Carcinization is the process by which a crustacean evolves into a crab-like form. The term was first proposed in 1916 by biologist Lancelot Alexander Borradaile who described it as “one of the many attempts of Nature to evolve a crab.” Carcinization is typically offered up as an example of convergent evolution, where similar features evolve independently among different species (such as flight independently evolving in birds, insects, and bats).
In a 2017 paper on carcinization in Anomura (a group of decapod crustaceans that includes hermit crabs), researchers suggested that carcinization is likely a result of corresponding internal structures of the crustaceans that evolve to have crab-like forms,
Within decapod crustaceans, Anomura is the phenotypically most disparate taxon, ranging from hermit crabs to squat lobsters to crab-like representatives. Curiously, not only did the crab-like habitus evolve independently from the ‘true’ crabs (Brachyura), it also evolved three times independently within anomurans. This process has been called ‘carcinization’ by the English zoologist Lancelot Alexander Borradaile in 1916. In this paper we summarize the results of our recent studies into the evolution of the crab-like morphotype (i.e. carcinization) and into possible structural dependences (i.e. coherences) between the external morphology of a crab-like habitus and inner organs. A micro-computer tomography and computer-aided 3D reconstruction-based comparison between the various crab-like taxa shows that amazingly similar structural coherences exist across all these lineages between the external characters of the crab-like habitus and internal characters. We were even able to trace complex coherence concatenations, or ‘coherence chains’, between various structures. These new findings greatly enhance our knowledge and understanding of the evolutionary transformation into a ‘crab’, particularly in the context of causal and coherence morphology. Although enormous morphological disparity is observed in the internal anatomy of the crab-like taxa, reflecting the fact that the evolution of the crab-like habitus was indeed convergent, various corresponding dependences are found across the different lineages between the external characters of a crab-like habitus/morphotype and inner structures. In other words, as a result of carcinization certain structural coherences led to the specific internal anatomical patterns found in crab-like forms. There is no reason to assume that ‘evolutionary tendencies’ or any such vague concept played a role.