The Christmas Island Red Crab ( Largest Migration of Crab), Christmas Island, Australian Territory in the Indian Ocean

The Christmas Island red crab (Gecarcoidea natalis), is a species of land crab that is endemic to Christmas Island and the Cocos (Keeling) Islands in the Indian Ocean. Although restricted to a relatively small area, it has been estimated that 43.7 million adult red crabs lived on Christmas Island alone, but the accidental introduction of the yellow crazy ant is believed to have killed about 10–15 million of these in recent years. Christmas Island red crabs eat mostly fallen leaves and flowers, but will occasionally eat other animals, including other red crabs (see cannibalism) if the opportunity arises.

Christmas Island red crabs are rather large crabs with the carapace measuring up to 116 millimetres (4.6 in) wide. The claws are usually of equal size, unless one becomes injured or detached, in which case the limb will regenerate. The male crabs are generally larger than the females, while adult females have a much broader abdomen (only apparent above 3 years of age) and usually have smaller claws.  Bright red is their most common color, but some can be orange or the much rarer purple.

Like most land crabs, red crabs use gills to breathe and must take great care to conserve body moisture. Although red crabs are diurnal, they usually avoid direct sunlight so as not to dry out. Despite lower temperatures and higher humidity, red crabs are almost completely inactive at night. Red crabs also dig burrows to shelter themselves from the sun and will usually stay in the same burrow through the year. During the dry season, they will cover the entrance to their burrows with a loose wad of leaves to maintain high humidity in their burrow and will virtually disappear for 3 months until the start of the wet season. Apart from their breeding season, Red crabs are solitary animals and will defend their burrow from intruders

For most of the year, red crabs can be found within Christmas Islands' forests, however, each year they must migrate to the coast to breed. The beginning of the wet season (usually October/November) allows the crabs to increase their activity and stimulates their annual migration. The timing of their migration is also linked to the phases of the moon. During this migration red crabs abandon their burrows and travel to the coast to mate and spawn. This normally requires at least a week, with the male crabs usually arriving before the females. Once on the shore, the male crabs excavate burrows, which they must defend from other males. Mating occurs in or near the burrows. Soon after mating the males return to the forest while the females remain in the burrow for another two weeks to lay their eggs. At the end of the incubation period the females leave their burrows and release their eggs into the ocean. This occurs precisely at the turn of the high tide during the last quarter of the moon. The females then return to the forest while the crab larvae spend another 3–4 weeks at sea before returning to land as juvenile crabs

Surveys have found a density 0.09–0.57 adult red crabs per square meter, equaling an estimated total population of 43.7 million on Christmas Island. Others have estimated that about 120 million are found on this island, but the basis for that claim is unclear.

During their annual breeding migration, red crabs will often have to cross roads, sometimes as many as 3 or 4, to get to their breeding grounds and then back to forest. As a result, red crabs are frequently crushed by vehicles and sometimes cause accidents due to their tough exoskeletons which are capable of puncturing tires. To ensure both the safety of crabs and humans, local park rangers work hard to ensure that the crabs can safely cross the island to the coast. Park rangers set up aluminum barriers called "crab fences" along heavily traveled roads. The crab fences funnel the crabs towards small underpasses called "crab grids" so that the crabs can safely cross under the roads. In recent years, the human inhabitants of Christmas Island have become more tolerant and respectful of the crabs during their annual migration and are now more cautious while driving, which helps to minimize crab casualties. Further, "a five meter high bridge has also been constructed at one point along the road to help the crabs move across the island and continue their migration.

For more details- Christmas Island red crab Migration