Sep 6, 2010

Crab Swarms Overtake Island—Mystery Solved : Pictures

Crab Swarm

Migrating Christmas Island red crabs congregate on a beach in the Australian territory of Christmas Island (map) in an undated picture.

A new study solves a longstanding mystery surrounding the crabs: how the the normally sedentary species has the stamina to "undergo one of the most arduous migrations on Earth," in the words of study co-author Lucy Turner.

When the wet season blows into Christmas Island (map) each year, millions of Christmas Island red crabs hike for several days, from a high rain forest plateau down to Indian Ocean beaches, where the crabs mate in burrows.

"It's an amazing feat—going from not being able to exercise for more than ten minutes to walking for several miles," said Turner, a biologist at the University of Bristol in the United Kingdom.

But by sampling circulatory fluid—the equivalent of blood—from migrating crabs, Turner and colleagues discovered that a surge in the crustacean hyperglycemic hormone works with glucose, an energy-producing sugar, to fuel the epic trek.

The crab's endocrine system also stockpiles enough sugar to allow the crabs to return to their forest homes, Turner added.

Invasion of the Crabs

Children watch migrating Christmas Island crabs (file photo), which overtake Australia's Christmas Island at the beginning of the wet season, in October or November.

The crab march "completely disrupts most people's daily lives," even shutting down major roads, study co-author Turner said. But Christmas Islanders have "learned to live with it."

Some residents are also "quite protective" of the 8-inch-long (20-centimeter-long) crustaceans--schoolchildren put up signs encouraging people to drive carefully, she said.

"Having said that, there are so many crabs, some get squashed."

Crab Swarm Spawns by the Sea

Once Christmas Island crabs arrive at their beachside mating spots, males dig burrows and defend them from marauders, study co-author Turner said (file photo).

A female mates in a male's burrow, then makes a beeline for the ocean. Her eggs hatch almost immediately after contact with the seawater, Turner said.

The baby crabs develop in coastal sea grasses, eventually coming onto land to repeat the life cycle.

Beach Trip

Of Christmas Island's 14 crab species, the Christmas Island red crab (above, crabs migrating in an undated picture) is the most easily spotted, according to the Christmas Island National Park website.

The crabs burrow in a variety of habitats, including coastal terraces, dense rain forests, and even domestic gardens, according to the website.

During the dry season, a crab will plug its burrow entrance with leaves to keep the space humid—and disappear for up to two or three months.

 Crab Standoff

During their 2007 fieldwork, Turner and colleagues had to deal with curious crabs (above, two Christmas Island red crabs in an undated picture).

The zealous climbers got into the research station and even the scientists' houses. "If you leave the door open, you're probably going to find one in there," she said.

The same went for the basketball-size coconut crab (see picture), the world's biggest land crab, of which Christmas Island has the world's largest population.

Playing Throug

Golfers, including these 1980s players, on Christmas Island have learned to cope with the annual crab invasion, Turner noted.

"There's a special rule for golf courses: If the crab knocks the ball one way, you have to go with it," she said.

Fortunately, Christmas Island crabs aren't just a nuisance. Their feces acts as a crucial forest fertilizer, and their burrowing turns and aerates the soil, according to the Christmas Island National Park website.

Crabby Tourist

For the best crab viewing, Australian officials advise tourists (pictured, a Christmas Island visitor in an undated photo) to arrive during the last quarter of the moon in either November or December.

The crabs breed on a strict timetable: Unwaveringly, females spawn before sunrise during the last quarter of the moon, according to the Christmas Island National Park website.

On Track for Disaster

Of the hazards the migrating crabs encounter, human infrastructure is the greatest (file photo).

Christmas Island has erected some "crab crossings": walls or plastic fencing along roads that detour  the crab swarms away from traffic, according to the Christmas Island National Park website.

Christmas Island crabs VIDEO

This island gets swamped with crabs every year as hundreds of millions of them migrate and reproduce during their mating season