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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 hatch within two to four
weeks. The larvae will emerge from the
sandy beaches and enter the water
during a high tide nearly a month later.
The larvae look like miniature adult
horseshoe crabs without tails.
After hatching, horseshoe crabs spend
their first few years of life on the tidal
flats and move out farther from shore as
they get older. Adults spend the winter
in deep bay waters and off-shore areas.
As spring approaches, the crabs move
en masse toward the beaches to
prepare for spawning.
Since horseshoe crabs have a hard shell,
they must molt to grow. Horseshoe crabs
will molt at least six times in their first
year of life and about 18 times before
they reach sexual maturity. Females are
generally larger than males and may
molt more than males to reach the larger
size. Once crabs are sexually mature,
which takes at least nine years, they
won’t shed their shells again. When the
male crab completes his final molt, his
first set of claws becomes modified into a
boxer-glove shape that he uses to clasp
onto a female for spawning. Adult crabs
may live another eight to 10 years,
making the total lifespan of a horseshoe
crab as long 20 years.
Physical Characteristics
The horseshoe crab has a unique and
primitive body structure. The body is
composed of three parts: the prosoma
(head), opisthosoma (central area) and
telson (tail). The horseshoe crab’s name
is derived from the prosoma, resembling
the shape of a horse’s shoe. The telson
helps the crab to flip itself over if waves
on the beach turn it over. As dangerous
as the telson may look, it is not
venomous or used as a weapon by
the crab.
Horseshoe crabs have several pairs of
eyes. Two large compound eyes on the
prosoma are sensitive to polarized light
and can magnify sunlight 10 times. A
pair of simple eyes on the forward side of
the prosoma can sense ultraviolet light
from the moon. In addition, multiple eye
spots are located under the prosoma,
with more on the underside of the tail.
Horseshoe crabs occasionally swim
upside down and may once have used
these eyes more than they do today.
Horseshoe crabs use book gills to get
oxygen from the water. The parts of the
gill are in small flaps resembling the
pages of a book. If these primitive gills
MARCH 3, 184
9
U.
S.
DE
PA
RT
MEN
T OF THE INTER
I O
R
stay moist, horseshoe crabs can remain
out of water up to four days. Crabs
stranded on the beach during spawning
bury themselves in the sand or fold
themselves in half to conserve water
until the tide rises again.
Horseshoe crabs have no jaws or teeth.
Instead, they have an impressive array
of spiny mouth bristles at the base of five
pairs of legs to maneuver food items
such as razor clams, soft-shelled clams
and marine worms into their centrally
located mouth. To chew its food, the crab
must simulate walking movements.
The Shorebird Connection
The largest population of horseshoe
crabs in the world is found in Delaware
Bay. During the spawning season, many
eggs are exposed to the beach surface by
wave action and the digging action of
mating crabs. Once an egg is exposed to
air, it can dry out quickly, preventing it
from hatching; however, it still plays a
vital role in the ecosystem. These
exposed eggs are the primary food
source for migrating shorebirds making
the journey from South America to the
Arctic along the Atlantic Flyway.
Delaware Bay hosts the second largest
population of migrating shorebirds in
North America. The shorebirds in the
Atlantic Flyway use only a few areas for
feeding and rest on their trip from South
America to the Arctic. More than a
million shorebirds fly nonstop from
places as far away as Peru, Suriname
and Argentina’s Tierra del Fuego – as
much as 5,000 miles. More than half of
the total flyway population of red knots,
ruddy turnstones and semipalmated
sandpipers depend on Delaware Bay’s
horseshoe crab eggs as a rich food
supply. Hundreds of thousands of other
migratory birds representing several
species also converge on Delaware Bay
during mid to late May.
Every year nature provides an incredible
spectacle in late May as these ravenously
hungry birds start arriving. Each bird
can double its weight in less than two
weeks by eating thousands of horseshoe
crab eggs. Then they fly on to their
summer breeding grounds in the Arctic.
The Medical Connection
Of all marine species, horseshoe crabs
have contributed the most to medical and
physiological research. Most of what we
know about human vision was drawn from
a Nobel Prize-winning scientist’s work
with cells found in horseshoe crab eyes.
Horseshoe crab blood plays a vital role
in human medicine. The straw-colored,
copper-based blood turns blue when
exposed to high concentrations of
oxygen. Horseshoe crab blood contains
primitive large blood cells called
amoebocytes. A clotting agent called
Limulus Amoebocyte Lysate is derived
from the amoebocytes of the horseshoe
crab. When the LAL comes in contact
with bacterial toxins, a clotting reaction
occurs. Pharmaceutical companies test
the sterility of vaccines, drugs,
prosthetics and other medical devices
using LAL. There is no synthetic
substitute for the LAL test.
Threats to Survival
Human development threatens
horseshoe crab survival in the United
States and in other parts of the world.
Human disturbance can adversely affect
spawning activities. Beach development
and shoreline modifications prevent
crabs from reaching sandy areas or
strand them once they reach spawning
areas. Recreational vehicle traffic on
beaches can crush the crabs and destroy
their nesting habitat. Oil spills from
barges or tankers also pose threats to
breeding and young crabs. These human
activities not only harm crabs, but also
interfere with feeding and resting
activities of millions of migrating
shorebirds at a critical time in their lives.
Horseshoe Crab Harvest and Management
In the late 1800s and early 1900s, up to 4
million horseshoe crabs were harvested
annually and used as fertilizer or animal
food. Currently, crabs are harvested for
bait in conch and American eel fisheries
on the Atlantic Coast. Horseshoe crabs
suffered a substantial increase in harvest
in the 1990s that spurred the need for
management on a coast-wide scale.
In 1998, the Atlantic States Marine
Fisheries Commission, representing 15
states from Maine to Florida, developed
a horseshoe crab management plan. The
ASMFC plan and its subsequent
addenda established mandatory state-by-
state harvest quotas and created the
1,500-square-mile Carl N. Shuster Jr.
Horseshoe Crab Sanctuary off the mouth
of Delaware Bay.
Active management, as well as
innovative bait conservation techniques
have successfully reduced commercial
horseshoe crab landings in recent years.
Conch and eel fishermen have been using
mesh bait bags in their traps, so they use
only a portion of one crab per trap,
compared to using a whole crab in each
trap. The bait bags have reduced the
demand for bait by 50 to 75 percent in
recent years. Research is also being done
to identify alternative baits for the conch
and eel fisheries to further reduce the
need for horseshoe crabs as bait.
Despite restrictive measures taken in
recent years, populations are not
showing immediate increases. Because
horseshoe crabs do not breed until they
reach nine or more years of age, it may
take some time before the population
measurably increases.
What the Future Holds
Unfortunately, we often learn the value
of a species after its population is
decimated. However, in the case of
horseshoe crabs, we know their
ecological role, importance to the
biomedical industry and importance in
the commercial fishery. We have the
opportunity to manage and protect
horseshoe crab populations at a
sustainable level and also provide crabs
for ecological and commercial uses now
and in the future.
For additional information contact:
U.S. Fish & Wildlife Service
Delaware Bay Estuary Project
302/653 9152
U.S. Fish & Wildlife Service
Maryland Fishery Resources Office
410/263 2604
www.fws.gov/northeast/marylandfisheries
Federal Relay Service
for the deaf and hard-of-hearing
1 800/877 8339
U.S. Fish & Wildlife Service
1 800/344 WILD
http://www.fws.gov
August 2006
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 hatch within two to four
weeks. The larvae will emerge from the
sandy beaches and enter the water
during a high tide nearly a month later.
The larvae look like miniature adult
horseshoe crabs without tails.
After hatching, horseshoe crabs spend
their first few years of life on the tidal
flats and move out farther from shore as
they get older. Adults spend the winter
in deep bay waters and off-shore areas.
As spring approaches, the crabs move
en masse toward the beaches to
prepare for spawning.
Since horseshoe crabs have a hard shell,
they must molt to grow. Horseshoe crabs
will molt at least six times in their first
year of life and about 18 times before
they reach sexual maturity. Females are
generally larger than males and may
molt more than males to reach the larger
size. Once crabs are sexually mature,
which takes at least nine years, they
won’t shed their shells again. When the
male crab completes his final molt, his
first set of claws becomes modified into a
boxer-glove shape that he uses to clasp
onto a female for spawning. Adult crabs
may live another eight to 10 years,
making the total lifespan of a horseshoe
crab as long 20 years.
Physical Characteristics
The horseshoe crab has a unique and
primitive body structure. The body is
composed of three parts: the prosoma
(head), opisthosoma (central area) and
telson (tail). The horseshoe crab’s name
is derived from the prosoma, resembling
the shape of a horse’s shoe. The telson
helps the crab to flip itself over if waves
on the beach turn it over. As dangerous
as the telson may look, it is not
venomous or used as a weapon by
the crab.
Horseshoe crabs have several pairs of
eyes. Two large compound eyes on the
prosoma are sensitive to polarized light
and can magnify sunlight 10 times. A
pair of simple eyes on the forward side of
the prosoma can sense ultraviolet light
from the moon. In addition, multiple eye
spots are located under the prosoma,
with more on the underside of the tail.
Horseshoe crabs occasionally swim
upside down and may once have used
these eyes more than they do today.
Horseshoe crabs use book gills to get
oxygen from the water. The parts of the
gill are in small flaps resembling the
pages of a book. If these primitive gills
MARCH 3, 184
9
U.
S.
DE
PA
RT
MEN
T OF THE INTER
I O
R
stay moist, horseshoe crabs can remain
out of water up to four days. Crabs
stranded on the beach during spawning
bury themselves in the sand or fold
themselves in half to conserve water
until the tide rises again.
Horseshoe crabs have no jaws or teeth.
Instead, they have an impressive array
of spiny mouth bristles at the base of five
pairs of legs to maneuver food items
such as razor clams, soft-shelled clams
and marine worms into their centrally
located mouth. To chew its food, the crab
must simulate walking movements.
The Shorebird Connection
The largest population of horseshoe
crabs in the world is found in Delaware
Bay. During the spawning season, many
eggs are exposed to the beach surface by
wave action and the digging action of
mating crabs. Once an egg is exposed to
air, it can dry out quickly, preventing it
from hatching; however, it still plays a
vital role in the ecosystem. These
exposed eggs are the primary food
source for migrating shorebirds making
the journey from South America to the
Arctic along the Atlantic Flyway.
Delaware Bay hosts the second largest
population of migrating shorebirds in
North America. The shorebirds in the
Atlantic Flyway use only a few areas for
feeding and rest on their trip from South
America to the Arctic. More than a
million shorebirds fly nonstop from
places as far away as Peru, Suriname
and Argentina’s Tierra del Fuego – as
much as 5,000 miles. More than half of
the total flyway population of red knots,
ruddy turnstones and semipalmated
sandpipers depend on Delaware Bay’s
horseshoe crab eggs as a rich food
supply. Hundreds of thousands of other
migratory birds representing several
species also converge on Delaware Bay
during mid to late May.
Every year nature provides an incredible
spectacle in late May as these ravenously
hungry birds start arriving. Each bird
can double its weight in less than two
weeks by eating thousands of horseshoe
crab eggs. Then they fly on to their
summer breeding grounds in the Arctic.
The Medical Connection
Of all marine species, horseshoe crabs
have contributed the most to medical and
physiological research. Most of what we
know about human vision was drawn from
a Nobel Prize-winning scientist’s work
with cells found in horseshoe crab eyes.
Horseshoe crab blood plays a vital role
in human medicine. The straw-colored,
copper-based blood turns blue when
exposed to high concentrations of
oxygen. Horseshoe crab blood contains
primitive large blood cells called
amoebocytes. A clotting agent called
Limulus Amoebocyte Lysate is derived
from the amoebocytes of the horseshoe
crab. When the LAL comes in contact
with bacterial toxins, a clotting reaction
occurs. Pharmaceutical companies test
the sterility of vaccines, drugs,
prosthetics and other medical devices
using LAL. There is no synthetic
substitute for the LAL test.
Threats to Survival
Human development threatens
horseshoe crab survival in the United
States and in other parts of the world.
Human disturbance can adversely affect
spawning activities. Beach development
and shoreline modifications prevent
crabs from reaching sandy areas or
strand them once they reach spawning
areas. Recreational vehicle traffic on
beaches can crush the crabs and destroy
their nesting habitat. Oil spills from
barges or tankers also pose threats to
breeding and young crabs. These human
activities not only harm crabs, but also
interfere with feeding and resting
activities of millions of migrating
shorebirds at a critical time in their lives.
Horseshoe Crab Harvest and Management
In the late 1800s and early 1900s, up to 4
million horseshoe crabs were harvested
annually and used as fertilizer or animal
food. Currently, crabs are harvested for
bait in conch and American eel fisheries
on the Atlantic Coast. Horseshoe crabs
suffered a substantial increase in harvest
in the 1990s that spurred the need for
management on a coast-wide scale.
In 1998, the Atlantic States Marine
Fisheries Commission, representing 15
states from Maine to Florida, developed
a horseshoe crab management plan. The
ASMFC plan and its subsequent
addenda established mandatory state-by-
state harvest quotas and created the
1,500-square-mile Carl N. Shuster Jr.
Horseshoe Crab Sanctuary off the mouth
of Delaware Bay.
Active management, as well as
innovative bait conservation techniques
have successfully reduced commercial
horseshoe crab landings in recent years.
Conch and eel fishermen have been using
mesh bait bags in their traps, so they use
only a portion of one crab per trap,
compared to using a whole crab in each
trap. The bait bags have reduced the
demand for bait by 50 to 75 percent in
recent years. Research is also being done
to identify alternative baits for the conch
and eel fisheries to further reduce the
need for horseshoe crabs as bait.
Despite restrictive measures taken in
recent years, populations are not
showing immediate increases. Because
horseshoe crabs do not breed until they
reach nine or more years of age, it may
take some time before the population
measurably increases.
What the Future Holds
Unfortunately, we often learn the value
of a species after its population is
decimated. However, in the case of
horseshoe crabs, we know their
ecological role, importance to the
biomedical industry and importance in
the commercial fishery. We have the
opportunity to manage and protect
horseshoe crab populations at a
sustainable level and also provide crabs
for ecological and commercial uses now
and in the future.
For additional information contact:
U.S. Fish & Wildlife Service
Delaware Bay Estuary Project
302/653 9152
U.S. Fish & Wildlife Service
Maryland Fishery Resources Office
410/263 2604
www.fws.gov/northeast/marylandfisheries
Federal Relay Service
for the deaf and hard-of-hearing
1 800/877 8339
U.S. Fish & Wildlife Service
1 800/344 WILD
http://www.fws.gov
August 2006