Wednesday, 22 December 2010

Christmas Island Red Crab

Crabe rouge de l'île ChristmasImage via Wikipedia
The time has come to really start ramping up the jolly. To help you out in that I can think of nothing better than going off to a tiny isolated island to check out a small cold blooded beast with a tough, unyielding integument that is red like Santa Claus' red bits, red like an unwrapped Ferrari and red like blood. Or maybe sorrel.

The island in question is Christmas Island, an Australian territory named after the day of it's discovery. Actually it's closer to the capital city of Indonesia than to any Australian coast, but that's history for you.

Today we meet the Christmas Island red crab, a name which covers the basics though I will add that this creature is found nowhere else in the world, just this one tiny isolated island. You could add the word 'land' in there too, for like our very own Coconut Crab, this is a terrestrial crustacean, though at just 12 centimetres long excluding claws, it can't compete in size. Actually coconut crabs can be found on the very same island alongside a dozen other land crabs, but none can be found in quite the numbers of the red crab. That number is estimated at 120 million. Not bad for a tiny isolated island.

As you might expect with that kind of number, the Christmas Island red crab is very important to their local ecosystem. They live in burrows that they create for themselves to keep moist and for shelter from the sun, but this also aerates and turns the soil for the benefit of plants and trees.

They are largely vegetarian, eating fallen leaves, fruits, flowers and seedlings. This means they help scatter seeds and almost dictate what will grow where through sheer force of hunger. They also eat dead birds and crabs though, even the introduced African Giant Snail isn't safe.

Of course, what goes in must come out, so the droppings of these crabs provide fertiliser galore.

Despite moisture being so important to them, Christmas Island red crabs aren't nocturnal, so all their activities are overseen by the sun. During the worst of the dry season they will block their burrow entrance with a bundle of leaves and disappear for months at a time. During the wet season however, it's a completely different story.

As the clouds sail over the island to silently herald the wet season, the beating rays of the sun cannot compete with the cumulus, the nimbus and the cumulonimbus. The overbearing heat evaporates beneath the insistent power of the rains. From burrows in forest floors emerge red crabs, unleashed by the cool, moist air. They gather in groups, they group into clusters and they cluster into columns. From dozens to millions to dozens of millions, an unstoppable force marches. Through undergrowth they reclaim land, over boulders they invade cities, across roads they paint the town red. Down cliffs, they finally meet their object. At the coast, where sea meets land, they finally meet their fate. For there, by the beach, they mate.

This annual march is regarded as one of the natural wonders of the world, which seems quite appropriate to me!

At the beach, larger males arrive first and dig and fight over burrows. Mating occurs once the females arrive, whereupon the males leave and the females take over the burrows and lay about 100,000 eggs in a brood pouch. After about 2 weeks, the eggs are ready to hatch. The mother walks into the sea, raises her (Santa) claws and dances a kind of jig to release the eggs. There's 100, 000 of them, so it's understandable that she'd celebrate a little.

Image Source
The eggs hatch immediately, revealing tiny red crab larvae. They develop there for about a month before they finally grow up enough to breathe air. So now they all start climbing out of the sea and migrating inland. They are less than a centimetre long at this point but still, we now have another gigantic mass of migration. There could be hundreds or thousands of young per adult! But sometimes there can be a lot less or even none at all, which must be pretty eerie.

An interesting point: early settlers didn't mention this extraordinary event. You'd think they would. Settlers also brought over the black rat, which brought over diseases that killed off the native Maclear's rat, which may have controlled the red crab population back then. Maclear's rat only ever lived on Christmas Island, so it's gone forever. This leaves the Christmas Island red crab almost indomitable. Almost.

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