DANOS, N.*; BRAINERD, E.L.; Harvard University; Brown University: Biomechanics of the skin during swimming in the American Eel, Anguilla rostrata.

We are interested in the functional morphology of the dermis because it has a highly organized arrangement of collagen fibers, an arrangement that appears in all vertebrates that locomote by axial undulations. In this study we measure the in vivo mechanical behavior of the skin of the American eel, Anguilla rostrata, and use these data to conduct uniaxial tensile tests in vitro at realistic strain rates and magnitudes. Using sonomicrometry, we measured in vivo strains along the longitudinal direction to be 0.21 ± 0.0059 and strains along the hoop direction to be 0.061 ± 0.0031. When in vivo strain rates were applied to uniaxial in vitro materials tests, we found a tendency for the material to become stiffer at higher strain rates demonstrating the effect of viscoelastic elements in the skin. The dermal collagen fiber angle of 45° led to a prediction of isotropic mechanical behavior along the longitudinal and hoop directions. Stiffness of the skin was similar, 28.21 MPa, 30.07 MPa and 30.21 MPa when stretched along the hoop, longitudinal and on-fiber directions respectively. However, the strain at which the skin began experiencing stress, a result of fiber reorientation, differed between the three directions. We interpreted this to be the result of higher density of vertical collagen bundles traversing the dermal lamellae on the skin lateral to myoseptal attachments. We also measured subcutaneous pressure and found subambient pressure on the convex side of the body simultaneous with superambient pressure on the concave side. We conclude that, given the absence of a constant pressure on the body walls, modeling of the eel body as a pressurized thin-walled cylinder is inappropriate.