Parrotfish Teeth: Stiff Biominerals Whose Microstructure Makes Them Tough and Abrasion-Resistant to Bite Stony Corals*

Parrotfish feed by biting stony corals. To investigate how their teeth endure such contact stress, we examine the composition, nano/micro- structure, and mechanical properties of the steephead parrotfish Chlorurus microrhinos tooth. Its enameloid is a fluorapatite biomineral with outstanding mechanical properties: the mean elastic modulus and hardness near the biting surface: 124 GPa and 7.3 GPa, resp., making it among the stiffest and hardest biominerals known; mean indentation yield strength is >6 GPa, and mean fracture toughness is 2.5 MPa.m^1/2. This combination of properties yields high abrasion resistance. Fluorapatite shows X-ray linear dichroism at the Ca L-edge, enabling polarization-dependent contrast mapping^1-2 of apatite, to quantitatively measure nanocrystal orientations. Parrotfish enameloid consists of 100-nm-wide, microns-long crystals, co-oriented and bundled into interwoven fibersm which decrease in average diameter from 5 µm at the back to 2 µm at the tooth tip. This size change is spatially correlated with an increase in hardness.³
1. PUPA Gilbert et al. PNAS 2011, 108, 11350
2. PUPA Gilbert et al. EPSL 2017, 460, 281
3. MA Marcus et al. ACS Nano 2017, DOI: 10.1021/acsnano.7b05044