Animals Keep Evolving Into Crabs

This is genuinely one of the weirdest things in biology. You know the meme about everything turning into a crab? It's sort of true, but the real story is way stranger. One small group of animals has been becoming crab shaped, then stopping, then doing it again for 250 million years. King crabs used to be hermit crabs. Some lineages went crab, went back, then went crab again. The 2021 research that finally counted all this found the crab body plan evolved at least five separate times and was abandoned possibly seven times or more. It's not a one way street toward crabhood. It's more like a dial that keeps getting turned both directions. What makes this so odd is that it's happening between close relatives, not distant ones like bats and birds both getting wings. These are all decapod crustaceans doing this to themselves over and over. The scientists who study it still can't tell you exactly why. There are good guesses involving protection and fitting into crevices, but nothing settled. After a quarter billion years of this pattern, the honest answer is we're still figuring it out. Chapters: 00:00 Not Actually Crabs 00:38 The Meme vs Reality 01:25 What Carcinisation Means 02:32 Why the Shape Works 03:55 True vs False Crabs 04:55 King Crabs Were Hermits 07:00 Crabs Keep Unevolving 08:50 Why This Keeps Happening