In the nostalgic classic film The Neverending Story, one of the engaging minor characters is a rockbiter, a giant who lives by eating rocks (and whose messy eating habits cause trouble for his friends). The rockbiter was designed to seem alien and his habits are supposed to seem fantastic.
But eating rock isn’t really that farfetched. In fact, there are very successful organisms on earth that get by doing just that, including the parrotfish. And now a new study of their teeth shows us the remarkable properties that allow parrotfish to eat stone with their teeth.
Why Eat Rock?
It’s not strictly true that parrotfish eat rocks. (They don’t say, for example, “A delicious-looking limestone rock . . . nice bouquet.”) They eat coral. Coral are tiny pulpy organisms similar in some ways to jellyfish. In the open ocean they would be vulnerable, so they protect themselves by building a tiny house out of calcium carbonate (which, yes, is the chief ingredient in limestone). This makes it very hard to eat the coral organisms, and it allows them to build the tremendous coral reefs that provide homes for so many other organisms. Millennia of coral polyps live and die to gradually build up the reefs.
In order to eat the coral, an animal has to chew through the calcium carbonate shell, and there are few animals capable of doing that. But one of the few is parrotfish, thanks to their truly remarkable teeth.
What Makes Parrotfish Teeth Special?
There are two main keys that give parrotfish their ability to eat coral. One is that they are made of a slightly different material. Human teeth are mostly made out of hydroxyapatite, but parrotfish teeth are made out of fluorapatite. Fluorapatite is stronger and harder.
But it’s primarily the structure of parrotfish teeth that give them their extraordinary properties. By utilizing interwoven microcrystals of fluorapatite, parrotfish teeth are able to get the most out of their hard mineral content. These crystals start wide and taper toward the end, but their interweaving helps them transfer force to make them strong.
Hardness measurements are in gigapascals (GPa) of pressure. A gigapascal is equal to about 145,000 pounds per square inch. Hydroxyapatite has a hardness of about 7.0 GPa, but human tooth enamel has a hardness of about 3.5 GPa. Fluorapatite has a hardness of about 7.3 GPa, but with their densely woven structure, parrotfish teeth have a hardness of 7.0 GPa, according to this new study. And that’s why parrotfish can eat coral skeletons (hardness 5.0 GPa), while humans can’t!
Even the Best Get Worn Out
Despite the fact that parrotfish teeth are remarkably hard, they’re still not durable enough to eat coral reefs with impunity. Eating coral is hard on parrotfish teeth. Unlike beavers, parrotfish teeth can’t keep growing. Instead, parrotfish are similar to sharks: as their teeth wear out, they are lost and replaced with a constant conveyor of teeth to make sure that a parrotfish can keep eating coral for the entirety of their lives.
And if parrotfishes’ remarkable teeth wear down, it’s natural that our teeth would, too. Of course, there are some factors that contribute to tooth wear in people, such as biting and chewing nonfood objects. But another important factor to consider is whether your bite is healthy. Unhealthy bite conditions like TMJ can cause your teeth to wear down excessively, which means they will have to be repaired or replaced before they should.