Don’t underestimate the incredible bite of the parrotfish

Parrotfish relentlessly grind up coral and deposit it as sand onto the seafloor, and can single-handedly produce hundreds of pounds of sand every year. But what gives this fish its powerful bite? Scientists have now identified a woven microstructure that enables parrotfish to chomp on coral tirelessly.

Study first author Matthew Marcus is a scientist at the Department of Energy’s Lawrence Berkeley National Laboratory. He works at the lab’s Advanced Light Source (ALS), a type of X-ray source known as a synchrotron light source. This method of investigation proved to be critical to the examination of parrotfish.

Marcus became intrigued with parrotfish during a trip to the Great Barrier Reef when he was reminded of the remarkable roles they play. Parrotfish primarily eat polyps and algae on the surface of coral skeletons, and help to clean up reefs.

“I was reminded that this is a fish that crunches up coral all day, and is responsible for much of the white sand on beaches,” said Marcus. “But how can this fish eat coral and not lose its teeth?”

Measured near the biting surface, the hardness of parrotfish teeth is about 530 tons of pressure per square inch. This is equivalent to a stack of 88 African elephants compressed inside of a single square inch of space.

Marcus teamed up with biophysicist Pupa Gilbert to analyze parrotfish teeth. Collaborators in French Polynesia contributed parrotfish beaks, and the team performed a series of structural studies to examine how they work.

Using specialized techniques at the ALS, the team set out to examine how the fine crystal structure of parrotfish teeth contribute to their incredible strength. The researchers were able to view the individual crystals and observed their intricately woven structure.

Fluorapatite, which contains calcium, fluorine, phosphorous, and oxygen, is the the mineral responsible for the crystal structure of parrotfish teeth. The study revealed that the fluorapatite crystals are assembled into interwoven bundles, which decrease in size toward the tip of each tooth.

Gilbert pointed out that conventional microscopes overlook the unique orientation of crystals within the enamel structure of teeth. According to Gilbert, the crystal orientation “tells a big story about how different teeth are specialized for different functions.”

In parrotfish, the continuously growing rows of teeth, which form a beaklike structure that constantly replaces worn teeth with new teeth, are also key to their specialized feeding behavior. Parrotfish have approximately 1,000 teeth situated in 15 rows, and each tooth is cemented to the others and surrounded by bone to form a solid beak.

“Parrotfish teeth are the coolest biominerals of all,” Gilbert said. “They are the stiffest, among the hardest, and the most resistant to fracture and to abrasion ever measured.”

The study is published online in the journal ACS Nano.
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By Chrissy Sexton, Earth.com Staff Writer