CORAL REEFS ARE DYNAMIC EQUATIONS, CONSTANTLY BEING BUILT UP AND TORN DOWN. The stony corals and the coralline algae mostly do the building. Major factors in the tearing-down side are parrotfish. And, especially, the teeth in parrotfish beaks.
Parrotfish don’t set out to tear down coral. As herbivores, they focus on eating the algae that live on the surfaces of coral polyps’ calcium carbonate exoskeletons, or corallites. And they work at this pretty continuously, scraping away on the calcium stuff, often taking out chunks at a time.
Now, new research has indicated that they can do it because the teeth in parrotfish beaks are composed of one of the strongest biomineral substances in existence.
PARROTFISH BEAKS / SANDY BEACHES
Chunking away at coral for algae often means ingesting substantial amounts of calcium carbonate, as well. Parrotfish jaws grind the calcium material into tiny grains that move into their digestive tracts. Separate pharyngeal teeth in their throats continue the process. The resulting find grains of calcium exit at the other end…as sand.
It’s true: Parrotfish poop out sand – a very fine, white form that makes up much of the seafloors and beaches of coral reef environs. It’s estimated that as much as three-quarters of the contents of a working parrotfish’s digestive tract at any given time is inorganic sediment. It’s estimated that a large parrotfish can produce more than a ton of sand in a year.
Because their powerful jaws bear pronounced resemblances to parrot beaks, parrotfish are…parrotfish. Scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (AKA Berkeley Lab) were struck by the apparent durability of parrotfish teeth on diving trips, and decided to study it.
Since they worked at Berkeley Lab, they had access to advanced technology for studying it – they invented much of it. They used teeth samples from a Pacific species called steephead parrotfish (Chlorurus microrhinos).
They found a material that makes a parrotfish’s teeth one of the hardest biomineral substances in existence. Called fluorappatite, it’s composed of calcium, fluorine, phosphorous and oxygen – tougher than metals like gold and silver.
REALLY, REALLY STRONG TEETH
Their research indicated that the tips of parrotfish teeth can tolerate extreme pressure – some 530 tons of pressure. This is equivalent to a the combined weight of 88 African elephants compressed to a square inch of space.
The research team relied on an advanced x-ray technique developed at Berkeley Lab called polarization-dependent imaging contrast mapping. With it, they were able to visualize the nanocrystal orientation of the fluorappatite crystals in parrotfish teeth samples. They found the crystals to be assembled in interwoven bundles – something like a chain mail pattern.
“Parrotfish teeth are the coolest biominerals of all,” says Pupa Gilbert, one of the researchers and lead author of the study published in the journal ACS Nano. “They are the stiffest, among the hardest, and the most resistant to fracture and abrasion ever measured.”
Beyond this, anatomy reinforces this biomineral strength. Parrotfish beaks have some 1,000 teeth structured in 15 rows, with each tooth cemented to the others and surrounded by bone to create a supersolid jaw. Hardness and stiffness increases toward the tip of each tooth. And, when they do wear down, the fish are constantly replacing old teeth with new ones.
IT’S THE TECHNOLOGY, STUPID
As divers who revel in watching parrotfish go about their work, we’re served by understanding what lets them munch and chunk so relentlessly. Parrotfishes’ importance to the reef habitat in keeping algae in check (and creating sand) can’t be overemphasized.
But, beyond their parrotfish beak findings, Gilbert and her colleagues are excited by the technological possibilities they represent. “The interwoven characteristic and the crustal orientations are completely open to be explored for the production of synthetic materials,” Gilbert says.
Future parrotfish research remains to focus on the pharyngeal teeth, not explored in this work.
But technologically, “the sky’s the limit at this point,” Gilbert adds. “This first observation of the mechanical properties in exciting, and now a lot more work can be done on the structural properties.”
PRINCIPAL SOURCES: “ X-Rays Reveal the Biting Truth About Parrotfish Teeth,” Berkeley Lab News Center; “Tough Teeth and Parrotfish Poop,” Ashley Gallagher, Smithsonian; “Feeding Humphead Parrotfish,” Blue Planet, BBC Earth; “The Many Species of Parrotfish – Colorful, Complex Lives,” Poseidon’s Web.