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A crab-like body has evolved at least five separate times in ten-legged crustaceans, and scientists keep finding the same pattern.
Researchers in Germany compared crab-shaped groups with their relatives, looking for clues about why evolution repeats itself.
Their work suggests that once a crustacean starts folding its tail under a wide shell, many internal parts reorganize in step.
That mix of predictability and odd detail is why this crab theory is still relevant over a century after the idea got its name.
Understanding carcinization
Biologists call this carcinization – separate lineages evolving crab-like bodies again and again. This phenomenon is a type of convergent evolution, in which unrelated species developing similar traits because they face similar pressures in nature.
The work was led by Dr. Jonas Keiler at the University of Rostock in northern Germany.
His research follows how crustacean bodies change as parts like shells, nerves, and blood vessels adapt to new ways of living.
Many crab-like forms belong to decapods, crustaceans with ten legs and jointed body segments, because that body plan supports strong claws.
A broad, low shell can help an animal push through crevices, resist biting predators, and protect its softer underside during fights.
Many crab-like species tuck the pleon, the tail section under the body, which frees up space for larger muscles.
Sideways walking, quick turns, and strong gripping all become easier when the center of mass stays low and the legs spread wide.
Carcinization and crab origins
A recent paper compared crab-like shapes in Anomura, a group including hermit crabs, king crabs, and squat lobsters.
Brachyura, the group biologists call true crabs, also carries the crab layout, but it sits on a different branch.
Researchers use habitus, overall body shape tied to how an animal functions, to compare crabs with crab-like impostors.
The study looked past claws and shells, focusing on internal morphology, the way body parts are shaped and arranged.
The researchers reported that many internal anatomical features depend structurally on the external traits of a crab-like body form.
When the outer body flattens, muscles, the ventral nerve cord, and even parts of the circulatory system may reroute or compress. The findings do not support the idea that vague evolutionary tendencies played any role.
Seeing through crab armor
Keiler’s team used micro-computed tomography – a type of X-ray scanning that builds three-dimensional slices – to map organs without cutting animals open.
The method works well for crustaceans because their hard shells hold delicate tissues in place during scans and later computer modeling.
Researchers can measure nerve paths, gill chambers, and stomach positions, then compare those maps across lineages that look alike outside.
That inside view helps separate true convergences from coincidences, and it shows where the crab form forces the same compromises.
King crabs and carcinization
In 1992, a study used genetic data to place king crabs inside a hermit crab group. That result suggested carcinization can happen when a shell-dweller gives up its borrowed housing and builds protection into its own body.
Some lineages still carry hints of that past, including asymmetry linked to fitting into spiral shells even after the shell habit ends.
Researchers keep testing which hermit crab relatives sit closest to king crabs, because different genes can point to different matches.
Parallel evolution and starting points
Biologists also talk about parallel evolution, when related lineages evolve identical traits by using similar inherited tools.
Crustaceans start with segmented bodies and jointed legs, so small changes in growth can flatten a shell and shorten a tail.
Developmental genes guide these changes, and they can limit which shapes remain stable once an animal molts and hardens its next shell.
That kind of constraint can make evolution look predictable, even when each lineage reaches the crab form by a different route.
Crab-like bodies cause confusion
Crab-like bodies can trick scientists who rely only on shells, because similar outlines can hide very different organ layouts.
Keiler’s paper described coherence chains, linked changes that spread from an outer feature to many inner structures.
A wider shell can move the muscles that power legs, and those muscle changes can also reposition nerves and blood vessels.
This tangle makes it hard to tell which internal traits evolved for their own benefit and which came along for the ride.
Lessons from carcinization
By tracking which parts change together, researchers can test whether evolution follows a few easy paths or many equally likely options.
Because crab-like bodies appear in separate lineages, scientists can compare independent cases and spot the same rules across oceans.
Questions remain about which habitats most strongly reward the crab build, and how quickly different lineages can reach it.
As scientists add better genetic maps and finer scans, they may finally pin down why crab-like solutions appear so often.
The study is published in Biological Journal.
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