Skin stiffness in ray-finned fishes: Contrasting material properties between species and body regions.

The skin of aquatic vertebrates surrounds all the mechanical lineages of the body and must, therefore, play an important role in locomotion. A cross-woven collagenous dermal design has converged across several clades of vertebrates. Despite this intriguing pattern, the biomechanical role of skin in swimming fishes remains largely unknown. A direct force transmission role for fish skin has been proposed, a hypothesis that is supported by the arrangement of the connective tissues linking the skin to the axial musculature. To evaluate this direct force-transmission hypothesis, we undertook hundreds of uniaxial tensile tests on skin samples from coho salmon (Oncorhynchus kisutch), Florida pompano (Trachinotus carolinus), and red snapper (Lutjanus campechanus). To do this, we developed highly precise, low-cost, custom-built material testing units. To augment our data, we also assembled a data set of skin stiffness of four additional species of actinopterygians fishes from previously published studies. We found that stiffness varies significantly between species and that the skin of our study species was increasingly stiff along a rostrocaudal gradient. Placing our results in the context of the limited body of previous work, we found that species with lower skin stiffness exhibit shorter propulsive wavelengths and low thrust production at the caudal fin and species with higher skin stiffness possess longer propulsive wavelengths and high thrust production at the caudal fin. In addition, we found that mean collagen fiber angle was close to 50° and that fiber angle was lower in posterior samples than in anterior and midlateral samples. Taken as a whole, our mechanical and morphological results support the hypothesis that the skin functions as an important direct force-transmission device in actinopterygians whereby muscular force generated in anterior myotomes is transmitted to the posterior of the body through the increasingly stiff skin.