Mechanical behavior of ctenoid scales: Joint-like structures control the deformability of the scales in the flatfish Solea solea (Pleuronectiformes).

Ctenoid scales protect the fish body against predators and other environmental impacts. At the same time, they allow for sufficient degree of flexibility to perform species-specific locomotion. The scales of the flatfish Solea solea were chosen to study the specific mechanical behavior and material properties of the ctenoid scales. Using scanning electron microscopy and micro-computed tomography, three-dimensional asymmetric structures of the stacked mineralized ctenial spines in the posterior field, which is a part of the scales exposed to the environment, were examined in detail. Nanoindentations on the surface of the ctenial spines indicated that the elastic modulus and hardness of these mineralized structures are about 14 GPa and 0.4 GPa, respectively. The spines appeared to be connected to each other by means of joint-like structures containing soft tissues. Bending tests showed that the ctenoid scales have two functional zones: a stiff supporting main body and an anisotropically deformable posterior field. While the stiff plate-like main body provides support for the whole scale, the deformable joint-like structures in the ctenial spines increase the deformability of the posterior field in downward bending. During upward bending, however, the spines prevent complete folding of the posterior field by an interlocking effect. STATEMENT OF SIGNIFICANCE: In contrast to the continuously mineralized cycloid scales, ctenoid scales combine two conflicting properties: They are hard to protect the body of fish against predators and other environmental impacts, yet flexible enough to allow for sufficient degree of body bendability for locomotion. To understand the structural background underlying this specific biomechanical feature, here we investigated the scales of the flatfish Solea solea. For the first time, we demonstrated the presence of joint-like structures within the scales, which increase scale deformability during downward bending, but prevent scale deformation during upward bending by interlocking. Our results shed lights on the material-structure-function relationships in ctenoid scales, as well as on their functional adaptations to the specific environment.