There are roughly 7,000 species of true crabs on Earth, and nearly all of them scuttle sideways. That quirky gait is so universal it seems inseparable from being a crab. But a growing body of research reveals a strange split: the compact, wide-bodied shape we recognize as “crab-like” evolved independently at least five times across distantly related crustacean lineages, while the sideways walk appears to trace back to just one ancestor, one time, deep in the evolutionary past.
Five paths to the same body, one path to the sideways stride
Biologists call the repeated emergence of crab-like anatomy “carcinization,” and it is one of the most striking examples of convergent evolution in the animal kingdom. Hermit crabs, king crabs, porcelain crabs, hairy stone crabs, and coconut crabs all arrived at a flattened, compact body with a reduced tail tucked beneath a broad carapace, despite descending from ancestors that looked nothing alike. A foundational 2002 study on mitochondrial gene rearrangements across decapod families, published in Proceedings of the Royal Society B, confirmed that these crab-like forms arose independently rather than from a single shared ancestor. The genetic evidence showed that rearrangements in mitochondrial DNA tracked with morphological shifts toward the crab shape in separate lineages, ruling out a common origin for the body plan itself. More than two decades later, that paper remains a cornerstone of the field.
Sideways locomotion tells a completely different story. A study published in eLife traced the lateral gait back to a single evolutionary event at the base of Eubrachyura, the clade containing the vast majority of species people recognize as true crabs. (eLife operates a reviewed-preprint model: submitted manuscripts undergo full peer review, and both the authors’ work and the reviewers’ assessments are published openly, but the journal does not issue a binary accept-or-reject decision in the traditional sense.) The group likely diverged during the Cretaceous period, more than 100 million years ago. From that single origin, sideways walking remained remarkably conserved, with only scattered species reverting to forward motion. The contrast is sharp: the body shape is a case of evolution repeating itself over and over, while the gait is a case of evolution doing something once and sticking with it.
The evidence from genes, fossils, and leg joints
Several independent lines of evidence support the claim that carcinization happened repeatedly. Molecular phylogenetics, fossil morphology, and developmental biology all point to the same conclusion. A peer-reviewed synthesis by Joanna Wolfe and colleagues, published in BioEssays in 2021, collated morphological, phylogenetic, and developmental data behind the recurring crab-like body plan and clarified which specific anatomical traits qualify a crustacean as “carcinized.” “The crab-like body form has evolved independently in at least five lineages,” Wolfe and her co-authors wrote, noting that shared developmental pathways, essentially genetic toolkits, may predispose certain lineages to converge on the crab form even when they start from very different body architectures.
On the locomotion side, biomechanical studies of leg skeletons in forward-walking and sideways-walking decapods have shown that skeletal adaptations in the leg joints differ systematically between the two gaits. Leg articulation patterns in sideways-walking species reflect a consistent structural template, one that aligns with a single evolutionary origin rather than multiple independent inventions. The eLife study built on this foundation by analyzing locomotion across a large sample of crab species and mapping gait evolution onto a phylogenetic tree, finding that the lateral stride traces back to one ancestral node.
This distinction matters because it separates two evolutionary processes often assumed to travel together. A body plan can converge repeatedly under similar ecological pressures, but the locomotion strategy that accompanies it does not necessarily follow the same pattern. In this case, the sideways walk appears to have been a one-time biomechanical innovation that preceded or coincided with the diversification of true crabs, then persisted as those lineages radiated into coral reefs, deep-sea vents, mangrove swamps, and freshwater streams.
What researchers are still debating
The fossil record for crab locomotion is frustratingly thin. Fossils preserve body shape well, especially the hard carapace and limb segments, but they reveal little about how an animal actually moved. The claim that sideways walking evolved once rests primarily on molecular phylogenetics and comparative anatomy of living species, not on direct fossil evidence of gait. If older fossils with preserved limb articulation surfaces were discovered, they could either strengthen or complicate the single-origin hypothesis.
The mechanisms driving carcinization also remain debated. Researchers agree that the crab body plan evolved independently in at least five lineages, but they disagree on why. Some point to shared developmental constraints: genetic toolkits that make the crab shape an easy morphological “destination” regardless of starting point. Others emphasize ecological selection, arguing that the flat, wide body offers advantages in habitats like rocky intertidal zones, where a low profile helps an animal wedge into crevices and resist wave action. The Wolfe et al. review noted that both explanations likely play a role, but the relative contribution of each is unresolved.
A related open question is whether the single origin of sideways walking acted as a kind of biomechanical lock-in. Once the lateral gait was established, did it constrain subsequent body plan evolution within Eubrachyura, channeling later diversification toward similar ecological niches? Or did the gait simply persist because it worked well enough that there was no strong selective pressure to abandon it? Consider the anomuran crabs: king crabs and porcelain crabs evolved crab-like bodies but retained forward walking, suggesting the body shape and the gait are not inevitably linked. Testing the lock-in hypothesis would require detailed comparisons of limb articulation across both carcinized and non-carcinized decapod clades, work that as of mid-2026 is still underway.
Why convergence and constraint tell different evolutionary stories
Established work on crustacean phylogeny, including a widely cited 2007 analysis, has helped clarify where major decapod lineages sit relative to one another, providing the evolutionary scaffolding on which more focused studies of body plan and gait are built. When mitochondrial rearrangements, broader phylogenetic analyses, and detailed locomotor studies all converge on the same pattern, multiple origins of the crab-like form but a single origin of sideways walking, the combined picture becomes more convincing than any one line of evidence on its own.
The core conclusions are well supported by primary data, not modeling or speculation. For the repeated origins of the crab body plan, mitochondrial gene rearrangement studies provide hard molecular evidence that different decapod families independently evolved crab-like morphology. For the single origin of sideways walking, the eLife study combined anatomical measurements of leg musculature and joint geometry with phylogenetic mapping across a broad sample of species.
But the details of why crab-like bodies are so evolutionarily attractive, and how tightly they are linked to the sideways stride, remain active areas of research rather than settled facts. The pattern itself is clear: evolution found the crab shape irresistible, reinventing it again and again across tens of millions of years. The sideways walk, by contrast, was a singular invention, one that proved so effective it has endured virtually unchanged since the age of dinosaurs.
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*This article was researched with the help of AI, with human editors creating the final content.
