Most of us are so used to seeing how crabs move that they instantly bring to mind their signature sideways crawl.
And as soon as we see them crawling sideways, we know that is simply how they operate.
It looks like it is an innate way of moving. There has been debate among scientists for years over this feature.
To answer the first question, though, you have to answer another. So where did this behavior actually begin?
What makes crab movement more complicated than it seems
Does every crab species sidewalk? It appears to be universal to all of them. However, closer examination of different species suggests otherwise.
There are some crab species that do not even exhibit lateral movement, while others demonstrate different types of behavior depending on their actions or environment.
These differences make it difficult to determine what “lateral” actually means.
If the sideways walk is the defining characteristic of crabs, then why is the same type of movement not exhibited by all species?
One of the first explanations for this behavior was based on anatomy.
The anatomy of most crab species provides for a flat body and appendages that radiate outward from the body and support lateral movement.
However, although understanding a process explains how it occurs, it may not explain why that process developed in the first place.
What scientists discovered when they started watching crabs
To understand this better, scientists observed crabs directly.
They studied approximately forty different types of crabs in a laboratory setting. The larger number of species helped reveal a more defined trend in their behavior as they were studied.
There were essentially two behavioral categories when examining how crabs moved. They would move horizontally, or they would walk forward
Researchers found there were almost no cases where animals used both kinds of movement together.
Instead, when scientists looked at how many species were exhibiting the two types of movement, there was a clear division among all those examined.
As such, it became apparent that the pattern they were seeing could be much greater than simple coincidence.
A recent study published in eLife supports the idea that this shift may have originated from a single evolutionary ancestor.
A single evolutionary shift that changed everything
Why did all true crabs begin moving sideways?
Evolutionary changes in movement habits typically don’t happen often and usually require coordinated anatomical and behavioral changes.
This leads to another question.
What was the single evolutionary moment behind crabs’ sideways walk?
Scientists now think that the sideways walk started with a common ancestor of all true, or brachyuran, crabs about 200 million years ago.
Every single one of the many crab species that moves this way is a descendant of that ancestor.
There are clear differences in movement and no transitional type of movement, so there is strong evidence that there was a single evolutionary event.
This is often described by researchers as a major innovation. Changes like this are very rare, and they can influence how an entire group survives and reproduces.
An ability such as rapid response to predators or moving easily through space without having to turn the whole body was probably an advantage for early crab species.
It likely gave them a better chance of surviving and spreading into new environments where they became dominant across multiple species.
Those advantages would also have made those early crabs less predictable in their movements and better able to adapt to changing environmental conditions.
A sideways step may appear to be nothing more than just another way of getting from one place to another.
However, this is anything but simple. It is a whole new form of adaptation in how true crabs survive and evolve.