Dr Michelle Dickinson: nanotechnologist on the research revealing why crabs walk sideways

If you’ve ever watched a crab dart across a beach, you’ve probably noticed something strange - that they almost never walk forward. 

Instead, they scuttle sideways with surprising speed and agility, a movement so iconic it has practically become the defining feature of crabs themselves. 

Now, scientists have published in the journal eLife that they believe they know where this unusual walk came from.  

Researchers studying crab movement discovered that sideways walking likely evolved only once in the history of “true crabs,” a group known scientifically as Brachyura.  

That’s remarkable because in evolution, useful traits often appear independently many times. Wings evolved separately in birds, bats, and insects. Streamlined body shapes evolved in dolphins and sharks despite them not being closely related. 

But sideways walking? Scientists think crabs only invented it once, and then stuck with it.

The researchers studied how 50 different crab species moved, filming them individually in specially designed arenas that mimicked their natural environments. When they mapped those movements onto the crab family tree, a striking pattern appeared - most modern crabs inherited their sideways movement from one ancient ancestor.  

At first glance, walking sideways seems awkward. 

But for crabs, it may actually be a survival superpower. 

Crabs have wide, flattened bodies with legs that naturally extend outward. Moving sideways allows them to move faster and more efficiently without constantly twisting their bodies. 

A sideways-moving crab can rapidly dart left or right in unpredictable bursts, making it harder for predators to anticipate where it will go next.  

Today there are nearly 8,000 known species of true crabs living in environments ranging from deep oceans to rivers, forests, and even land.  

The scientists believe this evolutionary shift happened shortly after one of Earth’s major extinction events: the Triassic–Jurassic extinction around 200 million years ago.  

At that time, the world was changing dramatically and nature was opening up entirely new ecological opportunities. Crabs may have arrived with the perfect new adaptation at exactly the right moment. 

Scientists often talk about “carcinization” the repeated evolution of crab-like body shapes across different crustacean groups. Oddly enough, becoming crab-shaped has happened multiple times in evolutionary history. 

But sideways walking appears to have been much rarer.  

That suggests some behaviours are surprisingly difficult to evolve, even if they turn out to be highly successful once they appear. 

It’s easy to dismiss a crab’s sideways scuttle as just a quirky detail of nature. 

But this study suggests it may have been one of the most important movement innovations in marine evolution. 

A single change in how an animal moved may have helped crabs spread across the planet, adapt to countless environments, and become one of the most successful groups of crustaceans on Earth. 

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Episode Transcript

Speaker 1 (00:06):
You're listening to the Sunday Session podcast with Francesca Rudgin
from News talks'b.

Speaker 2 (00:13):
Doctor Michel Didkinson joins me now with her science study
of the wake, and we're going to talk crabs today.

Speaker 3 (00:19):
Taking crabs not something I think about very often, but
when I saw this study, I was like, you're right,
nothing else does this. So the question is why do
crabs walk sideways? I thought about it and I'm like, actually,
nothing else.

Speaker 2 (00:33):
They just dart all over the place, don't they?

Speaker 3 (00:35):
They do? And I'd never thought that nothing else on
the planet moves like a crab. Published in the journal eLife,
you can find it online, and scientists basically were like
looking at crabs and they went, if you have ever
watched a crab dart across the beach, you've noticed that
they probably never walk forwards. They always dart, like you said, sideways,

(00:55):
and they don't really even dart. They scuttle, right, So
they're very crab crab word scuttle, but that really fast
and they have great agility. And so what they understood is, well,
why do they do it? And why does nothing else
walk around like this? And they went through this whole
bunch of studies and DNA and looking at evolution, and

(01:16):
they basically reckoned that, Oh yeah, the eyes on the
top of the head. I've thought the same thing too, Yeah, that.

Speaker 2 (01:21):
Was my media thought. I've always looked at them and gone,
it's got to be with their eyes a place.

Speaker 1 (01:24):
Yeah.

Speaker 3 (01:25):
Well, what they found is that that actually it's probably
something that's evolved only once in the history of true crebs,
which is unusual because usually in evolution you get these
useful traits and they appear over all sorts of creatures.
So if you think about birds, for example, wings have
evolved in birds, but also in bats and also in

(01:47):
insects like butterflies. Right, so something to use streamlined body
shapes for example. So dolphins and sharks have evolved that
type of streamline body shape, but they're very different creatures.
But sideways walking has only happened with the crab, and
it looks like it's only happened. It's been invented once
and then crab just stuck with it. So they studied

(02:09):
how fifty different crab species moved. They filmed them individually.
They had these specially designed cooled crab arenas that mimic
their natural environments. And when they map those movements onto
the crab family tree, they found that all of these
modern crabs have inherited their sideways movement from just one
ancient ancestor. So basically they think it's a survival superpower.

(02:31):
One ancestor did it, and all of those ancestors lived
and it stuck. And look, crabs are weird, right. They
have these wide, flattened bodies and these legs that really
extend quite far outwards. So if you think about it,
moving sideways really helps them to move faster and more
efficiency efficiently without having to twist their bodies, which makes sense.

(02:52):
But also they think that moving sideways and being able
to dart left or right quickly is really unpredictable, and
that is good, and that's what I don't like about
them totally. Yeah, but also, if you're a predator, you
can't figuret where it's going to get, right, if you're
a crab, you're more likely it's a survival she hints,
and then that gets passed down over all the worse
that survived. So yeah, and when I didn't realize is

(03:13):
there are almost eight thousand known species of crabs living
environments and they range from deep oceans, rivers, forests, and
even land. And they think because of this, this is
why crabs can live in such wide ranges of areas,
because they've actually been able to adapt and survive because
there's this weirdness about them, this sideways thing over many

(03:34):
different species, which just allows them to go over many
different areas. So the next time you look at a crab,
basically you might go, that's quirky. But this study is
basically one of the most important movement innovations in marine evolution,
and only one thing has figured it out, and that's
the grab.

Speaker 2 (03:51):
Isn't it interesting though? That now why the animals has
seen the value in the sideways.

Speaker 3 (03:56):
Yeah, and you know it's not really value to you
look at this study and go, oh, it makes sense now,
because I do look at them and go, you're just
a little.

Speaker 2 (04:01):
Bit like over the place, I hear a hot mess.

Speaker 3 (04:05):
Yeah, that hot mess. Mentley survived all of this time
and it.

Speaker 2 (04:09):
All makes perfect sense. Thank you so much, Michelle, appreciate it.

Speaker 1 (04:14):
For more from the Sunday Session with Francesca Rudkin, listen
live to News Talks it Be from nine am Sunday,
or follow the podcast on iHeartRadio.