Christmas Island Red Crab

“Here, There and Everywhere”

Lots of animals migrate, but few migrations are as sensational as that of the Christmas Island red crab. Prompted by rainfall and the phases of the moon, this little animal bestirs itself from its humid burrow and its solitary life and marches toward the sea as unstoppably as General Sherman. No one has yet figured out how a crab’s tiny brain figures out the phases of the moon or how it knows the precise moment, when the phase and the tides are just right, to release its eggs. Yet, it does know and has known for millennia. The result is a spectacle that fascinates the world.

Four Incredible Facts!

Here are some amazing facts about the Christmas Island red crab:

• The Christmas Island red crab is a land crab, but it still breathes through its gills and needs to stay moist. It will die if it dries out.
• It is endemic to Christmas Island and the Cocos islands in the Indian Ocean; that is, it is found nowhere else in the world.
• In order to keep migrating crabs from being run over and tires from being punctured, the Australian government has built bridges and tunnels for the crabs to cross in safety. In other places, cars are simply not allowed to be on the road during the migration.
• The weather during the rainy season is crucial to the migration. If it’s too dry, the crabs might wait another month before they venture from the forest to the coast.

Christmas Island Red Crab Classification and Scientific name

The Christmas Island Red Crab’s scientific name is Gecarcoidea natalis. The name comes from the Greek Ge, which means “earth,” and carcoid, which has something to do with “crabs.” Natalis is Latin for “birthday” and commemorates Christmas, the birthday of Christ. So the name can translate into something like “earth crab of the island named for Christ’s birthday.” There are no subspecies. A population has been established on the Cocos Islands, but it is of the same species.

The crab belongs to the Gecarcoidea genus, which contains only one other species, Gecarcoidea lalandii, a purple crab found on the Andaman Islands.

Appearance

The Christmas Island red crab is considered a large crab with a shell of nearly five inches wide and a weight of a little over a pound. Most of these crabs are a brilliant, eye-catching red color, though there have been crabs that are more orange or, rarely, purple. The males are bigger than the females, but their abdomens are narrower.

As the name of its order, Decapoda, suggests, the crab has ten legs. There are eight smaller legs, four on each side of the shell, and the two front legs have evolved into large claws called chelae. Unlike some other crabs, the chelae of the red crab are the same size unless one of them has been lost and is regrowing. Females tend to have smaller chelae than males.

Distribution, Population and Habitat

The Christmas Island red crab is found only on Christmas Island and the Cocos Islands in the Indian Ocean, which are now owned by Australia. Although these islands are quite small, scientists estimate that there are nearly 44 million to 50 million crabs, with most on Christmas Island. This is true even though a creature called the yellow crazy ant is responsible for killing millions of crabs. This rapacious ant was accidentally brought to the island from west Africa.

The red crab spends much of the dry season in its burrow in its rainforest habitat. The environment of the burrow is moist, a must for an animal that cannot dry out. It lives there alone and will drive off any trespasser. Sometimes, it will plug up the entrance of its burrow with vegetation or soil. The crab also molts in the safety of its burrow.

Unlike some fossorial animals, the Christmas Island red crab uses and maintains the same burrow for as long as five years, and the leaf litter that’s around the entrances creates a favorable area for plants to germinate. The crab’s droppings also provide fertilizer. This is how it contributes to its island’s ecosystem.

Predators and Prey

There are so many red crabs that it can’t be said that the species has natural predators, but it is sometimes taken by the huge coconut crab. Christmas Island red crab babies are most at risk for predation. While they are part of the zooplankton in the ocean, they are eaten by filter feeders like whale sharks, which seem to know the time of the breeding season because that’s when they show up on the coast. The tiny crabs are also eaten by all kinds of fish and birds.

The yellow crazy ant, which is an aggressive and invasive species, has not only killed millions of adult crabs but has replaced millions more. Though these ants don’t bite or sting, they invade the crab’s burrow and simply overwhelm it with sprays of formic acid. When the crab finally dies of exhaustion, the ants devour it.

Reproduction and Lifespan

The crab’s reproductive strategy has fascinated millions around the world. At the start of the rainy season, the usually solitary crab leaves its burrow and heads for the ocean. It does this even though it’s a land crab because land crabs still need to lay their eggs in water. Eventually, millions of these crabs make their way toward the coasts of Christmas and Cocos Islands. Depending on where the crab lives on the island, this journey can take about a week.

Males arrive first and dig burrows, and females join them later. After they mate in the burrow, the male leaves. The female starts producing eggs and keeps them in a special brood pouch on her abdomen for about two weeks. Then, before dawn, just when the high tide turns during the moon’s last quarter, she leaves the burrow and shakes her eggs into the water. Scientists have not figured out how the female knows to release her eggs at this precise time.

When the eggs hatch, the larvae join the clouds of plankton in the ocean. If they’re not eaten, it takes about a month for them to develop into tiny megalopae, then very tiny crabs. Then, they leave the water and make their way to the center of the island. This too can take a week or longer. They’ll hide under leaf litter, rocks, or tree branches. They will be old enough to join the yearly migration when they’re four or five years old. These crabs have a lifespan of 20 to 30 years.

Fishing and Cooking

Despite its great numbers, Christmas Island red crabs are not considered edible, at least not edible to humans.

Population

Despite the invasion of the yellow crazy ant, the population of the Christmas Island red crab is counted as over 44 million individuals, perhaps 50 million, and it helps that a female produces up to 100,000 eggs during the breeding season. The crab’s status remains of least concern. One of the facts that contribute to these great numbers is the extinction of Maclear’s rat, which used to keep the crab’s numbers in check.