The material properties of acellular bone in a teleost fish.

Several lineages of teleost fishes have independently derived skeletons composed solely of acellular bone, a tissue without obvious advantages over bone that has osteocytes in the matrix. There is no consensus for the functional role of acellular bone, as factors such as salinity, activity level and gross morphology have been shown to be poor predictors of acellularity. We used a three-point bending method to test the hypothesis that the material stiffness (elastic modulus) of acellular bone is higher than that of cellular bone, which could be evidence that material properties were a selective pressure in the evolution of this unusual skeletal material. The acellular ribs of Myoxocephalus polyacanthocephalus are curved, hollow beams that decrease in size both distally and posteriorly along the rib series. First and second moments of area decreased distally and caudally in all individuals. Young's modulus (E) ranged from 3.67 to 8.40 GPa, with a mean of 6.48 GPa. The flexural stiffness (EI) differed significantly between ribs, and the hollow cylinder morphology increased the flexural stiffness by 12.0% over a solid, circular cross-section rod with the same area. Contrary to our expectations, acellular bone is not stiffer by virtue of fewer lacunae but instead falls at the very low end of the range of stiffness seen in cellular bone. There remains the possibility that other properties (e.g. fatigue resistance, toughness) are higher in acellular bone.