Crabs Have Walked Sideways for 200 Million Years — And Scientists Just Found Out Why
Crabs have one of the most recognizable moves in the animal kingdom — that quick, scrambling sideways scuttle that looks like nothing else on Earth. But have you ever wondered how that unusual way of walking came to be? Scientists have just published a major new study with some surprising answers, tracing the crab's sideways walk back around 200 million years to a single ancient ancestor.
Researchers at Nagasaki University in Japan led a study that analyzed the movement of 50 different species of true crabs — the scientific group officially known as Brachyura, which contains about 7,904 species. That is an enormous group of animals, and they have spread into nearly every watery and coastal environment on the planet, from deep-sea trenches to tropical beaches to freshwater rivers. To collect their data, the scientists filmed each species for ten minutes inside circular plastic arenas that were set up to resemble each crab's natural habitat. The footage let them clearly observe which crabs walked sideways and which walked forward.
The results were revealing. Out of the 50 species observed, 35 primarily moved sideways and 15 mainly walked forward. To understand how this behavior developed over time, the researchers compared their observations against an evolutionary family tree, sometimes called a phylogeny, built using genetic data from 344 crab species. A phylogeny works like a detailed family history chart — it shows which species are closely related and helps scientists figure out when and how certain traits first appeared.
When the team mapped the walking behaviors onto this family tree, a clear picture emerged: sideways walking most likely evolved just once. It appeared in a single forward-walking ancestor near the base of a crab subgroup called Eubrachyura, and was then passed down through generations to nearly all true crabs alive today. This is remarkable because many other crab traits, including the rounded body shape crabs are famous for, evolved independently multiple times in different species — a phenomenon scientists call carcinization. Sideways walking appears to be much harder to develop, which could be exactly why it happened only once.
So why would sideways walking be such a big advantage? Scientists believe it has a lot to do with escaping predators. When a crab can dart left or right at full speed, it becomes much harder for a hungry fish or bird to predict where it will go next. Think of it like a running back in football who cuts sharply sideways — defenders have a much harder time making the tackle. That unpredictability may have given early sideways-walking crabs a real survival edge over their forward-walking relatives.
At the same time, the researchers note that sideways locomotion — the scientific word for a particular style of movement — is not easy to evolve. It requires specific leg arrangements and body proportions, and it could interfere with other important behaviors like burrowing into sand, finding food, or mating. That may explain why it is so rare across the animal kingdom, with only a handful of other animals, like crab spiders and certain insects, showing anything similar.
The timing of this evolutionary leap is also fascinating. Scientists estimate that sideways walking first appeared around 200 million years ago, right at the start of the Jurassic period — shortly after a major extinction event wiped out many species at the end of the Triassic period. The world was changing rapidly at the time: the supercontinent Pangaea was breaking apart, shallow marine environments were expanding, and new ecological niches were opening up. Researchers believe these changes created fresh opportunities for early true crabs to spread, diversify, and thrive.
Senior author Yuuki Kawabata summed it up well: sideways locomotion is a rare but genuinely innovative trait that may have helped set true crabs on the path to becoming one of the most successful and widespread groups of animals on the planet. The next time a crab scuttles past you on the beach, you are watching a 200-million-year-old survival strategy in action.
Source: SciTechDaily