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Chitons are constantly scraping their teeth on rocks to eat the algae off of them, but that means their teeth need to be pretty tough. And it turns out one species's teeth are the hardest, stiffest biominerals in any living thing we've seen!

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Sources:
https://www.pnas.org/content/118/23/e2020160118
https://www.nature.com/articles/s41598-018-37839-2
https://news.northwestern.edu/stories/2021/05/rare-mineral-from-rocks-found-in-chiton-teeth/
https://news.ucr.edu/articles/2019/01/31/magnetic-teeth-hold-promise-materials-and-energy
https://www.tandfonline.com/doi/abs/10.1080/00222933.2014.959574
https://www.materialstoday.com/biomaterials/articles/s136970211070016x/

Images:
https://www.flickr.com/photos/47445767@N05/15947947127
https://www.istockphoto.com/photo/gumboot-chiton-gm183967843-27618844
https://www.flickr.com/photos/33466410@N00/3258676083
https://www.nature.com/articles/s41598-018-37839-2
https://www.flickr.com/photos/odfw/8020038038
https://commons.wikimedia.org/wiki/File:Santabarbaraite-270526.jpg
https://www.researchgate.net/figure/Chiton-teeth-and-their-formation-A-Light-micrograph-of-the-radula-of-the-chiton_fig19_342396092
https://www.istockphoto.com/photo/gumboot-chiton-gm467679603-34055468
Thanks to Brilliant for supporting  this episode of SciShow.

Go to Brilliant.org/SciShow to  learn how you can take your STEM skills to the next level with  their interactive courses! [♪ INTRO]. If you’ve ever been to a tide pool,  you’ve probably seen a chiton, even if you didn’t know it.

Chitons are basically flat, oval-shaped  snails that creep along rocks in the ocean. They specialize in scraping algae off those rocks, using a tongue-like structure  embedded with two rows of teeth. And because their teeth are constantly  scraping rock, they’ve evolved to be made of something a lot stronger than calcium.

Chitons have teeth made of  materials like the metal magnetite. One species even uses a  metal that, until recently, was only thought to be found in rocks. Turns out, these animals make the  hardest, stiffest biominerals of any living thing scientists have seen.

And they’re showing us what life is capable of. The chiton’s tongue-like  structure is called a radula. And in terms of design, the  teeth embedded in the radula are surprisingly similar to human teeth.

They’re made of two pieces. You’ve got the softer, flexible  stylus, which is long and hollow, and connects the top of the tooth to the radula, similar to how the roots of our  teeth hold them into our jaws. Then, there’s the hard cusp.

This is what chitons use to scrape  algae off rocks, and it’s similar to the hard part of our teeth we use to crush food. Except, if we were grinding our teeth  on rocks, our teeth would wear down pretty fast, because our calcium-based  enamel is very soft compared to stones. So, chitons have cusps made of a  metal other than calcium: magnetite.

It’s actually the hardest, stiffest mineral  made by a living thing that we know of. But some chitons don’t even stop there. A study published in 2021 on the  gumboot chiton revealed that their stylus contains a different mineral,  and a very rare one at that.

It’s called santabarbaraite, named for an  area in Italy, not the city in California. Sorry to all you Santabarbarinos. This mineral was only thought to  exist in tiny amounts in rocks, but then, it was discovered  inside this chiton’s stylus, mixed in with a relatively more soft,  flexible material called chitin.

That’s chitin with an “i”, not to be  confused with the animal we are currently discussing: the chiton. Sounds the same,  but it’s “o-n”. Not confusing at all.

Researchers believe this mixture helps  make the gumboot’s stylus strong and flexible, while staying light-weight. A  perfect combination for a life licking rocks. That said, chiton teeth aren’t invincible:  Even magnetite teeth wear out, so the chiton’s radula is  constantly producing new ones.

For how they do it... it’s a process. The teeth actually start out  transparent, because they’re only made of proteins and that flexible chitin. Then, as they age, they turn  reddish-brown, and then black.

That color change happens through  a process called mineralization. Here, particles of iron in the  chitons' teeth are converted into an iron compound called ferrihydrite. Then, in the cusp, that ferrihydrite  is converted into magnetite, and the teeth become black and  reach their maximum hardness.

Overall, chitons are a great  example of just how much life can adjust to a specific niche. These flat, unassuming creatures are so  adapted to their rock-grazing way of life that they have evolved to make minerals  we’ve never seen an animal make before. Which, if you ask me, is pretty metal.

And something that’s even more  metal is learning about the chemistry metals can do with  today’s sponsor Brilliant! They’ve overhauled their courses  to be even more interactive. Like this one about chemical reactions  where you can work through all the bits and bolts of chemistry.

Including chemical kinetics where  you’ll learn how things like molecules interact and transform. If you’re interested, you can get  started at brilliant.org/scishow to get 20 percent off an annual Premium subscription. And checking them out helps us too, so thank you. [♪ OUTRO].