related than blue
crabs. Their tails provide balance as they slowly go about
their business on the Bay’s bottom. If by some misfortune
they get flipped askew (upside-down), the tails give them
purchase to right themselves.
But let’s go back to askew. That’s a human judgement. Limulus
polyphemus is way ahead of Homo sapiens in terms of longevity
on Earth, having appeared several hundreds of millions of
years before us. Adaptability is arguably this species’ strongest
quality. These crabs have found a stable ecological niche, a way
of life, not ambitious, but durable, despite major environmental
changes. They feed on benthic (bottom-dwelling) animals
like seaworms, small shellfish, and tiny crustaceans. Habitat
for adults (ten years of age or more) is the open Bay and the
continental shelf out to depths of 20-500 feet.
Turn one over and look closely. Each leg has a claw of sorts,
but no, they don’t bite, or move nearly as fast as a blue
crab, despite surprisingly complex nervous and circulatory
systems. The tiny front claws move food to the jawless
mouth, where stiff bristles and a sand-filled gizzard grind up
food, including even soft clam shells. They have the capability
to regurgitate larger shell fragments. If the second pair of
claws is shaped like boxing gloves, your specimen is a male.
The following four pairs of legs are all for scrambling over the
bottom, with the after pair especially adapted for pushing
sand to create burrows.
What brings horseshoe crabs inshore to our beaches is
spring mating. A male’s second set of claws is used to hold
onto the shell of a (larger) female and fertilize eggs that she
lays, while burrowing into the sandy water’s edge. He may
stay connected to a female for several days during the mating
season, as she will lay eggs multiple times. The best habitat
for spawning is a beach with loose sand that allows oxygen
between the grains for the eggs and baby horseshoes to
absorb as they develop. Ideal beaches are crescent-shaped
to damp out wave action. One problem that threatens our
horseshoe crabs is the loss of these beaches to riprap and
other forms of shoreline hardening.
The other threats come from harvest by humans. It is
significant enough that coastal management of horseshoe crabs
is overseen by the Atlantic States Marine Fisheries Commission
(ASMFC), just like other highly sought-after migratory species,
including rockfish and bluefish. Highly sought-after? Why? A
century ago, we dredged up the seemingly useless horseshoe
crabs by the millions and ground them up for fertilizer. It’s
hard to admit that we were ever that clueless about how our
Chesapeake’s ecosystem functioned, but in those days, we were
still blinded by the wonders of the Industrial Revolution.
Fortunately, we’ve taken a more thoughtful tack. Broken-up
horseshoe crabs make excellent bait for eel and conch pots.
About 10 years ago, this need for bait came into conflict with
the ASMFC’s management guidelines for horseshoe crabs.
However, a savvy marine scientist developed a breakthrough,
a small mesh bait bag for the eel and conch pots that
reduced the need for horseshoe crabs by 90 percent. That