Novel Defense Mechanisms in the Armor of the Scales of the “Living Fossil” Coelacanth Fish

Here, experiments and analysis revealing the unique defense mechanisms of the coelacanth, a lobe-finned fish termed a “living fossil”, are reported, as it was considered to be extinct until 1938, but extant for 400 million years. This defense is provided by primitive elasmoid scales having a rare double-twisted Bouligand structure of lamellae which provides extraordinary resistance to deformation without failure. Distinct from other elasmoid scales, the collagen fibrils in the coelacanth fish scales form bundles which are embedded in a matrix comprising fibers arranged perpendicular to the layered (double-twisted Bouligand) structure that provide added rigidity and resistance to deformation. Using in situ synchrotron small-angle X-ray scattering during uniaxial tensile testing, the deformation mechanisms of the collagen are identified in terms of fibril stretching, reorientation, sliding, bending, and delamination. The unique structure of the coelacanth scales, distinct from modern elasmoid scales such as the carp, provides a variety of mechanisms to arrest cracking, making it an outstanding damage-tolerant material to resist predator attacks.