Crabs Get A Safety Bridge For Passage To The Sea

A new crawlable surface has induced many of Christmas Island's crabs to use a bridge to get to the sea to breed. Kristy Falkner, supplied by CITA

The Christmas Island crab migration, considered a wonder of the planet, has begun, and this year the crabs have some assistance. Thousands of crabs are using the world's only purpose-built crab bridge to scuttle safely from the forests to the oceans to spawn.

The crabs of Christmas Island have evolved an unusual lifestyle for a crustacean, with 13 species of land crabs living most of their lives in forests. Once a year tens of millions of red crabs (Gecarcoidea natalis) make their way to the oceans to breed, which Australia's Department of the Environment says can only happen “before sunrise during the last quarter of the moon.” After mating the males head back inland while the females spend a fortnight at the beach before laying their eggs and then also returning to the forests.

With up to forty million crabs needing to go first one way, then the other, all of them doing it simultaneously, migration paths become covered with red scuttling bodies. Some roads are closed to accommodate the migration, but the busiest road on the island is more of a challenge, although the Christmas Island Tourism Association's Linda Cash admits it only has about 50 vehicles a day. “A lot of those are trucks from the phosphate mine,” Cash told IFLScience, “So they could do a lot of damage to the crabs.”

Road vehicels can do a lot of damage to the crabs, but their pincers can puncture tires as well, so seperation is important: Credit: Max Orchard.

Rangers from the Christmas Island National Park have built 31 underpasses on the island, accompanied by 20 kilometers (13 miles) of barriers to prevent crabs getting onto the roads. However, several years ago one location was found to be unsuited to tunneling. A bridge over troubled gravel was built instead. According to Cash: “In previous years the crabs didn't use the bridge much.” However, a redesigned surface the crabs find easier to climb has changed all that, and this year the bridge is awash with crabs.

“Sydney can have its Harbor Bridge and San Francisco its Golden Gate Bridge, but it's our crab bridge which is currently wooing tourists from all over the globe,” Cash said in a statement, that may have exaggerated the number of visitors to the remote island.

The arrival of the yellow crazy ant (Anoplolepis gracilipes) on the island led to a 30 percent reduction in crab numbers over recent decades, with dire implications for the local ecology. Fortunately the Park Authority's hard work to control the ant has shown some success and Cash told IFLScience: “Last season we had the biggest return of crab babies for twenty years.”

A season like that is important because in most years the spawning fails, with few, if any, babies returning. However, in a good year the eggs hatch into larvae immediately after being dropped into the sea. Four weeks later the young come ashore in a larval stage known as megalops. After a short period in rockpools the megalops turn into baby crabs half a centimeter (one fifth of an inch) across and return along the same paths their parents used.

The tininess of the baby crabs makes their return journey even more astonishing. Credit: Justin Gilligan

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