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Carcinization: Evolution’s Favorite Shape is Crab

November 21, 2022

Evolution is a very complicated field – and one of the bigger long-standing questions is crabs! Specifically, why animals keep evolving into them!

The phenomenon of animals, or specifically crustaceans, evolving into a crab-like shape has occurred so often it has a name: carcinization. The term was coined way back in 1916, by evolutionary biologist Lancelot Borradaile, and today it has risen back into public vocabulary through the power…of memes.

                             

“Either die a hero, or live long enough to become crab”

But memes aside, this phenomenon raises several questions:

Question 1: What is a crab?

(Wolfe et al., 2021)

Crabs – the animals we call crabs at least – are all a part of the evolutionary order Decapoda (meaning literally “ten feet”), under the phylum Arthropoda, subphylum Crustacea, and clade Meiura (Kazlev, 2022). “True crabs” all fall under the infraorder Brachyura. They include species such as the Japanese spider crab, blue land crabs, ghost crabs, snow crabs, etc., and can be most easily differentiated by having 4 pairs of walking legs.

Question 2: What is not a crab?

Or at least, who are the fakes?

“False crabs” are largely on the neighboring Anomura branch – hermit crabs, porcelain crabs, coconut crab, king crabs even, are all false crabs. They tend to have 3 visible pairs of walking legs, with the 4th pair often smaller or hidden under their shell.

To put it shortly, these are crabs:

These are not:

Get it? Nope

Question 3: How does one become a crab?

The process of carcinization is characterized by the crustacean’s carapace becoming flat and wide, while it’s abdomen or “pleon” shorten and curves underneath. The appendages that would normally form the tail of a lobster either shrink or disappear. Additionally, the plates that make of the underside, or ventral, part of the animal’s shell become fused into one. It’s important to note too, that not all these steps have to occur – many undergo “partial” carcinization, such as the hermit crab, who keeps their tail (Wolfe et al., 2021).

Final question: Why?

Why become a crab? Potentially, the crab form provides more protection and, surprisingly, flexibility for crustaceans (Green, 2020). With the pleon tucked under the carapace, there’s less surface area for predators to grab; and with a more compact body, crabs can scuttle around pretty quickly. They can get into smaller cracks than their more lobster-like compatriots.

Why are scientists studying this? Right now, carcinization serves as a frequent and recognizable pattern of evolution – AKA convergent evolution, where animals of a similar niche evolve similar features. For evolutionary biologists, this provides a potential wealth of data for seeing trends in how several species evolved, and predicting how even more may evolve in the future. Additionally, as noted by Wolfe et al., the fact that the widening of the carapace and curling of the abdomen evolves together so frequently implies that the two phenotypes may be linked developmentally, or even genetically!

Why is this a meme???

To be frank, this quirk of evolution is just…funny! Crabs are such a random, funky little animal – they alone are a sort of meme! So when people find out that this strange little creature, for some reason, keeps popping up in nature, it surprises them. Not only does a seemingly random process like evolution apparently have a pattern – the pattern is crabs; and it’s happened so often that there’s a name for it!

So in short:

Sources:

C_Mill24. (2022, October 20). Carcinization. Know Your Meme.  https://knowyourmeme.com/memes/carcinization

Green, H. (2020). Why Do Things Keep Evolving Into Crabs? PBS Eons. YouTube. https://www.youtube.com/watch?v=wvfR3XLXPvw.

Kazlev, M. A. (n.d.). Decapoda: Anomura. Palaeos Arthropods: Crustacea: Decapoda: Meiura. http://palaeos.com/metazoa/arthropoda/decapoda/meiura.html

Wikimedia Foundation. (2022, November 20). Carcinisation. Wikipedia. https://en.wikipedia.org/wiki/Carcinisation#:~:text=Carcinisation%20(or%20carcinization)%20is%20an,Nature%20to%20evolve%20a%20crab%22.

Wolfe, J. M., Luque, J., & Bracken‐Grissom, H. D. (2021). How to become a crab: Phenotypic constraints on a recurring body plan. BioEssays, 43(5), 2100020. https://doi.org/10.1002/bies.202100020

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Currently studying Biomedical Engineering at Georgia Tech, and has a passion for puzzles, biology, and just all sorts of quirky science facts

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  Timothy Durham was convicted of rape when a test showed his DNA genotype matched the DNA recovered from the crime scene. Eye witnesses testified that Durham was in a different state at the time of the incident, but he wasn’t exonerated from his 3000 year sentence until follow up DNA analysis showed that the preliminary forensic analysis used in the trial was misinterpreted (Thomas et al.). DNA testing serves an invaluable part in our criminal justice system, but it comes with complications and bias that can lead to wrongful convictions and misinformed juries.  

Background:

Forensic genetics aims to identify the origin of a biological sample. It can be traced back to Karl Landsteiner who discovered human ABO blood groups. With blood groups, suspects could be eliminated based on their blood type. For example, if a suspect had type A blood and there was type B blood left at the scene by the perpetrator, the investigators could rule out the suspect. Unfortunately, the reverse didn’t work. If the suspect and perpetrator both had the same blood type, it can’t be concluded that the suspect was guilty. Blood identifiers evolved into 8 distinct categories which improved the ability to eliminate suspects, but could only conclude that (if the system matched) it might be the perpetrator or this person might be related to the perpetrator (Jobling and Gill). The field evolved in the 1980s with the discovery of Minisatellites, lines of repetitive DNA that don’t encode proteins. Minisatellite patterns in DNA create what we know today as the DNA fingerprint, and it has a much higher accuracy rate than previous methods (Augustyn et al.). Around the same time, a breakthrough for victims of sexual assault was discovered. In the case of assault, bodily fluids of both the victim and the assaulter can get mixed. A process called differential lysis was created which could selectively increase the concentration of sperm in a fluid sample thereby allowing for accurate DNA testing (Jobling and Gill). Forensics evolved to use single-locus probe testing after several years because of its ease of interpretation. It looked at specific minisatellites that had high variability between subjects. This was the first DNA-based method to be used in a criminal investigation. Two murder homicides from several years apart were connected using DNA evidence. The evidence was then used to exonerate an innocent man, and the guilty party was found when he tried to evade mandatory DNA testing for all men in the small town. His guilt was then reinforced when the DNA profiles matched, and he gave a confession (Jobling and Gill). Finding DNA at a crime scene doesn’t guarantee that you’ll find a murderer. There are issues with non-pure samples and limited samples that make DNA work difficult, and accreditation of facilities and new techniques and take time (Jobling and Gill).  

Subjectivity of DNA Interpretation:

In a 2009 publication, the National Academy of Science discusses the possibility for subjective influence in lab based forensics. The interpretation bias happens in complex cases where, for example, there could be a mixture of DNA. A study published in 2011 looked specifically at this issue. A gang rape happened in Georgia, and the mixture in DNA from the case was used to prosecute one suspect after another suspect took a plea bargain to testify against him. With DNA mixtures, there are three possible conclusions: suspect excluded, suspect cannot be excluded, and inconclusive. If the suspect is excluded, it means that there’s little to no possibility that his DNA is part of the sample. If the suspect cannot be excluded, it means that there’s a possibility that his DNA is part of the sample. In the trial, the DNA examiners who knew the context of the case concluded that the suspect being prosecuted could not be excluded. The 2011 study gave the DNA evidence from this case and the DNA profiles of the suspects to 17 forensic examiners. These examiners had no context of the case, and they all worked independently. Of the 17, only 1 came to the same conclusion (suspect cannot be excluded) as the examiners who knew the context of the case. Twelve of the 17 concluded that the suspect who was prosecuted and convicted had no relation to the DNA found in the sample. Not only does this show that subjectivity occurs due to the context of the case, but the 17 examiners came up with different findings based on their past experiences meaning that not all DNA analysis is objective. Some organizations like the Scientific Working Group on DNA Analysis Methods say that quantitative statistical evidence should be presented with every categorical conclusion, but the US doesn’t require this (Dror and Hampikian).

False Positives:

There are two categories that cause wrongful identification in a DNA report: coincidental matches and false positives. Coincidental matches are DNA matches between two unrelated people. The probability of these matches is about 1 to 500,000 for white individuals and about 1 to 350,000 for black individuals. This small probability of coincidental matches makes people feel confident in DNA evidence. False positives, on the other hand, are matches between DNA and test DNA that result from issues in the forensics lab. There have been multiple cases of forensics labs accidentally switching victim and suspected rapist DNA which lead to convictions that were later overturned when the lab realized the error. There are also cases of misreading or misunderstanding a sample. In a case from 1993, a man was convicted of rape because someone interpreted a mixed sample of victim and perpetrator DNA as an individual sample. The conviction was again overturned. To prevent false convictions, retesting DNA is allowed, but it’s not an efficient solution. Many people are pushing for statistical information on the probability of false positives, so that the jurors can make accurate judgements based on DNA evidence (Thomas et al.).  

Conclusion:

DNA analysis is a useful tool for the criminal justice system, but subjectivity can be an issue in forensic testing. To combat wrongful convictions, there are organizations like The Innocence Project who use DNA testing to free the innocent, but reformed forensic testing standards should be put in place to prevent subjectivity and to ensure juries are correctly informed during trials. https://www.innocenceproject.org/    

Citation:

Augustyn , Adam, et al. “DNA Finerprinting.” Encyclopedia Britannica , 26 Apr. 2018, www.britannica.com/science/DNA-fingerprinting#ref153186. Dror, Itiel E., and Greg Hampikian. “Subjectivity and Bias in Forensic DNA Mixture Interpretation.” Science & Justice, vol. 51, no. 4, 2011, pp. 204–208., doi:10.1016/j.scijus.2011.08.004. Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.” Nature Reviews: Genetics, vol. 5, no. 10, Oct. 2004, pp. 739–751., doi:10.1038/nrg1455. Thompson, William C, et al. “How the Probability of a False Positive Affects the Value of DNA Evidence.” Journal of Forensic Sciences, vol. 48, no. 1, Jan. 2003, pp. 1–8., doi:10.1520/jfs2001171.