Things just won’t stop turning into crabs,
From claw to carapace –
They look as if they’re engineered in labs
Or zapped from outer space.
Except…the fishes show no cancric tug,
Nor do the worms or squids –
It seems it’s just crustaceans have the bug
To spawn such crabby kids.
Not counting woodlice, shrimps, or barnacles,
Nor the copepods –
But still, a fair few join the carnival,
In their squat new bods.
And as for them, the more derived they get,
The more the format grabs –
Converging on a winning set,
And walking sideways into crabs.
This meme relies on a fairly liberal definition of ‘crab’ – it seems to come down to three things – caws, an oval fused carapace, and an absent abdomen/tail (it’s actually tucked underneath). So hermit-crabs, for instance, certainly have the claws, but lack the other two (though when in a shell, they give the impression of them).
So, yes it happens, to the extent that the squat-lobster seems to be half-way through the process. But it’s also helped along by our wishful-thinking. Or, as I put is recently, plants won’t stop turning into trees.
Tellingly, other aquatic arthropods like dragonfly larvas and water spiders show no inclination to crab-up.