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A
close-up photo of a young female Blue Land Crab, Cardisoma guanhumi in defensive posture.
Click To Enlarge |
Those
of us who have lived in South Miami over 20 years can remember the land
crab migrations over Old Cutler Road.They would cross the road in such
great numbers that traffic would have to slow down to a crawl.
If you accidentally
ran a crab over your tire would be punctured by one of their overgrown
claws. These curious creatures are now on the verge of becoming history.
Over development has destroyed most of their unique habitat and the fact
that some cultures consider them a culinary item has not helped their
dwindling population, either.
The Blue
land crab, sometimes called the American land crab, is the largest species
of land crab in the Gulf of Mexico and Caribbean. Known scientifically
as Cardisoma guanhumi, it naturally occurs on the east coast
of the Americas from Florida to Brazil and is also found on Bermuda and
many of the islands in the Caribbean. There are five other species of
land crabs in the genus Cardisoma and they are found throughout the tropics,
from the east and west coasts of Africa to the Pacific coastal regions.
Although
these crabs appear to be completely terrestrial, they actually have modified
gills. They require water to "breathe" and when on land they
carry their own supply. Blue land crabs live in burrows that go down to
the water table. These excavated burrows, which have been recorded up
to ten feet deep, are sometimes their only source of water in arid areas.
The familiar
migrations of the Blue land crab begin at the onset of the rainy season.
This sometimes long (up to four miles from the ocean) and always dangerous
trek is undertaken by the females in order to release their larvae in
the ocean. Many animals, such as raccoons, foxes, and hawks consider these
crustaceans food items and prey upon the exposed crabs that no longer
have their burrows to dive into at the first sign of danger. (The larvae
are also a source of food for fish and other aquatic animals).
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Collected
land crabs
Click To Enlarge |
The breeding
cycle of the female crab coincides with the lunar cycle, with major migrations
preceding full moons and minor migration preceding new moons. Breeding
between males and females occurs one to two days before the full or new
moon and development of the fertilized eggs takes about 16 days. The minute
percentage of larvae that have survived their planktonic stage come to
shore as megalops (baby crabs) after 30 to 40 days in the ocean.
A hard exoskeleton
protects land crabs from the environment and most predators. Once a year
the Blue land crab seals itself in its burrow for two or three months
and molts its exoskeleton. This process, technically known as ecdysis,
is how the crabs grow. The old exoskeleton is shed and, before a new one
forms, they inflate their body by absorbing water, thereby increasing
their size. The new exoskeleton then begins to grow. Blue land crabs molt
once a year and do not appear to grow dramatically each time. In one study
a large land crab with a carapace (the large shell on the top) that measured
nine centimeters wide (or almost four inches in diameter) has been estimated
to be 20 years old.
Food for
the Blue land crab is mostly plant material. However, they have been known
to eat other things such as carrion and animal feces. In some areas they
have become agricultural pests because of their fondness for young plant
shoots. Areas where these crabs occur are characterized by a lack of seedlings
or plant debris. The voraciousness of the crabs actually is a selective
factor for the types of vegetation in the habitats where they occur (they
do not eat all vegetation, possibly because of plant toxins) and it has
been theorized that where land crabs occur, exotic plant species are unable
to become established.
These odd
creatures that once numbered in the millions and impressed all with their
massive migrations are now only plentiful in a few locations in South
Florida. Hopefully they will be protected for future generations and be
allowed to continue to occupy their niche as nature's recyclers.
Cardisoma
guanhumi after a rainy day in South Florida |
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