Bone in mammals is a nanocomposite of collagen and carbonated apatite (cAp) nano-platelets, permeated by a cellular pore network. In contrast, many neoteleost fish comprise anosteocytic bone, a cell-free bone type that resists millions of loading cycles and can remodel [1]. Neoteleosts, the largest group of extant ray-finned fishes, feature paired fins supported by segmented fin rays. Each segment consists of two articulated hemitrichs connected by collagenous tissue, enabling cyclic bending during locomotion, maneuvering, and stabilization [2]. Fins generate complex flow patterns and propulsion forces up to 25 mN, yet the stresses and strains within these rays remain poorly understood. In our project within FOR 5657 BioAntiFatigue, we compare pectoral and caudal fins of neoteleost fishes with anosteocytic bone against osteocytic species of similar size, to understand the microstructural reasons for the fatigue resistance.

Here we report on nanostructural and compositional investigations, performed on multiple fin rays from neoteleost fishes (Fig 1 A). Using synchrotron-based X-ray diffraction (XRD), we measured diffraction patterns from selected regions of hydrated and dried fin ray samples, which allowed us to calculate lattice d-spacings of the cAp (Fig 1 D). Changes in d-spacing between the wet and dry states provided insight into crystal strain states and possible stress release associated with collagen degradation. Complementary synchrotron X-ray fluorescence (XRF) mapping on the same specimens yielded information on the spatial distribution of mineral-associated elements (Fig 1 E). Together, these approaches allow us to correlate local compositional variations with nanoscale lattice parameters, thereby linking mineral chemistry to the mechanical function of fin ray bone.