Carcinization is a term coined by L.A. Borradaile in a 1916 report published in the Journal Contribution to Zoology. Borradaile, an evolutionary biologist studying crustaceans, noticed that genetically disparate crustaceans, especially in the order Anomura (King Crabs, Porcelain Crabs, Hermit Crabs), had evolved traits that were extremely morphologically similar to those typical in the infraorder Brachyura (true crabs). Anomura are genetically much more similar to lobsters and crayfish than to Brachyura; and yet, due to similar selective pressures, they have morphologically converged both their internal and external organs to be incredibly similar to one another.
Stability, in biological systems, is more or less impossible. Living organisms are constantly undergoing reactions and changes. Materials are processed, atoms are rearranged, waste is shed, nutrients are taken in. An organism is never the same for very long. This poses a problem for a biologist trying to understand an organism. If a biologist is trying to study one bacterium, how are they supposed to delineate it from its environment? If they were to only understand the bacterium as a collection of molecules, they would find it quickly spread across the petri dish as matter that was once part of the bacterium is expelled as waste. In place of their bacterium, they would see once foreign matter incorporated into the bacterium via whatever mechanism it decides is fit for feeding. So, the biologist must imagine some way that matter becomes part of the organism, and then is no longer part of it. The bacterium is not a collection of molecules, but a process that molecules participate in. A dance that molecules can step into and out of.
Now, let us imagine that a biologist is trying to understand a species. Species aren’t just sums of individual organisms, as members of a species die and are born all the time. Much like the molecules that make up the bacteria, the matter of each individual organism is being thrown into membership of the species as the organism is born and out of the species as it dies. A species is not a description of a portion of the earth’s biomass, but a description of a pattern that matter happens to arrange itself in. When a pile of matter composed of dead fish and marine snow in the Bering Sea forms a certain configuration of fibers, fluids, and other such polymers with ten limbs reaching nearly six feet across, we can say that it participates in the pattern Paralithodes camtschaticus, and is thus dubbed an Alaskan King Crab.
Why have I gone on a long tangent about how organisms and species are not hunks of matter, but processes and patterns? Because the crab is a very stable pattern. The first crab patterns were the Cyclida of the carboniferous period, arising almost 360 million years ago. It would be one thing if the true crabs of today were descended from Cyclida — it would make modern some sort of extension and variation of the ancestral patterns of Cyclida, changing with each successive repetition of the pattern. But no, Cyclida died out some time during the cretaceous period, which spanned from about 145 million years ago to 66 millions years ago. That means that the Cyclida pattern persisted for well over 200 million years before going extinct.
The real beauty is not just the long reign of Cyclida, but that the crab pattern surpasses just the first lineage of organisms to replicate it. The first modern crab, Eocarcinus praecursor, arose about 185 million years ago, and has been repeating itself ever since. Genetically, it is far distant from the Cyclida it lived alongside, but its body plan is strikingly similar. Convergent evolution at an extreme. Crab-like body plans have emerged more recently in the aforementioned Anomura five separate times. The crab pattern is so stable that other patterns run into it, slowly replicating its features to the point where porcelain crabs have more phenotypic similarities with distantly related true crabs of Brachyura than they do with closely related squat lobsters of Anomura.
Organisms and species, when viewed as collections of matter, are wildly unstable systems. Constantly undergoing reactions and increasing entropy, but when we view an entire species as a pattern, we can find some staggering amounts of stability. Crabs might be wildly unstable as organisms, most having shorter lifespans than us humans, but crabs as a species, as a pattern that persists in our universe despite mass extinctions and extreme deliciousness, have been around longer than any of us can imagine.