Why Does Evolution Turn Everything Into Crabs?

If you've been on the Internet for long enough, you've probably come across the meme that - sooner or later - everything turns into crabs. According to the meme, sooner or later - be you a fish or Danny DeVito - you are to become a crab. While this is, of course, just a fun exaggeration, it's based on some fun ideas. 

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For you see, everything in nature (well, thankfully just crustaceans) seems to want to become a crab. Just like tech bros repeatedly trying to invent a new type of transport and accidentally reinventing the bus, evolution seems to keep spitting out animals that look like crabs. 

First coined as a term in 1916, carcinization was originally defined as "one of the many attempts of nature to evolve a crab". It is now recognized as a classic example of convergent evolution.

Convergent evolution is when similar features evolve in species from different periods or regions that have a similar form or function, despite the last common ancestor of the animals or plants not having that particular feature. Think how echolocation has evolved in both whales and bats, and mechanisms for flight evolved in birds, insects, pterosaurs, and bats. (Get your own evolutions, bats, quit hogging up everyone else's).

Think also of how several different animals have evolved prickly protusions, including echidnas (of the monotremes), porcupines (rodents), and hedgehogs (erinaceinae). Despite appearances, the last common ancestor of the three lived during the dinosaur era; they just ended up with similar characteristics.

Convergent evolution essentially happens when animals and plants have to adapt to similar environments or ecological niches and end up with similar solutions. Crab-like forms are thought to have happened independently at least five times in decapod crustaceans, including porcelain crabs, hairy stone crabs, and coconut crabs.

A lot of things you might reasonably call crabs (because they look and act like, well, crabs) aren't actually crabs; they just evolved into something that looked like crabs independently.

The crab body plan is a sturdy, practical design. Its broad, flattened shape offers robust protection from predators, lets it scuttle away swiftly, and enables it to slip into tight crevices with ease. Those pinchy pincers are also highly effective tools to catch prey, fend off attackers, or just latch onto something. So when the natural world has an open niche for a “coastal scavenger” or “bottom-dwelling opportunist”, the crab-like blueprint tends to be very effective, very often.

During the Cretaceous period, creatures that looked more lobster-like in shape became more squashed, and their smaller back legs became longer and more crablike. The advantage seems to be that the crab shape allows them to walk and burrow more efficiently, with some crabs that can even climb trees thanks to the shape.

It's also possible that creatures with shorter tail segments survived more, due to both their maneuverability, but also because it gave predators less to latch on to.

So until we know more, that's why everything wants to be a crab.

An earlier version of this story was published in January 2021.