RED CRAB MIGRATION
Christmas Island red crab
The Christmas Island red crab, Gecarcoidea natalis, is a species of terrestrial crab endemic to Christmas Island in the Indian Ocean. Although restricted to a relatively small area, it is estimated that up to 120 million red crabs may live there, making it the most abundant of the 14 terrestrial crab species on Christmas Island. Christmas Island red crabs eat mostly fallen leaves and flowers, but will occasionally eat other animals, including other red crabs if the opportunity arises.
The carapace is up to 116 mm long, rounded, and encloses the gills. The claws are usually of equal size, unless one becomes injured or detached, in which case the limb will regenerate. During that time, it will be the smaller of the two. The male crabs are generally larger than the females, while adult females have a much broader abdomen and usually have smaller claws. The broader abdomen of the female Christmas Island red crab only becomes apparent in the third year of growth.
Christmas red crabs live in burrows, in order to shelter from the sun. Since they still breathe through gills, the possibility of drying out is a great danger for them. They are famous for their annual migration to the sea in order to lay their eggs in the ocean. During the migration, the crabs cover the routes to the coast so densely that they can be seen from the air. Volunteers shovel the crabs off the roads and, although no harm is intended, some of the countless millions of crabs inevitably get injured.
Early inhabitants of Christmas Island hardly ever mentioned these crabs. It is possible that their famous large population size was caused by the extinction of the endemic Maclear's Rat, Rattus macleari in 1903, which may have kept the crab's population in control.
An exploding population of the Yellow crazy ant, an invasive species accidentally introduced to Christmas Island and Australia from Africa, is believed to have killed 15–20 million red crabs in recent years.
"At the beginning of the wet season (usually October/November), most adult red crabs suddenly begin a spectacular migration from the forest to the coast, to breed and release eggs into the sea. Breeding is usually synchronized island wide. The rains provide moist overcast conditions for crabs to make their long and difficult journey to the sea. The timing of the migration breeding sequence is also linked to the phases of the moon, so that eggs may be released by the female red crabs into the sea precisely at the turn of the high tide during the last quarter of the moon.
"It is thought that this occurs at this time because there is the least difference between high and low tides. The sea level at the base of the cliffs and on the beaches, where the females release their eggs, at this time varies the least for a longer period, and it is therefore safer for the females approaching the water's edge to release their eggs. Sometimes there are earlier and later migrations of smaller numbers of crabs but all migrations retain this same lunar rhythm.
"The main migration commences on the plateau and can last up to 18 days. Masses of crabs gather into broad columns as they move toward the coast, climbing down high inland cliff faces, and over or around all obstacles in their way, following routes used year after year for both downward and return migrations. Movement peaks in the early morning and late afternoons when it is cooler and there is more shade. If caught in open areas, in unshaded heat, the crabs soon lose body water and die."