Pictures: Crab Swarms Overtake Island—Mystery Solved

Invasion of the Crabs

Photograph from Photolibrary

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." 

 (Related pictures: "A World of Crabs from One Tiny Island.")

Published September 3, 2010

Crab Swarm Spawns by the Sea

Photograph from Photolibrary

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 the waters around Christmas Island, eventually coming onto land to repeat the life cycle.

(See related picture: "She-Crabs Are World's Pickiest Daters.")

Published September 3, 2010

Playing Through

Photograph by Roger Garwood, National Geographic

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.

Published September 3, 2010

Crab Standoff

Photograph courtesy Mrinalini, Bangor University

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.

(See pictures of another coconut crab habitat, the Southern Line Islands.)

Published September 3, 2010

Beach Trip

Photograph from Photolibrary

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.

Published September 3, 2010

Crabby Tourist

Photograph by Roger Garwood, National Geographic

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.

(Learn more about the phases of the moon.)

Published September 3, 2010

On Track for Disaster

Photograph by Roger Garwood, National Geographic

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.

(Related: "New Public Road to Split the Serengeti?")

Published September 3, 2010