Unit Five
COAST/Horseshoe Crabs
ï›™Project Oceanography Fall 2001 81

COAST
Written by Willie Heard

On the cutting edge…
There are only four species of horseshoe crabs in existence in the world today.
These are Limulus polyphemus, Tachypleus gigas, Tachypleus tridentatus, and
Carcinoscorpius rotundi-cauda. These creatures are sometimes called “living
fossils” because they have changed little from their fossilized relatives; the
earliest species identified is approximately 450 million years old. Horseshoe
crabs are a valuable resource, commercially as a fertilizer and as a source of
calcium for enriching fowl grains and medicinally in identifying endotoxins. The
most persistent study on these animals has focused on the properties of their
blue blood. In 1977, The Food and Drug Administration of the United States
approved a new test for identifying endotoxins using Limulus Amoebocyte Lysate
(LAL) purified from horseshoe crab blood. In spite of their commercial and
medical importance, horseshoe crabs are threatened by the loss of living and
breeding habitats. This habitat degradation has resulted in a rapid population
decline over the last few decades.

Horseshoe Crabs

Lesson Objectives: Students will be able to do the following:
• Describe the difference between a true crab and a horseshoe crab
• Briefly discuss the life history of the horseshoe crab
• Give some medical and commercial uses of the horseshoe crab

Key Concepts: chitin, Limulus Amoebocyte Lysate (LAL), chitosan, exoskeleton,
pedipalps, molt, arthropod, decapod, chemoreceptors, pheromone

What is a Horseshoe Crab?

Even though the
horseshoe crab has
a hard shell and
numerous
appendages with
claws, it is not really
a crab. Horseshoe
crabs belong to the
phylum, Arthropoda, along with
crabs, insects, and other
invertebrates with jointed legs, but
their closest living relatives are
spiders and scorpions.
True crabs have two pairs of
antennae and a pair of mandibles, or
jaws; horseshoe crabs lack these
structures. Further, comparing the
legs of a true crab with the legs of a
horseshoe crab reveals another
significant difference. True crabs
classified as decapod crustaceans,
have five pairs of legs, which include
a pair of claws. Horseshoe crabs
have seven pairs of legs under their
helmet-like shells; five of these
seven pairs of legs are equipped
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with claws. In adult males, the
second pair of claws (pedipalps)
has a “boxing-glove” appearance
and is used to grasp females during
spawning. Horseshoe crabs also
have four simple eyes on the top of
their carapace instead of two as with

the true crab. Our North American
species has been named Limulus
polyphemus – Limulus meaning “a
little askew or odd” and polyphemus
after the giant cyclops in Greek
mythology.

Biology/Life History of the Horseshoe Crab

Horseshoe crabs are
among the world’s
oldest and most
fascinating creatures.
The earliest
horseshoe crab
species had already inhabited Earth
at least 200 million years before the
dinosaurs arrived or about 400
million years ago.

Today, there are four species of
horseshoe crabs in the world, but
only one is found on North American
shores. Our native horseshoe crabs
live along the Atlantic coast from
Maine south to Florida and the
Yucatan Peninsula. They are also
found in several places along the
eastern shores of the Gulf of Mexico.
The largest concentration of
horseshoe crabs is found in
Delaware Bay, located between
Delaware and New Jersey.
Horseshoe crabs are animals of the
temperate seas. During the cold
months, they lie half-buried in the
ocean sediments. Horseshoe crabs
have been observed mating from
April through December, although
the peak reproduction period occurs
during the highest tides in late May
and early June, at the time of the full
or new moon. Most spawning is at
night because of the protection
afforded by darkness.
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In spring, males arrive at the
shorelines first, followed by the
females a week or two later.
Females average being 30% larger
than males and attract the males by
releasing a pheromone, or natural
chemical stimulant, into the water.
Horseshoe crabs also use their
relatively good vision to help locate
potential mates.
Males patrol
the nearshore
waters and
use their
pedipalps to attach to the abdomen
of a female as she moves toward the
beach. She drags him to the water’s
edge and scoops out a series of five
to seven crude nests, where she
deposits tens of thousands of eggs.
The attached males and other males
that gather around the female
fertilize the eggs as she lays them.
The new eggs are about 1.5 mm, or
1/16 inch, in diameter, and are an
opaque, pastel green color. In a few
days, they double in size and the
outer layer peels away, leaving the
eggs transparent. Moisture from the
tides, and the warmth of the sun,
allows the eggs to hatch in the two-
week period between spring tides
(the higher-than-
normal tides that
occur at the new and
full moons.) After
hatching, the juvenile
horseshoe crabs dig their way “out
of” the sand. When born, the tiny
horseshoe crabs look very much like
their adult counterparts. They begin
life as miniature adults, about three
mm (1/8 inch) across, but lack a fully
functional digestive system and a
movable tail. For about a week, they
“swim about,” eating their yolk sac as
their digestive systems mature.
They swim upside down, moving
their legs and gills in a progressive
wave-like oscillation from front to
back.

Horseshoe crabs push their way
along the bottom, digging small
furrows in search of food. They use
their first pair of legs as feelers to
determine the presence of prey.
When the crab feels or smells a
worm, clam, or dead fish, one of the
claws picks it up and pushes it
toward the heavy, spiny projections
that surround the mouth; the
horseshoe crab
has no nose; but
the tiny hairs on
the spiny
projections around
its mouth are
chemoreceptors, allowing the crab
to smell prey. Since the horseshoe
crab has no jaws to chew its food, it
must bring all of its legs together and
use the spiny projections around its
month then the first set of legs to
crush the worm or clam. Horseshoe
crabs also have gizzards containing
sand and small bits of gravel to help
grind their food.

Horseshoe crabs continue to grow
for nine to ten years until they reach
maturity. The young horseshoe
crabs molt, or shed, their outer
skeleton (exoskeleton) often until
they reach sexual maturity, then
molting slows, occurring only about
once annually. The animals
increase in size by 25-30% with each
molt by pumping in water to expand
their new shells, which will harden in
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approximately 24 hours. Males are
sexually mature at their sixteenth
molt or ninth year. Females need at
least 17 molts and mature in their
tenth year. Unlike the blue crab,
which “backs out” of its old shell, the
horseshoe crab crawls forward out of
its shell through a split that develops
along the junction of the dorsal (top)

and ventral (bottom) surfaces. No
one really knows how long
horseshoe crabs live, but some
scientists believe that 30 years is
possible. Generally, an animal
which does not begin reproduction
until age nine or older would have a
life span enabling reproduction for a
number of years.
Commercial Importance of Horseshoe Crabs

Though the eggs and flesh of
Limulus polyphemus are not toxic,
they are not eaten by people today.
Years ago, indigenous American
people did eat the lump of meat in
the abdomen which moves the tail.
They also used the shell to bail water
from their canoes and the tails as
spear tips. More
recently,
horseshoe crabs
were used as
fertilizer and as
feed for chickens
and hogs.
Chicken and hogs fed on horseshoe
crab developed a bad taste, so using
horseshoe crabs as a food source
was discontinued.

Today horseshoe crabs are
important to people for their use in
medicine. For over fifty years
scientists have used horseshoe
crabs in eye research. Scientists
can easily study the large eyes and
optic nerve (the nerve that sends
signals from the eye to the brain) of
the horseshoe crabs. Scientists
have learned a great deal about how
human eyes function from research
on horseshoe crab eyes.

Chitin is a substance found in the
shells, or exoskeleton, of
horseshoe crabs, as
well as other
arthropods, such as
lobsters, crabs, shrimp,
spiders, beetles, and mosquitoes.
Chitin has received a lot of attention
from scientists because it is non-
toxic and biodegradable. When chitin
is processed, another substance,
chitosan, is produced and can be
used as a raw material to
manufacture a variety of important
products.

Contact lenses, skin creams, and
hair sprays can also be
manufactured from chitin. Chitin can
be used to remove lead and other
harmful chemicals from wastewater.
Chitin joins the fight against fat when
added to foods. It has the ability to
bind with fats and
then pass them
through the body
without being
absorbed. Besides
absorbing fat to
promote weight
loss, chitosan also inhibits “bad”
cholesterol uptake and boosts “good”
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cholesterol. Other chitosan uses
include: promoting the healing of
ulcers and lesions; serving in anti-
bacterial action; acting as an antacid;
helping to control high blood
pressure; and treating and
preventing irritable bowel syndrome.
Lastly, chitosan can be made into
string used to
suture surgical
wounds and in
wound dressings.
People are less
likely to experience an allergic
reaction to the chitosan-based
stitches, which dissolve slowly, and
the dressings actually promote
healing.

In 1950, Frederick Bang discovered
the blue-colored blood of the
horseshoe crab contained special
cells that react to certain kinds of
disease causing bacteria. Limulus
Amoebocyte Lysate (LAL) was
found in Factor C, the first enzyme to
be activated by the endotoxin from
gram negative bacteria. LAL is now
used as a fast and effective way of
testing drugs to make sure they are
free of these harmful bacteria before
they are administered to people.
Pharmaceutical companies used the
LAL to test sterility of antibiotics and
kidney dialyzers, to detect some
cancers, and to detect spinal
meningitis.

Sometimes during the peak summer
months, more than 1,000 crabs per
week are harvested for their blood.
Up to one-third of the animal’s blood
is painlessly removed by medical
researchers. The cost of horseshoe
crab blood has reached $15,000
dollars per quart. A horseshoe crab
collector can make up to $1,000
dollars a night.
.




















Unit Five
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Activity: Horseshoe Crab “Hands On”

The horseshoe crab is compared to the giant cyclops (polyphemus) in Greek
mythology. Yet, despite this frightening comparison, the horseshoe crab with its
large spiny body and spiked tail is not poisonous, but completely harmless.

Objectives: Students—in groups of two or four—will be able to do the following:
• Touch and hold the horseshoe crab.
• Identify the external anatomy of the horseshoe crab.
• Identify the gender of the horseshoe crab.

Materials:
• Live horseshoe crabs, male and female
(There are a number of vendors for living horseshoe crabs, however, one
vendor is: Dynasty Marine Associates, 10602 7th Avenue, Gulf Marathon, FL
33050. Phone: 305-745-7666. Horseshoe crabs are approximately $12 each
depending on size and seasonal price variations.)
• Molts from various sizes of horseshoe crabs
• Paper towels
• Hand soap

Procedures:
1. The instructor should review the history of the horseshoe crab and research
uses, as well as discuss the safe handling techniques for these animals.
2. Removing the live animal from its tank, the instructor should present the
animals to the class, identifying the major external, anatomical structures.
3. Students should be given time to carefully feel the tail, legs and pinchers of
both the live animals and the molts.
4. The instructor should review, using male and female animals, the
distinguishing characteristics of the claws between the two sexes.
5. Students should wash hands thoroughly at the conclusion of the class.

Extensions:
1. Compare the horseshoe crab with a true crab such as the blue crab
(Callinectes sapidus).
2. Locate recent and historical research reports utilizing horseshoe crabs by
accessing via internet the web sites of Delaware Sea Grant College Program.
3. Construct a paper model of the horseshoe crab using Dr. Bill Hall’s published
model available through Delaware Sea Grant College Program.
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Activity: Biodiversity Quiz

Objectives: Students will be able to do the following:
• Identify which niche horseshoe crabs fill.
• Define “niche”.
• State the importance of horseshoe crabs in research.

Materials:
• Live horseshoe crabs
• Paper
• Pencil

Procedure:
Two camps of animal rights activists argue concerning the use of marine
invertebrates for biomedical purposes. Some individuals believe passionately
that research involving living subjects of any kind is cruel and unnecessary.
Other individuals believe invertebrates can be substituted for warm-blooded
research animals, i.e., mammals. A list of uses for horseshoe crabs has been
provided below. Explore your value system by ranking the uses from most to
least acceptable. Tell which uses should be continued and discontinued, and
why. Be prepared to explain your answers.

1. Horseshoe crab nerve cells are large, accessible, and good for modeling
human nervous system mechanisms. Horseshoe crabs’ relatively large and
simply constructed compound eyes have an easily accessed optic nerve; and
because they are easy to keep in the laboratory, horseshoe crabs have
contributed greatly to human eye research.
2. American Indians began using horseshoe crabs thousands of years ago.
They ate the abdominal meat, the tail muscles, and possibly other parts.
Shells were not discarded, but used to “bail out” dugout canoes. Spear tips
were constructed from horseshoe crab tails, and Indians buried the crabs as a
time released, high-nitrogen fertilizer.
3. American settlers also used horseshoe crabs for fertilizer, leading to an
industry that lasted until the 1950s. A few farmers continue the practice today.
Horseshoe crabs have also been used as stock feed, but imparted a fishy
flavor to hogs and chickens. Horseshoe crabs’ eggs are still used as eel bait
in a small fishery supporting U.S. and European markets.
4. The chitin of horseshoe crab shells is extremely pure, and chitin-coated
sutures reduce healing time in humans by 35-50%. A Japanese company
spins pure chitin dressings for burns, surface wounds, and skin-graft donor
sites.
5. Limulus Amoebocyte Lycate (LAL) is a clotting agent extracted from
horseshoe crab blood. LAL is the standard test for injectable and intravenous
drugs, during which it clots in the presence of toxins and impurities. LAL is
also used to diagnose diseases such as spinal meningitis. Large horseshoe
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crabs are caught and bled for LAL production through a stainless steel tube
inserted into the circulatory system. After 24 hours, the crabs are returned to
the bays where they were caught with little serious harm. (Source:
. Used by permission.)

Possible Extension:
1. Devise a food web with the horseshoe crab at the center of the web.

Unit Five
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Student Information: Horseshoe Crabs

Even though the
horseshoe crab has a
hard shell and
numerous appendages
with claws, it is not
really a crab.
Horseshoe crabs
belong to the arthropod phylum
along with crabs, insects, and other
invertebrates with jointed legs, but
their closest living relatives are
spiders and scorpions.

True crabs have two pairs of
antennae and a pair of mandibles, or
jaws; horseshoe crabs lack these
structures. Further, comparing the
legs of a true crab with the legs of a
horseshoe crab reveals another
significant difference. True crabs
classified, as decapod crustaceans,
have five pairs of legs, which include
a pair of claws. Horseshoe crabs
have seven pairs of legs under their
helmet-like shells, with five pairs of
legs equipped with claws. In adult
males, the second pair of claws
(pedipalps) has a “boxing-glove”
appearance and is used to grasp
females during spawning.
Horseshoe crabs also have four
simple eyes on the top of their
carapace instead of two as with the
true crab. Our North American
species has been named Limulus
polyphemus – Limulus meaning “a
little askew or odd” and polyphemus
after the giant cyclops of Greek
mythology.

Chitin is a substance found in the
shells, or exoskeletons, of horseshoe
crabs, as well as other arthropods,
such as lobsters, crabs, shrimp,
spiders, beetles, and mosquitoes.
Chitin is non-toxic, biodegradable,
and is processed to produce another
substance called chitosan
(aminopolysaccharide) that can be
used to produce a variety of
important products. Horseshoe
crabs grow larger by molting, or
shedding their outer shell, which is
actually their skeleton
(exoskeleton).

Too often, we learn the value of
something only after it is gone. In the
case of the horseshoe crab, we now
know its value and as a result, many
people are concerned about the
horseshoe crab’s future. We should
learn from the experience of the
Japanese, whose horseshoe crab
species is considered endangered.
The medical value of the horseshoe
crab is easily measured
economically, yet environmentally,
the picture is much more complex
when we consider the millions of
shorebirds and other animals—fish,
turtles, and the entire estuarine food
chain—that depend on horseshoe
crab eggs as a food source.



Unit Five
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Horseshoe Crab Vocabulary

Carapace-the front top portion of the shell of the horseshoe crab

Chelicerae-the first pair of pincers located in front of the horseshoe crab’s mouth

Chemoreceptors-tiny hairs located on the spiny projections that surround the
horseshoe crabs’ mouths that allow them to “smell” prey

Chilaria-appendages located in the rear of the horseshoe crab’s mouths

Chitin-a cellulose-like substance found in the shells, or exoskeleton of
horseshoe crabs, as well as other arthropods

Chitosan-processed chitin; used in a variety of manufactured products

Decapod-creatures that have five pairs of jointed legs

Dorsal-top portion of the carapace where the shell splits during molting

Exoskeleton-the outer structure (skeleton) of the horseshoe crab’s body

Gnathobases-the heavy, spiny projections that surround the horseshoe crab’s
mouth

Limulus Amoebocyte Lysate-a clotting agent extracted from horseshoe crab
blood

Molting-the process by which crabs grow by shedding their exoskeleton

Pedipalps-the second pair of claws of the male which has a “boxing glove”
shape and which are used during spawning to grasp the female’s abdomen

Pheromone-a natural chemical produced by female horseshoe crabs that acts
as a sexual stimulant to attract males

Prey-an animal that is hunted for food

Telson-the long, spike-like tail of the horseshoe crab

Ventral-the bottom portion of the carapace where the horseshoe crab’s shell
splits during molting