WATCH: Millions of Red Crabs Swarm Christmas Island Every Year

Tied to the lunar calendar, this massive migration usually begins during the island's wet season, when millions of crustaceans travel from the rain forest to seaside breeding grounds to mate
<p>When the wet season returns, red crabs begin a legendary mass migration to their seaside breeding grounds on Christmas Island, Australia.</p>

When the wet season returns, red crabs begin a legendary mass migration to their seaside breeding grounds on Christmas Island, Australia.

Photograph by National Geographic Television
Common Name:
Christmas Island Red Crab
Scientific Name:
Gecarcoidea natalis
Type:
Invertebrates
Diet:
Omnivore
Group Name:
Stream
Size:
5 inches
IUCN Red List Status:
Not evaluated
Current Population Trend:
Unknown

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.

Habitat and Diet

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.

Migration

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.

Reproduction

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.

This photo was submitted to Your Shot, our photo community on Instagram. Follow us on Instagram at @natgeoyourshot or visit us at natgeo.com/yourshot for the latest submissions and news about the community.
This photo was submitted to Your Shot, our photo community on Instagram. Follow us on Instagram at @natgeoyourshot or visit us at natgeo.com/yourshot for the latest submissions and news about the community.
Photograph by Kirsty Faulkner, National Geographic Your Shot

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