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What are those white crabs I see on the beach,
and how can they live out of water?
June 28, 2004
Now you see ’em, now you don’t. That’s
the disappearing act used by ghost crabs.
These rapid racers spend their adult lives on sandy bays
and ocean beaches. Scientists believe the crabs are in
the midst of an evolutionary change, evolving from a past
existence as a sea animal to a future as a land animal.
Between their ability to move so fast and their perfect
camouflage, you might catch a glimpse of a ghost crab during
the day, scampering across the beach to disappear into
its burrow. However, they are nearly impossible to see
unless they move.
These tricky travelers are much more active at night, and
when we say they can move we mean move – as much
as six feet per second! And, they can run sideways, forward
or backward. Their Greek scientific name, Ocypode quadrata,
means “fleet of foot.”
Like aquatic crabs, ghost crabs have gills. So
how can they live on the dry upper beach? This
is where their evolutionary transition comes in:
they must go to the ocean several times a day to
give their gills a dunking. Also, females still
enter the ocean to lay eggs. The larvae become
free-floating plankton and drift for four to six
weeks until the lucky survivors roll back onto
shore as match-head sized crabs.
As the crabs mature, they move farther up the
beach to dig their burrows. They reach maturity
at two years, and adults may tunnel a quarter mile
away from the beach and burrow four to six feet
deep. Burrow openings can be as large as three
inches in diameter.
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Now you see ’em, now you don’t.
That’s
the disappearing act used by ghost crabs.
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Even as adults, the square bodies of ghost crabs are relatively
small, seldom measuring more than two inches. It’s
all those legs that make them look larger. Their shells
are thin, and their white pincers are fairly weak and sometimes
tinted lavender. Like most crabs, their periscope eyes
are mounted on stalks and can rotate 360 degrees. They
can retract their eye stalks into grooves in the front
of their shells for protection.
Ghost crabs scavenge much less than other crabs. They
feed on live prey, such as mole crabs and coquina clams,
and even capture turtle hatchlings and other small animals.
They will also eat dead flesh and beach debris.
These fleet-of-foot crustaceans are more common on undisturbed
beaches, where off road vehicles and human footsteps don’t
crush their burrows. However, they do have natural predators.
As free-floating larvae, they are eaten by many animals
in the sea. When they are small, shorebirds attack them,
and raccoons will eat them at any size. Fish can also snap
them up when they go to the water to wet their gills.
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