Carcinogenesis – why does evolution turn crustaceans into crabs?

Hydrobiologists aside, most people will think of the image of a ten-legged crustacean when they hear the phrase crustacean. More specifically: crayfish, lobster, shrimp or crab. In Poland, the first association will still be the crayfish, but on a global scale, the crab is more likely to come to mind, as it belongs to the most numerous group of decapods. This is the result of an evolutionary process called carcinogenesis. Thanks to it, most decapods have the characteristic shape of a crab, regardless of their actual taxonomic position.

Polish Cancer

Folk taxonomy (folk taxonomy) is the division of organisms present in the popular consciousness. Sometimes it diverges from the systematics adopted by specialists. Biological taxonomy has universal ambitions – biologists from one country should agree with colleagues from another part of the world. In Poland, crabs have always been much less well known than crayfish (not only has access to the sea historically not always been easy, but these animals are relatively rare in the Baltic).

The knowledge of others had to be reinforced, and so Latin texts began to be translated, and Cancer became Cancer. This is the case, for example, in the book On Herbs and the Power of Herbs. .. by Stefan Falimirz, where Cancer/cancer was described in the chapter on fish and is a river animal. In the picture posted there, it has an oblong shape, mustache and tail, but in reprints of foreign books it used to be round. This discrepancy applies not only to biology, but also to astronomy and astrology, and tropicus cancri became Tropicus cancer in Polish.

Discrepancies in nomenclature around the world

Meanwhile, Cancer is the Latin name for crab, and crayfish were referred to more by the Greek-derived name Astacus or Astacos. Linnaeus named Cancer astacus, placing it in the same genus as the shore crab(Cancer maenas). As new species became known, not long after Linnaeus, some crabs were separated into genera such as Astacus and Carcinus. The shore crab (today Carcinus maenas) was moved to the latter. In the genus Cancer, the pocket crab(Cancer pagurus) remained. In English-speaking biology, there is a tradition of preserving folk names, even when they can be very confusing in terms of affinities (hence crayfish are still called crayfish, as if they were fish, nowadays the etymology has blurred, but it can be translated as crab fish), so also species of the genus Cancer and Carcinus are called crab.

Horsetail crabs are called Horseshoe Cra bs (literally horseshoe crabs), although they are more closely related to spiders and scorpions than crustaceans. English-speaking scientists therefore generally use scientific names, referring to Latin, and in popular science texts it is easy to find the adjective true, indicating the convergence of folk taxonomy with scientific ones. In the Polish tradition and after changes in systematics, other names are more boldly used. If necessary, these are neologisms and Carcinus is sometimes called crawfish.

Carcinogenesis, carcinogenesis and carcinology

Carcinus is a name taken from Greek, where it meant crab. Ancient physicians, because of the similarity to this crustacean, named certain tumors the same way. The Romans adopted this synonym, referring to tumors as cancer. Polish translators did the same with the name of the animal and also interpolated it to cancer, even though in shape it more closely resembles a crab. In modern medical terminology, the name cancer is reserved for one type of malignant tumor, and the nameacarcinogenesis for its formation, unlike the more general name “oncogenesis.”

In colloquial language, cancer has become synonymous with neoplasm, and there is talk of carcinogenic effects or carcinogenesis with other types of cancer as well. The confusion is further increased by plant medicine (phytopathology), where, for obvious reasons, cancer means something different than in vertebrate medicine. Thus, carcinogenesis is linguistically related to carcinogenesis, but is a completely different process. As an aside – the branch of zoology devoted to crustaceans is carcinology.

Carcinogenesis is a physiological process occurring at the scale of the organism, meanwhile, carcinogenesis, which can be called scraping or crabbing, is an evolutionary process. It involves the various developmental lineages of decapods, starting with the shrimp body form (here again, folk taxonomy diverges from scientific taxonomy), moving through the crayfish/homara form and ending with the crab form. Evolution does not stand still, and the optimal form under certain conditions ceases to be so after they change, so there is also decarcinogenesis, the process by which crabs turn into other forms(crayfish), and this is not about taxa, but forms that at first glance show similarity.

Carcinogenization with examples

Carcinogenesis occurred mainly in two groups of tenth-graders: Brachyura and Anomura. The former are called crabs in Polish and true crabs in English. For the latter, the name soft-shelled crabs is used in Polish literature. The word Brachyura can be translated as short-shelled, because representatives of this group, as a rule, have abdomen short and wrapped under the cephalothorax so that it cannot be seen. This is one of the features of carcinnation. The other two concern the carapace (carapace and plastron), which becomes a dorsally flattened can with a shape closer to circular than elongated. In soft-bodied abdomens, the abdomen is not always shortened, and carcinogenesis manifests itself by hiding the abdomen.

Soft-bodied crabs include hermit crabs (Paguroidea), known precisely for the softness of their abdomen (although adult palm crabs have armor on them). In the not-so-distant Polish popular literature they were known as hermit crayfish. However, in English-language literature they are called crabs(Hermit Crabs), and nowadays the version of hermit crabs is also most easily found in Polish texts. Scientists, if they use the Polish name at all, stay with hermit crabs. Their closest relatives are the Lithodoidea, for which the name crabs was coined in Polish-language literature, and are known as King Crabs in English-language literature. There are more such crabs in this group.

They differ from the real ones in anatomical details (usually the most prominent is the different number of visible crotch legs). Crabs have the most crab-like features. Porcellan cra bs (Porcellanidae) also have quite a lot, but their sister group, Galatheidae, is not called crabs in the English-language literature, butSquat Lobsters. In contrast, two other families of squat lobsters, Chirostylidae and Eumunididae, are related to the Kiwaidae family, about which English-language authors disagree whether they are Yeti Crabs or rather Yeti Lobsters. This shows that popular nomenclature based on outward appearance does not match the actual affinity.

Research on crabs continues

The existence of crab-like characters differing in anatomical details noticeable only to specialists, as well as intermediate links, allows us to formulate two hypotheses. The common ancestor of Brachyur and Anomur (merged into the Meiur group) may have had a crab form, but some of its descendants underwent decarcinization (astacy) and today less resemble the model. However, more supports the hypothesis that all modern crabs are the result of decarcinization. Fossil traces also support this. The common ancestor of crabs from the Brachyura and Anomura groups would today be considered a shrimp or crayfish, not a crab.

The hypothesis has a century-old history – in 1916. Lancelot Alexander Borradaile put it forward as a plausible scenario for the history of Porcellanopagurus – a genus of hermit crabs that look more like ordinary crabs than typical hermit crabs. Currently, it is still being analyzed, with an article by Joanna Wolfe and colleagues published in 2021 pointing out various points in the evolution of Meiur that are manifestations of carcinogenesis and decarcinogenesis. The latter also occurred in crabs proper, making one of their families, the Raninidae, known asFrog Crabs, in appearance similar to the Hippoidea of the Anomura group, otherwise known asMole Crabs. Finally, it appears that carcinellation occurred independently at least five times (two in Brachyur and three in Anomura), and decarcinellation seven times.

Carcinogenesis is an example of convergent evolution, or convergence. Since certain traits are optimal in a given environment, different organisms evolve to perpetuate them. A textbook example of this process is also known from aquatic life – fish shape is adopted not only by fish, but also by ichthyosaurs, cetaceans or penguins. This shape is largely determined by hydrodynamics. This applies not only to shape – their bellies are more often lighter than their backs, making it difficult to spot against the sky or bottom. Apparently, the shape and behavior of crabs are for some reason beneficial to macrozoobenthic life.

Biologists can’t really pinpoint with any certainty what evolutionary advantage carcinogenesis provides. Crabs proper and supposed crabs have lost through it the typical escape strategy of other decapods, namely retreating by crawling. However, running sideways makes them more pivotable while still being able to see the danger lurking above. Evolutionarily, this is similar to the transition from bipartite to radial symmetry, as in echinoderms, tunicates and cephalopods.

However, not everyone is in danger of being criminalized

Carcinogenesis in the full sense of the word applies only to crustaceans, actually – decapods. The mere flattening of the body, shortening of the tail part and covering it with a shell, such as in bugs, beetles and turtles, are similar but nonetheless slightly different processes. Therefore, the sensational tabloid headlines that all organisms, including humans, will one day turn into crabs can be put between fiction, without the adjective: scientific.

Photo. main: Rod Long / Unsplash

In the article, I used, among other things. z:

[1] Lancelot Alexander Borradaile (1916). Crustacea. Part II. Porcellanopagurus: an instance of carcinization In British Antarctic (“Terra Nova”) Expedition, 1910 Natural History Report. Zoology, Vol. III, No. 3, Pp. 111-126
[2] Joanna M. Wolfe, Javier Luque, Heather D. Bracken-Grissom (2021) How to become a crab: Phenotypic constraints on a recurring body plan. BioEssays 43 (5), https://doi.org/10.1002/bies.202100020