Carcinization: Explaining the Repeated Evolution of Crab-like Forms

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Carcinization: Explaining the Repeated Evolution of Crab-like Forms

Clip title: Why Do Things Keep Evolving Into Crabs? Author / channel: PBS Eons URL: https://www.youtube.com/watch?v=wvfR3XLXPvw

Summary

This video explores the fascinating evolutionary phenomenon known as carcinization, where various crustaceans independently evolve into a crab-like body shape, even if they aren’t “true” crabs. The main topic revolves around why this particular form has been so evolutionarily successful, leading to its repeated emergence across different lineages.

The video introduces Paralomis debodeorum, an ancient ancestor of modern-day king crabs, highlighting that despite its crab-like appearance, it belongs to the infraorder Anomura, which are considered “wannabe crabs.” True crabs belong to the infraorder Brachyura. Both these groups originated from longer-bodied, lobster-like decapod crustaceans, which date back about 365 million years ago. Over vast spans of time, these diverse decapod groups repeatedly underwent a transformation where their long, vulnerable tails (pleons) shortened and tucked under their bodies, while their front armor (carapace) broadened and flattened, eventually resembling what we recognize as a crab. This process was termed “carcinization” by Lancelot Alexander Borradaile in 1916.

Scientists have been researching this evolutionary convergence for over a century. Studies indicate a significant “Cretaceous Crab Revolution” in the Mesozoic Era, around 145 to 66 million years ago, where many true crab groups emerged and diversity shifted towards crab-shaped species. The primary explanations for the success of the crab form include enhanced mobility and predator evasion. The compact, rounded, and flattened body allows crabs to walk, run, swim, and burrow more efficiently, and even climb trees. Losing the long, exposed tail also makes them less vulnerable to predators.

However, the video also touches upon instances of “decarcinization,” where some crustaceans have lost their crab-like features, demonstrating that evolution is not always a one-way street. Despite these exceptions, carcinization remains a remarkable example of convergent evolution, underscoring the adaptive advantages of the crab body plan. This versatility in locomotion and lifestyle across diverse habitats – from deep-sea environments to beaches, fresh water, and even land – explains why so many crustaceans have independently evolved to look like crabs.

Video Description & Links

Description

Check out our podcast Eons: Mysteries of Deep Time: http://ow.ly/2J4450Iu69U

For some reason, animals keep evolving into things that look like crabs, independently, over and over again. What is it about the crab’s form that makes it so evolutionarily successful that non-crabs are apparently jealous of it?

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Thanks to Franz Anthony for all of the ancient crab (and “crab”) illustrations featured throughout this episode! http://franzanth.com/ And special thanks to Dr. Jo Wolfe, Dr. Javier Luque, and Dr. Katie Collins for assisting Franz with research for those illustrations.

And thank you to Dr. Rodney Feldmann for providing a photo of the Paralomis debodeorum fossil!

Produced in collaboration with PBS Digital Studios: http://youtube.com/pbsdigitalstudios

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References: https://docs.google.com/document/d/1q9McRu8tVceDumDFTWyrkVr3D15b3-DHp2ZYyd2sCuc/edit?usp=sharing

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Carcinization: Explaining the Repeated Evolution of Crab-like Forms

Carcinization: Explaining the Repeated Evolution of Crab-like Forms