Christmas Island Red Crabs: Characteristics, reproduction and habits

It is time to learn about  the Christmas Island Red Crabs or Gecarcoidea natalis in scientific terms, which could be  called also «the travelling crabs.»

These creatures are land crabs that  are endemic to Christmas Islands and the Keeling Islands in the Indian Ocean. Moreover they are quite important in their homelands, since around 120 million individuals cover the base of the rainforest and play a crucial role in the structural determination of the ecosystem.

 Christmas Island Red Crabs Main Characteristics

These large crabs are active during the day, but prefer to lie in the shade because they can die dehydrated due to the suffocating heat of direct sunlight. It is common to see them digging around the fallen leaves, seeds, fruits and flowers recycling nutrients and collaborating with the morphology and composition of the native flora.

They are also very popular because of their annual mass migration, which they carry out in order to lay their eggs into the ocean

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 Christmas Island Red Crabs Anatomic Features

These crustaceans are fairly large crabs whose shells measure up to 116 millimeters (4.6 in) in width. Their pincers are usually the same size, unless one is injured or separated, in which case such limb will regenerate.

Male Christmas Island Red Crabs are generally larger than females, while adult females have a much wider abdomen and  smaller pincers.

In terms of colors, we can point out that bright red is the most common tonality for this species. But don´t be surprised if you find an orange or purple specimen.

Habitats

Like the majority of land crabs, these crustaceans employ gills to breathe and must be very careful to conserve their bodies´ moisture.

They are characterized by being diurnal creatures, although they avoid direct sunlight so they do not dry out. The Christmas Island red crabs are almost completely inactive at night, in spite of the lower temperatures and higher humidity. They usually dig burrows to protect themselves from the sun. During the dry season, they tend to cover the entrance of their burrows with a bundle of loose leaves, to maintain a high humidity in such places.

Then they remain there for 3 months until the beginning of the rainy season.

When the rainy season arrives in October or November, they begin their legendary migration in the direction of the mating grounds by the sea, moving in colorful and massive waves that flood and surpass all obstacles, including roads (which require tunnels for crabs or temporary closures) and even cliffs facing the ocean.

How Christmas Island Red Crabs Reproduce?

For most of the year, red crabs can be found within the forests of the Christmas Islands, however, each year they must migrate to the coast to breed. The beginning of the rainy season ( from October to November) allows these crabs to increase their activity and stimulate their annual migration. The right moment of their migration is also related to the phases of the moon.

When the tide rise up, between the waning quarter and the new moon, the sea level of the beach presents less variation and offers a simpler approach, a factor so important that if the time were to be delayed, the migratory crabs would postpone the spawn until the next lunar cycle

During their journey, the red crabs leave their burrows and travel to the coast to mate and reproduce.

This usually requires about a week, and male crabs usually arrive before females. Once on the shore, male crabs dig burrows, which they defend from other males. Then, mating occurs in or near the burrows. Shortly after mating, the males return to the forest, while the females remain in the burrow for another two weeks to spawn. As soon as the incubation period finishes, the females come out of their burrows and throw their eggs into the ocean. This occurs precisely at the change of high tide during the last quarter of the moon. Then the females return to the forest, while the crab larvae spend another 3-4 weeks in the sea before returning to the land as juvenile crabs.

Life cycle

The eggs released by the females hatch on contact with seawater. Then the clouds of crab larvae swirl near the coast until they are washed into the sea, where they remain during 3 or 4 weeks.  During this time, the larvae go through several larval stages, and eventually become shrimp-like animals called megalopae. The megalopas gather near the coast for 1-2 days before transforming into young crabs of 5 mm (0.20 in) wide. Young crabs leave the water for a 9-day trip to the center of the island.

During the first three years of their lives, young crabs remain hidden on rocky outcrops, fallen tree branches and debris on the forest floor. Red crabs grow slowly, reaching sexual maturity around 4-5 years. Then they begin to participate in the annual migration. During their first growth phases, red crabs will molt their shells several times. Mature red crabs molt their shells once a year, usually in the safety of their burrow. (See our article crabs to learn about the different molting patterns of other species of crabs

Christmas Island Red Crabs Alimentation Habits

Christmas Island red crabs are usually opportunistic omnivorous scavengers. They often eat fallen leaves, fruits, flowers and seedlings, and dead animals in some occasion (including other red crabs,) .The non-native African terrestrial snail is also another food option for these crabs. Red crabs have virtually no competition for food due to their dominance of the forest floor.

Christmas Island Red Crabs Population

According to several studies there is a density of 0.08-0.50 red crabs per square meter, which equates to an estimated total population of 43.7 million on Christmas Island. There is less information available for the population in the Cocos (Keeling) Islands. According to genetic evidence, it seems that Cocos Islands Red Crabs are relatively recent immigrants to Christmas Island, and for conservation purposes the two can be managed as a single population.

Christmas Island Red Crabs Predators

The whale shark is one of the few natural predators of the Christmas Island red crabs. Adult red crabs do not have natural predators on Christmas Island.  However, it is believed that an explosive population of the crazy yellow ant, which is an invasive species accidentally introduced to Christmas Island and Australia from Africa, has killed around 15.000 million red crabs in recent years. This number represents around a quarter of the total population of this species.

Their name comes from their characteristic frenetic pace without direction that makes them seem ‘crazy’. These ants do not bite or sting, nor are they particularly large, so human beings have not paid much attention to them, but what they lack in size, they compensate with chemical weapons: formic acid. They are an enemy like fog or sand, against which force does not work.

In order To reduce the impact of the yellow ants and preserve the ecosystem of the National Park, the Government of Australia, carried out a comprehensive field study in 2002, to locate exactly where the super colonies were located. More than 900 sites covering more than two thousand hectares were detected. In 2009, a helicopter began distributing fipronil-based baits. The population the crazy ants on the island dropped by 98% in a few days.

However, the insecticide is lethal also for twenty other species on the island. In addition, park officials use the poison only if an ant colony has reached super-colony status, so as not to compromise biodiversity. So there will always be smaller colonies where the poison cannot be applied.

The solution: micro-wasps

Chemical control has slowed the decline of the red crab population, but it is not a realistic long-term solution. As always, only Nature can restore balance. And this is where the last character in the story appears: the Tachardiaephagus somervillei, a parasitic micro-wasp, about one-tenth of what its scientific name is in writing.

The small wasp, originally from Malaysia, does not bite or build nests, but hunts the larvae of an insect that produces a sugary substance called honeydew, which makes mad ants ‘crazy’ and represents their main food. In an experiment in which the yellow ants were deprived of their precious honeydew, the density of the colony was reduced by 80 percent in just a few weeks. The approval of the Australian Government to the import plan of the wasps is expected for this year.

Probably the little wasp will be the savior of the placid Christmas Island Red Crab. In the meantime the crazy ants don´t stop and move fast. They have already landed in northern Australia.

Christmas Island Red Crabs Migration Journey

We must start mentioning that the surprising thing about this phenomenon is that every year there are more than 100 million crabs that cross the island. This journey becomes almost like a national holiday. Believe it or not, the inhabitants help the crabs to cross the highways and roads.  The government blocks some of the roads of the country out, to prevent the crustaceans from being crushed by the wheels of the vehicles. Even fences and «crabs bridges» have been built on the roads, in order to redirect these small animals to safer places. Unfortunately, it has not been possible to prevent a few thousand of these crabs from dying each year, when they´re trying to reach their goal.

The most important of all this, is that we should praise the measures taken on Christmas Island, where a clear affection for these animals is demonstrated. It is an example that we all follow, whatever the place where we live, and the species that we are taking care of. Remember that biodiversity is essential for our planet. We are the main predators of all animals and plants.

Finally , Let´s enjoy this video