Ask anyone to explain how crabs move and you’ll get the same answer. It’s “horizontal”. It’s the distinctive feature that makes a crab unmistakably a crab.
But there are questions that have actually puzzled biologists for decades. When did crabs start doing this, how often did it evolve, and what did it do to crabs as a group?
New research published in e-life is the most comprehensive look at how crabs walk to date, and the resulting answers are truly surprising.
Why crabs walk sideways
Biologists have studied crab anatomy in detail for decades. Still, their movements were not well measured. Most people thought that all crabs walk sideways, but that’s not entirely true.
“Despite the wealth of information available about real crabs, data on their locomotor behavior are sparse,” says study lead author Hiroki Kawabata, an associate professor at Nagasaki University.
“Most species of true crabs move sideways, but some groups walk forward. This raises some interesting questions: When did their sideways movement begin? How many times has it evolved over the years, and how many times has it reversed?”
Answering these questions required careful observation of many species, something never before done on this scale.
Photograph dozens of types of crabs
The research team collected 50 species of crabs from various environments.
Some come from tidal flats, others from aquariums and fish markets. Each species lived in environments tailored to their natural habitats, including sand, freshwater, and saltwater.
The researchers placed each crab inside a circular arena. After a short rest, the fence was removed and crab movements were recorded for 10 min.
This simple setup allowed us to compare behavior under controlled conditions.
Measuring crab movement
The team digitized each recording. They tracked two points on each crab’s body every frame. Next, we measured the direction of movement relative to the body.
The results were clearly divided. Of the 50 species, 35 walked sideways and 15 walked forward. Few species showed mixed behavior.
Statistical analysis confirmed this pattern. Crabs did not form loose ranges. Instead, they are divided into two distinct groups. Crabs either walk sideways or they don’t.
A clear pattern emerges
The researchers then mapped these results onto a large-scale evolutionary tree built from the genetic data. A pattern became apparent.
Sidewalking appeared only once in the evolution of crabs. It began with a group called the Eubracula, which includes most of today’s crabs. The early groups continued to advance.
This means that all sideways walking crabs share a common ancestor that lived about 200 million years ago.
“This single event stands in sharp contrast to the repeated carbonizations that occurred between decapod species. It highlights that while body shape may converge multiple times, changes in behavior such as side-walking may be rare,” Kawabata said.
Some crabs are back
Evolution has not proceeded in only one direction. Researchers found at least six cases in which crabs reverted to forward walking.
These include the Japanese spider crab, the soldier crab, and the pea crab. Each group has adapted to a different lifestyle, reducing the need for a quick escape.
Walking sideways has clear benefits, but the benefits can be lost when conditions change.
Advantages of lateral movement
Lateral movement gives crabs a huge advantage. They can move to the left or right at the same speed without rotating their body.
This ability makes their movements less predictable. Predators cannot easily guess which direction the crab will go.
Experiments using crab-like robots support this idea. You’ll find that lateral movements are faster and more efficient in the widebody shape.
promoting diversity
The evolutionary impact of sidewalking becomes clear when looking at the number of species. The groups that have adopted this movement include several thousand species.
In contrast, related groups that continued to advance included far fewer species. This suggests that lateral migration helped the crab expand into many environments.
“Lateral migration may have contributed significantly to the ecological success of real crabs,” Kawabata says.
Crabs spread across their habitat
There are approximately 7,900 types of real crabs. This is a very large number compared to closely related species such as Anomura and Astacidea, which have much fewer species.
These crabs live almost everywhere. They can be found on land, in rivers, lakes, and even in the deep sea.
Crab-like body shapes have also appeared many times in different animal groups over millions of years. This repeating pattern is called carbonization.
When change makes sense
Crabs that return to forward walking often rely on other strategies. Alligator crabs travel in large groups. Spider crabs use camouflage. Pea crabs live among other animals.
In these cases, speed is less important. Protection comes from behavior and environment, not from running away. This explains why side walking is not always necessary.
Many animals look like crabs, but they don’t walk sideways. Good examples include red king crab and coconut crab.
This shows that body shape is not the only determinant of movement. Behavior evolves under its own pressure. Just because it looks like a crab doesn’t mean it moves like a crab.
for a moment
Sidewalking originated about 200 million years ago. This period was followed by large-scale mass extinctions.
Events like this reshape the ecosystem and create new opportunities. The new locomotion style may have helped the crabs take advantage of these open gaps.
Still, scientists need more data to distinguish between behavioral and environmental influences.
“Disentangling the relative roles of innovation and environmental change requires further analysis of trait-dependent diversification, fossil-based timelines, and performance tests linking lateral movement in true crabs to adaptive advantages,” Kawabata said.
rare innovation
Sidewalking is rare in the animal world. Only a few other species exhibit similar movement patterns.
This makes the crab example particularly interesting. A single behavioral change had a huge impact over millions of years.
“These results highlight that lateral movement in true crabs is a rare but innovative trait that may have contributed to their ecological success,” Kawabata said.
“Such innovations can open new adaptation opportunities, but they are still constrained by phylogenetic history and ecological context.”
The research will be published in a journal e-life.
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