Arya News - The internet has discovered carcinization: how different crustaceans keep evolving into crabs. It`s disturbing.
Here’s what you’ll learn when you read this story:
Carcinization is the repeated evolution of a crab-like body plan in different crustacean lineages.
New genomic work is adding molecular and time-calibrated context to a story long told through anatomy.
A 2026 locomotion study suggests true crabs’ sideways walk may have evolved once, rather than reappearing every time a lineage got crab-shaped.
Evolution has a crab problem. Over time, several crustacean lineages have arrived at a broad, low, crab-like body even when they didn’t begin as true crabs. Scientists call that pattern “carcinization,” and we’ve had fun with the idea that given enough time, everything wants to become a crab .
That doesn’t mean every animal is secretly destined to become a crab. But it does mean the crab shape—a tucked abdomen, a widened body, and a compact crab-like form—has turned out, again and again, to be a useful solution.
Now, a new reviewed preprint on living true crabs has found that the sideways walk most people associate with crabs likely evolved once, from a forward-moving ancestor, near the base of Eubrachyura.
The term “carcinization” comes from the Greek , meaning crab—the same root behind words like carcinoma and carcinogen. English zoologist Lancelot Alexander Borradaile coined the word in 1916, and it’s stuck because the pattern itself is so odd: crab-like forms didn’t arise only in “true” crabs. They also evolved independently in several anomuran crustaceans, the group that includes hermit crabs, king crabs, and squat lobsters, according to a 2017 paper in the
So, how does carcinization happen? Well, that part is pretty simple. Animals that live in similar habitats face obstacles that can shuttle them all toward the same evolutionary advantages. cites the marsupials as a key example, where despite having one critical difference from their “placental” counterparts in other parts of the world, the marsupials often correspond closely to these other animals.
Animals can evolve separately but end up evolving toward other species, too, or even spontaneously evolve the same characteristics in totally separate groups. Birds and bats can both fly using mechanical wings. Birds and mammals are both warmblooded, but both evolved from groups that were not.
New research points to a messier mix of ecology, body architecture, developmental limits, historical accident, and environmental change. A January 2026 study in analyzed 42 anomuran mitochondrial genomes, including three newly sequenced taxa, to refine anomuran relationships and test carcinization-related patterns. Its time-calibrated analyses placed key divergence events for newly sampled lithodid and pagurid lineages around the Eocene-Oligocene transition, a period the authors treated as a plausible backdrop for diversification and adaptation.
The 2017 paper’s key point still holds: crab-like form didn’t evolve only in “true” crabs. It also appeared several times within Anomura, which is useful for studying convergence. Now, researchers have more than shape to work with. Genomes, locomotion data, and a stronger fossil benchmark give scientists a wider view of what repeats, what doesn’t, and what still can’t be pinned to one cause.
Meanwhile, on the fossil side, a February 2026 J paper benchmarked the decapod fossil record at 4,225 fossil species, 1,284 genera, and 193 families. It found diversity peaks in the Late Jurassic, Late Cretaceous, Eocene, and Miocene, and flagged trends such as carcinization and decarcinization as questions that depend on better fossil accounting.
The internal anatomy is also part of the puzzle. The 2017 paper described how the crab-like exterior connects to internal anatomy, including structures tied to the nervous and circulatory systems. Some similarities show up across crab-like lineages, while other organs still differ in shape and size.
Moreover, the crab-shapedness of the groups can make it hard to trace what came about from interacting internal systems as opposed to, well, the crab shell:
“Some of the internal anatomical characters studied herein are structurally dependent on the external characters of a crab-like habitus. Since morphological coherence can also exist between internal anatomical structures, the coherence chains which can be traced back to the external characters of a crab-like habitus are relatively complex in some cases (indirect coherences).”
But, of course, hermit crabs don’t have a “habitus,” the biological term for a body shape or casing type that affects your health or biology context. And, the researchers say, majestic and extremely spiky king crabs evolved hermit crabs.
The crab wonders may never cease.
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