The horsehoe crab, a close relative of the extant trilobite, is the
oldest living fossil in Maryland, living here for approximately 360 million
years. Horseshoe crabs are bottom-dwelling organisms that belong to the
largest group of all living animals, the phylum known as arthropods. The
presence of chelicera (pincer-like appendages), 5 pairs of walking legs
and book gills, and lack of jaws and antennae make horseshoe crabs more
similar to spiders, ticks and scorpions than to "true" crabs.
At one time, there were many species of horseshoe crabs, however, only
four have survived. Three of these can be found along the shores of Southeast
Asia and nearby islands. In America, they range intermittently from the
Yucatan peninsula to northern Maine. Each major estuary along the coast
is believed to have a discrete horseshoe crab population that can be distinguished
by adult size, carapace color, and eye pigmentation. Along the Atlantic
coast, horseshoe crabs are most abundant between Virginia and New Jersey
with Delaware Bay at the center of the species distribution.
Horseshoe crabs molt or shed their shell to grow. Molting occurs several
times during the first two to three years and about once a year afterwards.
Molting occurs approximately 16 to 17 times over a period of 9 to 11 years
before sexual maturity is reached and once mature, it is believed they
no longer molt. Females reach maturity one year later than males and consequently,
go through an additional molt. Mature horseshoe crabs then repeat what
has occurred for years, an annual spring migration to inshore spawning
areas. If a horseshoe crab can survive the rigors of spawning, it may live
to 18 years of age.
Horseshoe crabs are well known for their highly visible mating activities.
Spawning in the Chesapeake usually begins in late May when large numbers
of adults move onto beaches to mate and lay eggs. The peak in spawning
activity usually coincides with the full moon and evening spring tides.
Adults prefer beach areas within bays and coves which are protected from
rough water. Eggs are laid in clusters or nests along the beach, usually
between high and low tide. Several nests are made during one beach trip
and females will return on successive tides to lay more eggs. Females can
produce approximately 88,000 per year. Egg development usually takes about
a month and once hatched, larvae usually swim around in the shallow intertidal
areas near the beaches where they were spawned until they settle to the
bottom and molt. Juvenile horseshoe crabs spend their first and second
summer on the intertidal flats and then begin moving offshore.
Adult horseshoe crabs feed mainly on marine worms and shellfish including
razor clams and soft-shelled clams. Because they lack jaws, horseshoe crabs
crush and grind their food items using the spiny bases of their legs and
then push the small food particles into their mouths. Horseshoe crabs can
tolerate a wide range of temperatures and can survive in low oxygen environments.
As long as their book gills are kept moist, horseshoe crabs can survive
out of the water for extended periods of time, especially to spawn.
Horseshoe crabs play an important ecological role in the food web for
migrating shorebirds and juvenile Atlantic loggerhead turtles. Delaware
Bay is the principal breeding location for horseshoe crabs and is also
the second largest staging area for shorebirds in North America. If timed
correctly, shorebirds will arrive in the Delaware-Chesapeake region at
the peak of horseshoe crab mating in mid-May and June. These migratory
birds rely on horseshoe crab eggs to replenish their fat supply during
their trip to Canadian breeding grounds. Because these areas are regularly
used by large numbers of shorebirds at specific times of the year, they
are particularly vulnerable to disruption. The collection of horseshoe
crabs by hand from spawning beaches disturbs the feeding of shorebirds.
A decrease in the number of horseshoe crabs may leave a large portion of
migrating shorebirds without the necessary food resources to complete their
trip to arctic breeding grounds.
Adult horseshoe crabs also form a significant part of the diet of juvenile
Atlantic loggerhead turtles, a threatened species that utilizes Chesapeake
Bay as a summer nursery area. Horseshoe crab eggs and larvae are also a
seasonally preferred food item of many invertebrates and finfish, including
all crab species, whelks, striped bass, white perch, American eels, killifish,
silver perch, weakfish, kingfish, silversides, summer flounder and winter
The horseshoe crab also provides a medical compound critical to maintaining
the safety of many drugs used in medical practice. Limulus Amebocyte Lysate
(LAL), a product produced from the blood of a horseshoe crab is used by
pharmaceutical and medical device manufacturers to test their products
for the presence of endotoxins, bacterial substances that can cause fevers
and even be fatal to humans. All injectable drug products and all medical
devices (such as replacement hips and artificial hearts) implanted into
humans must be safely tested for the presence of endotoxins.
Horseshoe crabs have also been to benefit cancer research. Endotoxins
are known to inhibit the growth of cancer cells. Therefore, the ability
of the LAL test to detect cancer cells could lead to controlled cancer
therapy with endotoxins. In addition, another substance found in horseshoe
crab blood may have the potential for diagnosing leukemia. This substance
reacts with red and white human blood cells, including cancerous white
blood cells in leukemia patients. Furthermore, a New Jersey Sea Grant project
has recently discovered a rare protein in horseshoe crab blood that traces
and binds with vitamin B12. These findings led to the development of a
accurate, cost-efficient testing kit for detecting vitamin B12-related
deficiencies and diseases, which may include pernicious anemia, gastric
and intestinal damage, and even mental disorders.
Although they are neither valued for their beauty nor their delectable
taste in seafood dishes, they are very important to the Chesapeake Bay
as an integral part of the marine ecosystem and for their use as a medical
compound. In fact, without proper management and protection, man may manage
to wipe out a resource that has survived not only dinosaurs and woolly
mastodons, but dramatic destruction of their living and spawning environment.
Horseshoe Crab Fun Facts:
The Indians of Roanoke Island, North Carolina tipped their fish spears
with horseshoe crab tails, and their remains have been found in the kitchen
middens of coastal Indians.
By recording electrical impulses from the crab's optic nerve in its lateral
eye, many principles underlying the functioning of all visual systems was
discovered and gave Dr. H. Keffer Hartline a shared part of the 1967 Nobel
Horseshoe crabs can swim, awkwardly and upside down, but spend most of
their time rummaging along the bottom.
Despite their armament of spines, tail spike and wriggling, weakly clawed
feet, horseshoe crabs are harmless!