Casts of crabs make their way from the forests of Christmas Island over to the ocean to lay their eggs. (Credit: Courtesy of Parks Australia)

In preparation for the upcoming peak migration season for Australia’s Christmas Island red crabs (Gecarcoidea natalis), officials recently completed the third aerial baiting program to eliminate the crab’s most destructive predators — the yellow crazy ant (Anoplolepis gracilipes). This invasive species of ant — accidentally introduced to the Island between 1915 and 1935 — sprays formic acid into a crab’s eyes, which blinds and eventually kills it. The ant then feeds on its victim. In the absence of a natural predator, colonies and supercolonies of ants have been responsible for reducing the Island’s red crab population by one-quarter to a third over the last several years. Still, in previous rounds of aerial baiting, officials have successfully eradicated about 99 percent of targeted supercolonies, allowing the Island’s 45 million red crabs to continue their annual migration (Parks Australia, Christmas Island National Park: Yellow Crazy Ants).

The migration — commonly referred to as “one of the wonders of the natural world” — will begin in another month or so, when millions of red crabs will begin their annual crawl from the forest to the coast to lay their eggs in the ocean. The island’s red crabs are critical to its rainforest ecosystem: Their droppings serve as fertilizer for the forest floor, while their burrowing helps aerate the soil. Further, the crabs’ selective feeding on seeds and seedlings helps maintain the island forest’s unique composition (Parks Australia, Christmas Island National Park: Red Crabs).

Signs such as this remind Islanders to watch out for red crabs during the species’ annual migration. (Credit: Courtesy of Parks Australia)

The crab migration season tends to correspond with the onset of the wet season and typically lasts from October through December. During that time, endless casts of red and orange crabs are seen on Christmas Island’s beaches, school playgrounds, and streets. In an effort to protect these colorful crustaceans from accidental encounters with humans (and their vehicles), staff with the Christmas Island National Park, which covers 63 percent of the Island, has erected crab-crossing signs to warn drivers to slow down and watch out for the migrants. Further, red crabs run the risk of dehydrating when forced to cross areas that have been cleared of forest cover. To keep that from happening, conservationists have also set up plastic walls and fences to direct crabs to crab bridges and underpasses that will help get them across quickly and safely.

As always, authorities will track this mass migration and provide updates on crab movements on the local radio as well as public notice boards.