The asymmetrical crab

Although The Gambia was a bird-focussed trip, rules are made to be broken.

Nature tends to be symmetrical: creatures have two hands, four feet, six legs, eight tentacles etc. West African Fiddler crabs, for example, have eight legs and two front pincers, and the females look the same on both sides, unsurprisingly.

BUT, the males are in no way even-handed: one claw is huge and one is tiny. Here is a typical male Afruca tangeri, abut 50mm (2″) wide and 25mm (1″) long, feeding in the tidal mudflats of the mangrove swamps:

The tiny claw is used to delicately select minute portions of food, and the huge claw is used to fight other males:

but also to attract the female, who goes for a nice big claw (and also for the most impressive claw-waving dance.)

Their Portuguese name is ‘boca-cava-terra‘, meaning “mouth-dig-earth”. The small claw is used for feeding, finding food in the mud, and then carrying it to the mouth:

They filter through the muddy mouthful, and then drool out the unwanted residue; their mouth is just below where the two white markings converge:

I took a video of them feeding, all the while keeping their stalked eyes on the other guy. (Notice how the left-hand one occasionally uses his smaller pincer t0 knock off the accumulated debris from his chin.)

The male population is roughly 50:50 left-clawed vs. right-clawed, and depending on their relative claw placement they seem to have techniques for fighting face-to-face or side-by-side. This video shows a different species in combat:

The local Green Monkeys forage for these crabs. Here is a youngster, who came away from the mud-flats empty-handed:

PS Imagine if we used our left arm only for fighting, and our right hand only for eating. Sounds awkward? In fact there are indeed human cultures in which only the right hand is used for food, so we are not as distinct from the fiddler crab as you might imagine.

PPS Now that I think about it, crabs are ornery in other ways too. Most of the world’s creatures navigate the world forwards, but crabs famously scuttle along sideways. I wonder if their brains are just wired differently?

One thought on “The asymmetrical crab”

  1. If I were a female crab, I know it would be a comforting feeling to know that my man-crab has a big, powerful claw to crack anything that comes our way on the sand.


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