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.’, ‘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.
Christmas Island sees a major spectacle every year, around the beginning of the wet season (usually October / November).
100 million red crabs of Christmas Island begin their march, leaving the forests, heading towards the coast driven by the basic instinct: to reproduce.Tides of red carapaces, enough to be seen from the air, suddenly begin a spectacular migration from the forest to the coast, to breed and release eggs into the sea. The rainy seasons offers the right humid conditions for crabs to make their long and difficult journey to the sea.
Description
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.
Ecology
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 highway 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.
Population
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.
When to go & Weather
The timing of this event is linked to both the rainy season and the phases of the moon.
The migration, dipping, mating, and egg-brooding sequences take place before the females spawn. All these events occur in the 4 weeks preceding spawning, and one cannot pinpoint the precise date of the beginning of the migration before spawning.
Some believe the phases of the moon also affect the timing of the migration, 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 level of the sea at the base of the coast and on the beaches varies the least and this means it is safer for the females approaching the water to release their eggs. All migrations are timed with the same lunar rhythm, even if they occur earlier or later.
How long does it last?
The main migration can last up to 18 days. Tides or “streams” of thousands of crabs move towards the coast, crawling over all obstacles in their way, negociating the same precise route used the previous year for both the “outbound” and “return” journeys. The crabs must not be caught for long in open areas where it’s hot, because the heat causes them to loose water and die. They’re quite smart little buggers, so the top moving period is in the early moist mornings and late afternoons when the shade protects them and the temperature is lower.
Impact of Humans
Many crabs perish during the migration due to the impact of human activity, in particular by drivers.
The island has worked hard in coming up with measures to reduce the number of crabs killed every year: Road signs, Barriers, Road closures, Underground tunnels, Diverted traffic and a variety of “crab crossings” have all been introduced in the recent years.
Source/info: http://en.wikipedia.org/wiki/Christmas_Island_red_crab
Christmas Island Website: http://www.christmas.net.au/
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