By Rick Newton
On the list of the Nature
Conservancy’s top migrations is that of the horseshoe crab (Limulus
polyphemus). It is truly a wonder to experience. Horseshoe crabs are
called a living fossil; they have been around for over 350 million
years virtually unchanged. It is not really a crab as it is
associated more closely with spiders and scorpions. Worldwide there
are four species of horseshoe crabs but only one in the United States
on the east coast.
In late spring and early summer,
mainly around the times of full and new moons and on the hide tide
cycle, horseshoe crabs migrate from their wintering grounds to local
beaches to lay their eggs. The female crab, usually with a male crab
grasping on to the female’s shell with a pair of modified legs
resembling boxing gloves, buries herself into the sand laying a
cluster of around 4,000 eggs. Over several nights the female may lay
as many as 100,000 eggs. About a month later, the eggs will hatch,
and tiny horseshoe crabs will spend the first few years of life on
tidal flats and marshes.
 |
| Horseshoe crabs molt many times before reaching maturity. |
Because horseshoe crabs have a hard
shell they must molt to grow. They will molt around six times in
the first year and up to eighteen times before reaching sexual
maturity. Once the crabs reach sexual maturity, which takes about
nine or ten years, the molting stops. When the male crab completes
its final molt, the front claws take the shape of boxing gloves that
he uses to grab on to the female for spawning. A horseshoe crab's
lifespan is believed to be 20 – 30 years.
Horseshoe crab bodies are composed
of three parts: prosoma (head), opisthosoma (central area), and
telson (tail). Horseshoe crabs cannot hurt you. Many people think
the tail is some kind of stinger, but it is mainly for allowing crabs
to flip themselves over should they get turned upside down.
Horseshoe crabs have ten eyes, and much of the research on human
vision has been accomplished using horseshoe crabs. Horseshoe crabs
have book gills to get oxygen from the water and can live on land for
up to four days if they get stranded. Their food consists of razor
clams, soft-shelled clams, and marine worms.
Horseshoe crabs are important for a
few reasons. First, shorebirds migrate from South America to the
Artic. Most need to stop and rest and feed on their travels north to
their summer breeding grounds. Their migration coincides with
horseshoe crab spawning. Eggs that are exposed to air by wave or
boat wake action or by the digging action of other crabs, quickly dry
out and won’t hatch. But these eggs are the primary food source
for the migrating birds allowing them to double their body weight in
less than two weeks.
Not just a fossil
Second, horseshoe crab blood plays
a vital role in human medicine. Their copper-based blood, which
turns blue when exposed to oxygen, contains blood cells called
amoebocytes. A testing reagent called LimulusAmoebocyte Lysate (LAL)
is derived from the amoebocytes of the horseshoe crab. The LAL is
used to test the sterility of vaccines, drugs, and other medical
devices. However, recent biomedical developments have shown that a
synthetic compound may be an alternative to using horseshoe crab
blood, thus saving hundreds of thousands of crabs from being bled. You can learn more here.
Horseshoe crabs have few natural
predators except for seagulls or raccoons that may feed on an
overturned crab. Major threats are from harvesters (who sell crabs
as bait for conch, whelk and eel), human disturbance, and loss of
habitat due to beach development or shoreline modifications as
communities harden the shoreline to deal with rising sea levels.
Avalonia’s Sandy Point Nature
Preserve is one of the primary spawning areas for horseshoe crabs
with hundreds of crabs coming to the beach on peak cycles.
Volunteers from Avalonia, Mystic Aquarium, and others support Project
Limulus (Sacred Heart University & USFWS) in southeastern
Connecticut. These volunteers are citizen scientists counting,
measuring, and tagging horseshoe crabs during the early spring and
summer.
Be a citizen scientist
Anyone can help by just walking the
beaches and looking for tagged crabs. If you see a tag on a dead
crab, remove the tag and report the tag number, date, time and
location to the number on the tag, or provide this information via
the internet at: http://www.fws.gov/crabtag/
If you see a tag on a live crab, just write down the tag number and
report it as above, leaving the tag on the crab. If you see any
crab upside down, just flip it over by grasping it by the side of the
shell (not the tail).
 |
| If you see a flipped Horseshoe crab on the beach. |
 |
| Give it a hand and turn it over by its shell. |
Tagged horseshoe crabs – Groton /
Stonington area – 2009 to 2017
Note: some of the
decline in tagged crabs is due to budget reductions to the USFWS with
fewer tags being distributed. In general, however, volunteers are
seeing fewer crabs each year.
You can read more about Horseshoe crabs in The Underwater
Secrets of Horseshoe Crabs, here.
photographs by Rick Newton
No comments:
Post a Comment