Christmas Island red crabs

Christmas Island red crabs are famous throughout Australia and the world for their bright red color and for their spectacular annual migration to the sea. Millions of crabs become rivers of red as they move from the island’s interior rainforests to the ocean to breed and lay eggs.

During migration, red crabs climb over and around obstacles in their way, following the same migration paths every year. The round-trip journey can last up to 60 days, and begins at the start of the wet season, usually October or November in that part of the world.

Christmas Island red crabs live on Christmas Island in the Indian Ocean off Australia’s northern coast, about 240 miles (380 kilometers) south of Java, Indonesia.

Red crabs must keep their bodies moist, so they wait for the rainy season to provide conditions that are ideal for the difficult journey. The main migration activity takes place in the early morning and late afternoon, when it’s cooler and there’s more shade.

Male red crabs begin the charge to the ocean and are joined by females along the way. Larger males arrive at the ocean first, after five to seven days, but are soon outnumbered by females.

The red crabs’ breeding season is linked to the phases of the moon. The female red crabs release their eggs into the ocean at the turn of the high tide during the last quarter of the moon, which ensures the lowest sea level for the longest period of time. A single female can lay up to 100,000 eggs. The eggs hatch on contact with the water.

Most of the millions of hatchlings, called larvae, are eaten by fish and other sea creatures. After about a month, surviving larvae transform into young red crabs only 0.2 inches (5 millimeters) across.

The baby red crabs trek inland, a journey that takes about nine days, where they disappear to live among rocky outcrops and in underground burrows.

In some years, no baby red crabs emerge from the ocean. But a few very successful years—just one or two every ten years or so—are enough to maintain the Christmas Island red crab population, estimated at some 120 million.

FAST FACTS
The scientific name for the Christmas Island red crab is Gecarcoidea natalis.

An adult Christmas Island red crab’s body shell, called the carapace, can measure up to 4.5 inches (116 millimeters) across. Males are usually bigger than females, with larger claws.

To help protect migrating red crabs, the Australian government closes roads along popular migration routes.

Red crabs eat leaves, flowers, fruits, and seedlings, though some may eat dead birds and other dead crabs.