By Alison Campbell 10/11/2020


This is a post of two parts: the interesting tale of convergence involving crab-like creatures, and the very poor – nay, crappy (because I like the alliteration) – headline on a popular article about it.

Part 1: the history of carcinization in crustaceans, described in this 2017 paper in the Biological Journal of the Linnean Society (Keiler, Wirkner, & Richter, 2017).

But first, the terminology! Crustaceans are a group of arthropods: invertebrate creatures that also have an external skeleton. (You could say that they are crunchy spineless animals with multiple legs.) And carcinization describes the way that several different 10-legged crustaceans have evolved a crab-like body form. That is, there are ‘true’ crabs, & then there are the wannabes.

There are a lot of different taxa of 10-legged crustaceans, but one group (taxon) stands out from the others in terms of the range of different body forms it includes. Called the “Anomura¹”, it contains hermit crabs, squat lobsters, and a range of species that look very like your average crab, including king crabs & the “hairy stone crab“. (‘Squat lobsters’ are smaller than ‘true’ lobsters, which include our NZ crayfish, and are flattened dorsoventrally, meaning their bodies are flattened rather than rounded. [Analagous to the difference in shape between cockroaches and grasshoppers.])

So, ‘true’ crabs and the crab-like creatures classified in the Anomura are in two different-but-related taxonomic groups. But, it seems that the crab-like critters didn’t evolve from crabs – Keiler & his colleagues note that the earliest-known member of the group resembled a squat lobster. It’s been dated to around 260 million years ago, very soon after the split between crabs and their differently-tailed brethren.

Nonetheless, after that split, the crab-like body form evolved 3 separate times, an example of convergent evolution: porcelain crabs, king crabs, and the strange little hairy stone crab. The interesting question is, what’s the adaptive significance of this form? The authors comment that this “has been debated at length”, and it seems that there are several possibilities: folding the tail under the body may protect it from predators, but may also make the animals more mobile. It also means that crabs, with their dorsoventrally-flattened bodies, can easily slip under rocks and into crevices. And as anyone who’s watched a crab move around on the beach or in a rock pool will know, crabs are really good at walking sideways. (This would actually make a really interesting Schol Bio question.)

Possibly it’s a combination of all of these. But, whatever the selection pressures, the authors note that the three different crab-like lineages evolved at different times and from different ancestral groups: king crabs from ancestors of hermit crabs, and the others (separately) from an ancestral squat lobster. They also comment that the similarities in form are more than skin-deep: there are similarities in the form of some of their internal organs as well, including structures in the circulatory and nervous systems. “Endless forms most beautiful²”, and all that.

Part 2: the popular title. Now, the article at Popular Mechanics is a good one – but the title! Animals Keep Evolving Into Crabs, Which Is Somewhat Disturbing. But I suppose Creatures with a similar overall body plan (crunchy shell, lotsa legs, two pairs of antennae) keep evolving into crabs doesn’t have the same ring about it.

¹ A little bit of arcane taxonomical detail: all the members of the Anomura are “differently-tailed”: the prefix anom- means different, & –ura means tails. The ‘true’ crabs are in Brachyura, meaning “short-tailed”, and crustaceans with long tails are grouped into Macrura.

² “… endless forms most beautiful …” – a quote from Darwin’s The Origin of Species.

 

Keiler, J., Wirkner, C.S., & Richter, S. (2017) One hundred years of carcinization – the evolution of the crab-like habitus in Anomura (Arthropoda: Crustacea). Biological Journal of the Linnean Society121 (1): 200-222  https://doi.org/10.1093/biolinnean/blw031

 

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