Photo: Close-up of a crab

The red crab is a Christmas Island, Australia, original found nowhere else in the world—some 120 million individuals cover the rain forest floor.

Photograph by WaterFrame, Alamy

Map

Map: Red crab range

Red Crab Range

Fast Facts

Type:
Invertebrate
Diet:
Omnivore
Size:
5 in (12.7 cm)
Group name:
Streams
Protection status:
None
Did you know?
Red crabs can stop traffic, puncture tires, and wander through any open doors during their migration to the sea.
Size relative to a tea cup:
Illustration: Crab compared with tea cup

The red crab is a Christmas Island, Australia, original found nowhere else in the world. But on its home turf it is a very significant species—some 120 million individuals cover the rain forest floor and play a major role in determining the structure of the ecosystem.

These large crabs are active during the day but prefer to stay in the shade and can die in the moisture-robbing heat of direct sunlight. They scavenge on fallen leaves, seedlings, fruits, and flowers, recycling nutrients and helping to determine the spread and composition of native flora.

Most of the year red crabs are solitary dwellers of the burrows they dig throughout the forest. During the dry season they retreat into these shelters to retain body humidity and essentially remain there for two to three months.

But when wet season returns in October or November they begin a legendary mass migration to their seaside breeding grounds, moving in colorful waves that wash over all obstacles including roads (necessitating crab tunnels and road closings) and even seaside cliffs.

The annual trek is also intimately tied to the lunar schedule. The crabs arrive at the coast and mate at such a time that the females can produce eggs and develop them in burrows for a dozen or so days before releasing them into the sea precisely when high tide turns between the last quarter and new moon. During this period sea level on the beaches varies the least and offers an easier approach, a factor so important that if weather delays the migration crabs will put off spawning until the next lunar month.

Red crab eggs hatch right away, and young live as larvae in the sea for a month before returning to the shoreline, molting into air breathers, and slowly returning inland to begin the cycle anew.

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