Cryptochiton stelleri are a type of mollusk known as a chiton that have earned the nickname “wandering meatloaf” for their reddish-brown color and ovoid shape. That is not the end of their weirdness, however, with the discovery their teeth are made of an iron-based mineral previously only seen in rocks.
In a taxonomic group that includes octopuses and oysters, you have to be a bit strange if you want to stand out. Getting around on a single muscular foot and feeding on algae, bacteria, and sometimes small invertebrates, chitons might not seem that different from marine snails. However, Dr Derk Joester of Northwestern University was intrigued by their teeth.
Chitons scrape their teeth against the rocks on which they live to get their food. Although they grow new teeth as the old ones wear down they'd probably struggle to keep up if they had the sort of dentition favored by most of the animal kingdom. Instead, chitons have evolved one of the hardest materials known to science and attached it to a soft radula, which resembles a tongue. The combination of an ultra-hard object supported by a living shock absorber works well, but Joester found that instead of a sudden transition, they make the shift from hard to soft gradually.
In Proceedings of the National Academy of Sciences, Joester and colleagues collaborated with synchrotron and electron microscope specialists to study C. stelleri (also known as the gumboot chiton) teeth in unprecedented detail.
They report the presence of an amorphous iron hydroxy phosphate mineral known as santabarbaraite. "This mineral has only been observed in geological specimens in very tiny amounts and has never before been seen in a biological context," Joester said in a statement. Other creatures might have missed a trick in not evolving santabarbaraite; "it has high water content, which makes it strong with low density. We think this might toughen the teeth without adding a lot of weight."
Flecks of santabarbaraite are distributed through C. stelleri's upper stylus, a hollow structure that connects the head of the tooth to the radula. Joester compares it to the root of human teeth. Over a distance of less than a millimeter, the stylus becomes 3–8 times as hard at the top as the bottom.
Such a material might be useful to humans even if we never scrape rocks for our food. Joester and co-authors decided to try and make an ink they could put in a 3D printer and build hard formations from. Provided it was not allowed to stand too long between being mixed and used for printing, Joester found the ink could be used to build up extremely hard structures.
"Mechanical structures are only as good as their weakest link, so it's interesting to learn how the chiton solves the engineering problem of how to connect its ultrahard tooth to a soft underlying structure.” Joester said. “This remains a significant challenge in modern manufacturing, so we look to organisms like the chiton to understand how this is done in nature, which has had a couple hundred million years of lead time to develop."