To end, the week, we have some stunning videos from BBC Earth and other places, depicting the migratory behavior of the red crabs of Christmas Island, a small Australian territory (135 km², pop. 1692 ) near Indonesia: encircled below. Their vernacular name is The Christmas Island red crab, the Latin binomial is Gecarcoidea natalis, and they are endemic to that island and the Cocos (Keeling) Islands in the Indian Ocean.
The life cycle of this crustacean is described here, but the videos are more impressive. Here are the basic facts:
The migration starts with the first rainfall of the wet season. This is usually in October or November, but can sometimes be as late as December or January.
Red crabs all over the island leave their homes at the same time and start marching towards the ocean to mate and spawn. Male crabs lead the migration and are joined by females along the way.
The exact timing and speed of the migration is determined by the phase of the moon. Red crabs always spawn before dawn on a receding high-tide during the last quarter of the moon. Incredibly, they know exactly when to leave their burrows to make this lunar date.
However, because crabs wait until the first rainfall to start their trek, they sometimes have to hurry. If the rains arrive close to the optimal spawning date, they will move rapidly. But if the rain comes early they may take their time, stopping to eat and drink on their way to the coast.
If it begins raining too late to make the spawning date, some crabs will stay in their burrows and migrate the following month instead.
And from BBC Earth, narrated by Attenborough. Note that the crabs breathe through gills, which must be kept moist. They live on land, but in moist habitats, but a remnant of their evolutionary origin is their need to go back to the ocean to spawn. Note that they walk sideways.
Note that they mate during the migration, too, so it’s not just females who are drawn to the sea at this time. A similar video, but also showing one of their predators and some of the other dangers they face.
I especially like this video because it shows how the island’s human inhabitants care for and protect the crabs:
Just taking a moment to let it sink in how the moon is timing this – plain, simple, but never fails to astonish.
Thanks I’ve always loved the story of these crabs.
“Note that the crabs breathe through gills, which must be kept moist.”
This reminded me of when we used to vacation at the beach. The tourist traps always had hermit crabs for sale, and my daughter always begged me to buy one. She loved those crabs and always took good care of them (or, as best we knew how), so she was always crushed when, after a month or two, they died.
It wasn’t until years later that I learned why – it was because the humidity in the average home is far too low for crabs.
I don’t know if they’re still selling these crabs at tourist shacks, but if they are, it should be a legal requirement that they inform purchasers that crabs need high humidity in order to survive and thrive.
It seems that everyone there is wholeheartedly on the crabs side, despite obvious inconveniences to the public. I don’t think they’d be as well-received in the US, as individals states could act differently.
Amazing!