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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.

 


Now you see ’em, now you don’t.

That’s the disappearing act used by ghost crabs.

 

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.