Unique three-dimensional structure of a fish pharyngeal jaw subjected to unusually high mechanical loads.

We examine the structure of the bone of the pharyngeal jaws of a large fish, the black drum (Pogonias cromis), that uses its tooth-jaw complex to crush hard-shelled bivalve mollusks. During mastication huge compressive forces are concentrated in a tiny zone at the tooth-bone interface. We report on the structure of this bone, with emphasis on its contact with the teeth, at different hierarchical levels and in 3D. Micro-CT shows that the molariform teeth do not have roots and are supported by a circular narrow bony rim that surrounds the periphery of the tooth base. The lower pharyngeal jaw is highly porous, as seen by reflected light microscopy and secondary electron microscopy (SE-SEM). Porosity decreases close to the bone-tooth interface and back-scattered electron (BSE-SEM) microscopy shows a slight elevation in mineral density. Focused ion beam - scanning electron microscopy (FIB-SEM) in the serial surface view (SSV) mode reveals a most surprising organization at the nanoscale level: parallel arrays of mineralized collagen fibrils surrounding channels of ~100 nm diameter, both with their long axes oriented along the load direction. The channels are filled with organic matter. These fibril-channel arrays are surrounded by a highly disordered mineralized material. This unusual structure clearly functions efficiently under compression, but the precise way by which this unique arrangement achieves this function is unknown.