Why is everything turning into crabs?

In this month's 'quirks of evolution', and a very unusual convergence occurring in the animal kingdom. Let me take you far back in time to the prehistoric year of 2018...

It’s morning at the mouth of the River Tamar in the south west of England. Conditions are quiet and still. Cutting through the fog that is hanging just above the water is a small fishing vessel, and aboard it are 4 doe-eyed wannabe marine biologists including yours truly because, whilst this boat is used to fish, it is also used to teach.

But it is also used to fish, and a net full of the river's occupants is slowly drawn up the back of the boat. The irony that, to preserve marine species you must kill some of them in a net, is not lost on us. The unlucky landing this time around features fish, rays, and, in adjacent pots, crabs.

This abundance and diversity was noted by one of the marine biologists on the boat as he remarked to another ‘I didn’t know we had so many crabs.’

‘Yes,’ came a disembodied voice. ‘And there’ll be even more soon.’

Enter stage right, the captain of the boat. White wispy hair, a face like stained oak, and a woolly hat older than most religions. Strolling past the children masquerading as scientists, and with all the whimsy of an abattoir, he muttered, ‘we’ll all be crabs one day’.

That memory will stay with me beyond any heartbreak, endeavour, or achievement in my life. What on Earth did he mean by that? At the time, in all honesty, I thought it was just an offhand comment about how we shall all die and become food for crabs. You know, classic ship banter amongst friends.

It was only years later that I discovered the truth.

Carcinisation, later dubbed the ‘return to crab’ phenomenon by strange parts of the internet, was a term coined in 1916 by zoologist Lancelot Alexander Borradaile. It refers to a case of convergent evolution, involving the process of non-crab-like animals evolving crab-like features over time. Or, in Lancelot’s words: "the many attempts of Nature to evolve a crab".

The theory suggests that over generations, a bunch of distant crab relatives, like lobsters or shrimp, decided to ditch their disgusting, non-crab-like body plans and start resembling crabs. There are lobsters and hermits out there masquerading as crabs, indeed 5 separate groups of non-crab crustaceans have been exhibiting this phenomena.

So why do this? Why go to all this trouble? Well, because crabs are perfect.

Crabs have a hard exoskeleton that protects them from predators. This also helps them retain water, which is handy when you live in the salty ocean. Their bodies are broad and flat, making them efficient swimmers, and their legs are adapted for both walking and swimming. If you’re a shoreline creature, it really pays to be able to live on land and in sea. Crabs also have specialised claws for grabbing food and defending themselves. Even the small ones can pack a noticeable nip. It makes sense, then, that other organisms have taken note of their bauplan and followed suit.

But the extent to which they have done so is remarkable. Convergent evolution, when two or more organisms evolve the same thing, is very common. Sometimes there is one way of doing things that is both effective and worth the energy expenditure to develop. But to assimilate an entire other organism really is something special. But is it all surprising? Well, perhaps not. Crabs have been around for over 200 million years in one form or another, surviving a couple of extinction level events along the way. There are over 7000 species of true crab alive today, that’s more than all of mammals combined. They’re schooling us. This is a winning formula that we are sleeping on, arrogant as we are in our soft fleshy prisons. There is one man with the requisite to understand what is necessary for us to survive as long as 200 million years, and he might still be operating a boat near Plymouth. Maybe, it was like 7 years ago, I don’t know. What I do know is that the past was crab, the present is crab, and you would be simply naive, a gormless rube, to think the future is anything other than crab.