The Crabs of Christmas Island

In 1615, about 200 miles south of the Indonesian islands of Java and Sumatra, Europeans first spied a 52-square-mile spit in the distance. Nearly three decades later, Captain William Mynors sailed aboard the Royal Mary for the East India Company. On 25 December 1643, Mynors spotted the same isle and decided to anoint it Christmas Island. In 1688, two Europeans stepped onto Christmas Island for the first time, finding it uninhabited.

Of course, Indigenous peoples from the region might have visited Christmas Island before Europeans, but we have no recorded evidence of serious exploration or occupation before the 19th century. The large gap in exploration arrives largely because of the island’s ruggedness, as sharp cliffs ring most of the perimeter. Phosphate deposits attracted the attention of the British Crown, who annexed the island in 1888.

After an invasion by the Japanese during World War II, Australia managed to coax the administration of Christmas Island from Britain for the sum of $20 million. Since 1958, the island has officially been an external territory of Australia. Today, approximately 1,600 humans call Christmas Island home, most of them in the wonderfully named capital of Flying Fish Cove. Wilayah Pulau Krismas, as it’s known in Malay, is so sparsely populated by people that Flying Fish Cove sometimes goes by the sobriquet “The Settlement.”

This dearth of humanity and its isolation in the Indian Ocean have created an evolutionary Petri dish out of Christmas Island. On this contained blob are many species found nowhere else on the planet. One such critter produces an awe-inspiring spectacle that some have called a “wonder of the natural world.”

Christmas Island's spot on the globe - graphic by TUBS
The thriving metropolis of Flying Fish Cove - photo by David Stanley
The imposing cliffs of Christmas Island - photo by David Stanley

We associate crabs with the sea, but some species have evolved to become what scientists term terrestrial crabs. Some of these crustaceans have all but abandoned the water to live their best lives on dry land. One such denizen happens to call Christmas Island home.

The Christmas Island red crab inhabits just its namesake isle and one set of atolls to the south, the Cocos Islands. What the species lacks in geographic diversity it attempts to make up for in sheer numbers. At one point, more than 40 million red crabs existed on Christmas Island. Unfortunately, humans accidentally introduced a predator that has culled about a third of the crabs. Unless one has studied the crabs of Christmas Island, the identity of this invasive creature might surprise. The beautifully named yellow crazy ant can overwhelm crabs despite a massive difference in size. These ants garnered their appellation due to the bizarre movements they make when provoked.

The yellow crazy ant poses a significant barrier to the Christmas Island red crab as it makes an annual journey from its quarters in the forest to its ancestral home in the ocean. The crabs migrate each year to mate. Researchers have termed this movement one of Earth’s greatest migrations. When one sees millions of red crabs scurrying across Christmas Island, it’s easy to see why!

The Christmas Island red crab - photo by DIAC images

For some of the crabs, the migration might cover just a few miles. This short span can produce quite a gauntlet for the crabs, however. In addition to crazy ants waiting to munch on them, the crabs have to fight biology and climate. Though they have become landlubbers, these crabs still require a lot of water to survive. The rainforests of Christmas Island provide ample moisture, but the open grasslands and rocky shores might as well be deserts to the crabs. Some dehydrate or suffocate on the way. Another impediment arrives because of human infrastructure. Christmas Island is relatively undeveloped, but roads crisscross the land and automobiles can become just as deadly as invasive ants.

Like the puffling patrol of Iceland’s Heimay, the people of Christmas Island have become stewards to the crabs. They do their best each year to cater their modern lives to the ambling crabs. Roads shut down and volunteers bear rakes to usher crabs across the pavement. When one sees millions of red crabs scurrying across Christmas Island, it’s easy to see why they help!

Crabs on their annual migration - photo by Ian Usher

Longtime readers of the project will recall our investigation of wildlife bridges, which marked the first time Christmas Island appeared in our annals. Some places have crafted rather ingenious thoroughfares for fauna, but the folk of Christmas Island have taken it up a notch.

The video above illustrates some of the infrastructural methods they have crafted to aid the crabs. They built temporary fences to guide the crabs to safe crossings; they constructed underpasses for the roads; and they assembled a glorious, lofty bridge.

The fortunate crabs that reach the beach have not even completed half the journey.

Males arrive first. At the shore, they dig love bungalows in the sand. When the females arrive, crabs mate and the males head back to the forest, crossing bridges, underpasses, and roads in reverse. The females remain for another two weeks, incubating their eggs.

One spectacular attribute of this migration relates to timing. The crabs use the moon! The journey begins during the third quarter of the moon cycle, which allows them to line up ready eggs with a high tide. Since these land crabs have lost the ability to swim, entering the ocean is usually a death warrant. Yet, the crab’s eggs still need to be released into the water to survive. By timing the tides, the female crabs can catch the water closer to their burrows. They take a few steps into the water, release their eggs, and get out of Christmas Dodge. 

Ironically for land crabs, these crustaceans spend their first several weeks alive in the ocean. When the eggs hit the ocean water, they immediately hatch. The youngsters eventually grow into a larval stage called a megalopae, which somewhat resemble shrimp in the water. Each time the babies change from one form to another, they moult. When megalopae are finally ready to become tiny crabs, they gather at the shore, drop one last skin, and emerge from the ocean measuring just 5 millimeters in length!

Millions of these munchkins must then recreate the second half of the journey their parents just undertook. This first trip is either just as thrilling to the nature lover as the movements of adults or nightmare fuel, depending on your perspective:

Christmas Island Indian Ocean Red Crab Megalope stage - photo by Chook keeper
A red crab megalopa - photo by Christ

The juveniles that make it to the forest will then bide their time, as Christmas Island red crabs grow rather slowly. Not until they are four or five years of age will they commute to the ocean via the crab bridge.

Though the humans of Christmas Island have become more and more tolerant of these unique inhabitants, the crabs have another protection on their side. Australia designated two-thirds of the island as a National Park in 1980. The yellow crazy ant is a legitimate threat to the crabs, but, absent the human threat so dangerous to many species across the globe, they seem to be in decent shape when it comes to extinction. This lack of imminent danger is a rarity in the modern world when it comes to endemic species on isolated islands.

Like the puffins of Heimay, the crabs of Christmas Island present us with another data point of how animals can thrive when humans realize we’re all part of a massive ecological web. Though it’s a cheap usage of an easy cliche, these red crabs and their vitality are a Christmas miracle!

Further Reading and Exploration

Christmas Island National Park – Official Website

Christmas Island – Geoscience Australia

Red crab migration – Parks Australia

Christmas Island Is 63 Percent National Parkland…And Has More Crabs Than People – Smithsonian Magazine

Red Crab Migration – National Geographic

Ecology and Behavior of Gecarcoidea natalis, the Christmas Island Red Crab, During the Annual Breeding Migration – Adamczewska/Morris/The Biological bulletin

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