Australia's Christmas Island used a bridge to solve road mortality for 50 million migrating crabs; the result transformed dangerous transit routes into safe canopy crossings

Every year, something extraordinary happens on Christmas Island. Millions of bright red crabs leave the forests together and begin a long march toward the ocean, crossing roads, streams, rocks and beaches along the way. The journey is essential. The crabs are heading to the coast to mate and release their eggs, following a remarkable timetable controlled by rainfall, the moon and the tides. But there is a problem in their path-roads.

With tens of millions of crabs moving across the island at once, traffic can become a serious obstacle. So Christmas Island rangers have built an unusual solution above one of the island’s busiest roads, while also creating dozens of underpasses to help the migrating animals continue toward the coast.

The five-metre-high “crab bridge” has become more than a crossing. It has also become a tourist attraction, offering a remarkable view of one of the world’s great wildlife migrations. The report is taken from Christmas Island National Park, The AP, The BBC and ABC News.

Why do millions of crabs cross the island?

The red crab migration begins with the first rainfall of the wet season, usually in October or November, although it can sometimes begin as late as December or January.

The rain provides the moisture the crabs need for their journey. Once conditions are right, red crabs across Christmas Island leave their forest homes and start moving toward the ocean.

The moon then helps determine what happens next.



Red crabs spawn before dawn during a receding high tide in the last-quarter phase of the moon. Their movement is therefore tied not only to rainfall but also to a very specific lunar and tidal window.

If rain arrives close to the ideal spawning date, the crabs can move quickly. When rainfall comes earlier, they may travel more slowly, stopping along the way to eat and drink. If the rain arrives too late for the spawning opportunity, some crabs remain in their burrows and make the journey the following month.

The scale is enormous. Around 50 million red crabs may take part in the annual migration.

How does the crab bridge help?

As the crabs make their way between the forest and the coast, roads become unavoidable obstacles. Christmas Island rangers have responded by putting up roadside barriers to prevent crabs from being crushed beneath vehicles. They have also constructed 31 underpasses across the island.

At one particularly important point, a five-metre-high bridge allows the crabs to cross above the road.

The structure has become known as the “crab bridge,” and its unusual purpose has made it part of the spectacle for visitors as well as a practical route for the migrating crabs.

Christmas Island Tourism Association marketing manager Linda Cash said aside from the practical use, the bridge had also become a tourist attraction.

“Sydney can have its Harbour Bridge, and San Francisco its Golden Gate bridge, but it's our crab bridge which is currently wooing tourists from all over the globe,” Ms Cash said.

“The ubiquitous crabs take the most direct route possible from their burrows in the rainforest terraces down to the coast, turning the roads, golf course and beaches into a crimson carpet.”

The island’s roads may also be closed during parts of the peak migration, sometimes at short notice, to protect the crabs as they travel.

What happens when the crabs reach the sea?

The largest male crabs generally arrive first, followed by the females. After crossing the island, the crabs enter the sea to replenish moisture before mating.

Male crabs then move to lower terraces and dig burrows. Because so many crabs are concentrated in the same areas, the burrows can be extremely close together, and males may fight over them.

Females join the males to mate in or around the burrows. After mating, the males take another dip in the sea before beginning their return journey inland.

The females remain behind in moist burrows while their eggs develop.

A female can produce as many as 100,000 eggs, carrying them in a brood pouch. About two weeks later, when the moon reaches its last quarter, the egg-bearing females leave their burrows and gather along the shoreline.

In some places, as many as 100 crabs can occupy a single square metre of beach or rock.

Before dawn, as the high tide begins to recede, the females enter the sea and release their eggs. Spawning can continue for five or six consecutive nights.

What happens to the baby crabs?

The eggs hatch almost immediately after reaching the water. The newly hatched larvae gather close to shore before waves and the outgoing tide carry them into the ocean. Over roughly the next month, they pass through several larval stages before becoming prawn-like megalopae.

These gather in pools near the shore for a day or two. Eventually, they develop into tiny crabs and emerge from the water. At only about 5 millimetres across, the young crabs then face another difficult journey. They begin moving inland, taking around nine days to reach the plateau, where they remain hidden among rocky outcrops and forest debris during their first three years.

Most never make it that far. Fish, manta rays and enormous whale sharks feed on the larvae during the annual feast. In most years, few or no baby crabs successfully emerge from the sea. Once or twice a decade, however, huge numbers survive, helping maintain the island’s large red crab population.

The migration therefore depends on a delicate sequence of rainfall, moon phase, tides, spawning and survival.

For the adult crabs making the journey, the island’s unusual network of barriers, underpasses and the five-metre-high bridge provides ways to negotiate one of the most difficult obstacles created by human activity.

And for visitors, the sight is unlike almost anything else. During migration, roads, beaches and even the golf course can appear to become a moving crimson carpet as millions of red crabs follow the same ancient route toward the sea.

The migration is also Christmas Island’s biggest tourist attraction. Drumsite, Flying Fish Cove, Ethel Beach and Greta Beach are among the easiest places to watch the crabs and their spawning activity.

Public notice boards and local radio provide updates on crab movements, while visitors are asked to follow signs and the advice of national park staff.

FAQs

When do Christmas Island red crabs migrate?
Usually during October or November after the first wet-season rains.

How many crabs migrate?
Around 50 million red crabs can take part.