Christmas Island red crab Facts for Kids

Quick facts for kids Christmas Island red crab


Megalopae of the crab
Scientific classification

Distribution map of Christmas Island red crab

Female red crab about to release eggs into the sea

The Christmas Island red crab (Gecarcoidea natalis) is a special type of land crab. It lives only on Christmas Island and the Cocos (Keeling) Islands in the Indian Ocean. Millions of these red crabs once lived on Christmas Island. But an insect called the yellow crazy ant was accidentally brought to the island. These ants have sadly killed many millions of red crabs.

Every year, Christmas Island red crabs make a huge journey. They travel from the forest to the sea to lay their eggs. Even though the ants are a big threat, the red crab was not listed as endangered by the International Union for the Conservation of Nature (IUCN) as of 2020.

What Do Red Crabs Look Like?
How Do Red Crabs Live and Behave?
How Many Red Crabs Are There?
Red Crabs and Humans
Images for kids
See also

What Do Red Crabs Look Like?

Christmas Island red crabs are quite large. Their top shell, called a carapace, can be up to 11.6 centimetres (4.6 inches) wide. Their claws are usually the same size. If a claw gets hurt or falls off, it can grow back!

Male crabs are generally bigger than females. Adult female crabs have a much wider lower body (called an abdomen). You can only see this difference when they are older than 3 years. Females also usually have smaller claws. Most red crabs are bright red. But some can be orange, and a few rare ones are purple.

How Do Red Crabs Live and Behave?

Daily Life of a Red Crab

Like most land crabs, red crabs use gills to breathe. This means they need to keep their bodies moist. They are active during the day. However, they usually stay out of direct sunlight so they don't dry out. Even though it's cooler and more humid at night, they are almost completely still.

Red crabs also dig burrows. These burrows protect them from the sun. They usually stay in the same burrow all year. During the dry season, they cover the entrance to their burrow. This helps keep the inside humid. They can stay in their burrow for 3 months until the wet season begins. Most of the year, red crabs live alone. They will even defend their burrow from other crabs!

Amazing Migration and Breeding

For most of the year, red crabs live in the forests of Christmas Island. But each year, they travel to the coast to breed. The start of the wet season (usually October or November) makes them more active. This also starts their yearly journey. The timing of their trip is also linked to the moon phases.

During this migration, red crabs leave their burrows. They travel to the coast to find a mate and lay eggs. This journey usually takes at least a week. Male crabs often arrive at the coast before the females. Once they reach the shore, the male crabs dig new burrows. They have to protect these burrows from other males.

Mating happens inside or near these burrows. Soon after mating, the male crabs go back to the forest. The females stay in the burrow for about two more weeks. During this time, they lay their eggs. They keep the eggs in a special pouch under their body. This helps the eggs grow.

When the eggs are ready, the females leave their burrows. They release their eggs into the ocean. This happens exactly when the tide is high during the last quarter of the moon. After releasing the eggs, the females also return to the forest. The baby crabs, called larvae, spend about 3 to 4 weeks in the sea. Then they return to land as tiny young crabs.

The Red Crab Life Cycle

When the female crabs release their eggs into the sea, they hatch right away. Clouds of tiny crab larvae float near the shore. Then they are carried out to sea. They stay there for 3 to 4 weeks. During this time, the larvae change several times. They eventually become tiny, shrimp-like creatures called megalopae.

The megalopae gather near the shore for a day or two. Then they change into young crabs, only 5 millimetres (0.2 inches) wide. These young crabs then leave the water. They make a 9-day journey to the middle of the island. For their first three years, the young crabs hide. They stay in rocks, fallen branches, and other things on the forest floor.

Red crabs grow slowly. They can start breeding when they are about 4 to 5 years old. That's when they join the yearly migration. When they are young, red crabs shed their outer shell (moult) many times. Adult red crabs moult once a year. They usually do this safely inside their burrow.

What Do Red Crabs Eat?

Red crab eating dry leaves.

Christmas Island red crabs eat many different things. They are omnivores, meaning they eat both plants and animals. They are also scavengers, eating things they find. They mostly eat fallen leaves, fruits, flowers, and young plants. But they will also eat dead animals. This can even include other red crabs! They also sometimes eat human rubbish. The giant African land snail, which is not native to the island, is also a food choice for them. Red crabs have very little competition for food. This is because they are so common on the forest floor.

Who Are the Red Crab's Predators?

The whale shark is one of the red crab's few natural predators.

Adult red crabs have no natural predators on Christmas Island. However, the yellow crazy ant is a big problem. These ants are an invasive species. They were accidentally brought to Christmas Island from Africa. It's believed they have killed 10 to 15 million red crabs in recent years. This is about one-quarter to one-third of the total crab population! In total, the ants have caused 15 to 20 million red crabs to move away from their homes on Christmas Island.

When red crabs are in their larval stage, millions of them are eaten. Fish and large filter-feeders eat them. These include manta rays and whale sharks. These big ocean animals visit Christmas Island when the red crabs are breeding.

Coconut crabs, also called robber crabs, have also been seen eating red crabs on Christmas Island.

How Many Red Crabs Are There?

Studies have found many red crabs on Christmas Island. There are about 0.09 to 0.57 adult red crabs per square metre. This means there are an estimated 43.7 million red crabs on Christmas Island! Less is known about the population in the Cocos (Keeling) Islands. But there are fewer crabs there. Scientists believe the Cocos (Keeling) red crabs came from Christmas Island. So, for conservation, they can be managed as one group.

Red Crabs and Humans

During their yearly breeding journey, red crabs often have to cross roads. This means many crabs get run over by cars. Their hard outer shells can even puncture tires, sometimes causing accidents. To keep both crabs and people safe, local park rangers help the crabs. They set up aluminium barriers along busy roads. These barriers guide the crabs towards small tunnels under the roads. This way, the crabs can cross safely.

Other special structures help the crabs too. There's even a five-metre-high "crab bridge"! In recent years, people living on Christmas Island have become more understanding of the crabs. They are more careful when driving during the migration. This helps to reduce the number of crabs that get hurt.