What Is a Horseshoe Crab?
The horseshoe crab belongs to the large group
of invertebrates (animals without backbones) called Arthropods.
This group also includes lobsters, crabs, insects, spiders,
and scorpions. Even though it looks crab-like, with a hard
shell and claws, the horseshoe crab is more closely related
to scorpions and spiders.
Horseshoe Crab Taxonomy
Phylum: Arthropoda — joint-legged
Subphylum: Cheilcerata — animals
with no jaws
Class: Merostomata — mouth
surrounded by legs
Subclass: Xiphosura — from
the Greek Xiphos meaning sword and ura
Order: Xiphosurida — sword-tailed
Family: Limulidae — one
living member, Limulus
Genus: Limulus — from
the Latin, meaning somewhat
oblique, odd, or askew
and referring to the
sideways placement of
the compound eyes
Species: polyphemus — from
the Greek, meaning
to the simple
eyes on the front
of the shell
A Closer Look
The body of a horseshoe crab is divided into three parts:
the prosoma, opisthosoma and telson,
or tail. The prosoma is the front, semicircular part of the
horseshoe crab and combines the head and thorax under a hard
shell or exoskeleton. The opisthosoma is attached to the prosoma
with a hinge. The shell protects the gills and two genital
pores located under the horseshoe crab.
The top or dorsal surface of the shell has ridges
and depressions. These are locations where muscles are attached
to the inside of the shell. Two large compound eyes are located
on the prosoma, with other light receptors scattered all over
Eating "On the Run"
pairs of leg-like appendages are found under the shell. These
are used for gathering and eating food as well as for moving.
A horseshoe crab pushes its way along the bottom using its
clawed and pusher legs. As it moves, its first pair of appendages,
called chelicerae, feel around for clams and worms.
When the chelicerae find food, one of the claws picks it up
and pushes it toward the gnathobases, the bristly
area near the base of the walking legs. The horseshoe crab
lacks jaws to chew its food, so as it moves, the gnathobases
work to tear and shred the worm or clam. The horseshoe
crab has no nose, but "smells" with tiny hairs on
the gnathobases that act as chemoreceptors. Bits
of food that get caught on the bristles are pushed into the
crab's mouth with either the chelicerae, or the chilaria,
small degenerate legs located behind the pusher legs. A horseshoe
crab also has a gizzard containing sand and small bits of gravel
to help grind its food.
Like many animals with hard shells, a horseshoe crab must
molt or shed its shell as it grows. Before molting, a new
shell begins to form. When this new shell is ready, the
horseshoe crab absorbs water through its gills, making
The old, hard shell cannot expand and
splits in the front where the top and bottom join. The
horseshoe crab crawls
out the front, leaving the old shell behind. It takes about
24 hours for the new soft shell to harden. With each molt,
the horseshoe crab increases in size by an estimated 25-30%.
|This horseshoe crab was in the process
when it died. Photo courtesy of NOAA Photo Library
Community on a Crab Shell
A horseshoe crab is virtually a "walking hotel," with
any number of creatures living attached to its shell — barnacles,
blue mussels, slipper shells, bryozoans, sponges, flatworms,
diatoms, fungi, and bacteria. While most of these hitchhikers
have little or no effect on the day-to-day life of the horseshoe
crab, certain fungi and bacteria can degrade the shell over
time. Although the crab's armored shell appears indestructible,
daily wear and tear can leave it with small cuts or scratches
that allow fungi and chitinase bacteria to gain a foothold.
Specifically, fungi and chitinase bacteria attack these abrasions
and gradually "eat" through
the shell, exposing the horseshoe crab to additional microbes,
which are eventually fatal.
addition, several species of flatworm glide around the bottom
of the horseshoe crab, eating scraps of food that the crab misses
in its haphazard method of feeding. The worm cements its eggs
to the horseshoe crab's gills. As a result, small portions of
the gill that surround the eggs become rigid and stiff, causing
the formation of hairline cracks that allow deadly bacteria
to invade the horseshoe crab's system.