Why is Everything Evolving into Crabs? - The Bizarre Phenomena of Carcinisation

Ellen Foster discusses the incredible, peculiar, and confounding phenomenon of carcinisation.

Okay, maybe not everything is evolving into crabs. But the strange coincidence of multiple species independently evolving into crab-like forms has been circling the internet for the past few months, causing many people to question if we are also destined for crabification…

Although it’s not in our distant future, such a huge variety of crustaceans have developed crab-like features over time that the official term for crabification, ‘carcinization’, was coined by the English zoologist Lancelot Alexander Borradaile in his 1916 paper ‘An instance of carcinization’1. He goes on to describe the process as "one of the many attempts of nature to evolve a crab". This process is an example of convergent evolution: the independent evolution of similar features in species over different periods of time, without the last common ancestor having any similar features2. The process occurs naturally as species evolve to suit their ecological niche, such as the similarities between dolphins and sharks despite their different taxonomic classes.

When thinking of a crab, you may picture a hard shell, flat body, and oval shape like a king crab. However, a king crab, despite its name, is actually part of the Anomura or “false crab” family, from which most carcinized species evolve. True crabs are members of the Brachyuran family, such as the Dungeness Crab. A 2017 study from the Biological Journal of the Linnean Society discusses how different species evolved such similar hardened shells and flat shapes as expected but, surprisingly, also developed similar vascular and nervous systems, making the species even more alike than pure appearance3.

Although the evolution of all these specific features can clearly be seen, biologists are still uncovering the mystery of what makes the crab such an advantageous form. Are crabs the ideal form? Are all species destined for crab-like features? Probably not, but the process of evolution still has elements to be discovered, so who knows!

References:

1. Borradaile, L.A. (1916) ‘Crustacea. Part II. Porcellanopagurus: An instance of carcinization. Natural History Report’, Zoology, 3, pp. 111–126.

2. Stern, D.L. (2013) ‘The genetic causes of convergent evolution’, Nat Rev Genet, 14, pp. 751–764. doi: https://doi.org/10.1038/nrg3483

3. Keiler, J., Wirkner, C.S., and Richter, S. (2017) ‘One hundred years of carcinization – the evolution of the crab-like habitus in Anomura (Arthropoda: Crustacea)’, Biological Journal of the Linnean Society, 121 (1), pp. 200-222. doi: https://doi.org/10.1093/biolinnean/blw031

From SATNAV Issue 24, page 3.