Meeting Abstract
Slack skin is a proposed adaptation for the peculiar knotting mechanism in hagfishes, as the loose covering enhances flexibility of the poorly attached body core. Hagfish skins are biological composites comparable in strength and stiffness to the taut skins of other fishes. The skins of all hagfish species are comprised of a fibrous dermis arranged between a superficial thin slimy epidermis and a thick fatty hypodermis, however, the material properties of the skins differ across species; Pacific hagfish skins are anisotropic, being more compliant in the circumferential axis than in the longitudinal axis, while the skins of Atlantic hagfish are isotropic. However, these data were gathered from uniaxial tensile tests to failure, whereas fish skins normally operate under biologically relevant stresses and strains, cyclically absorbing and releasing energy without mechanical failure. Furthermore, knot tying involves concurrent bending and twisting along multiple body axes. Here, we 1) measured a non-destructive property, resilience, of hagfish skins strained by 10% in longitudinal and circumferential axes, and 2) measured the strength, stiffness, extensibility, and toughness of skins simultaneously strained along both body axes. Hagfish skins return only 30% of the energy absorbed and do not return to their original length, and skins shown to be isotropic under uniaxial tension are anisotropic under equibiaxial tension. The anisotropy in Atlantic hagfish skins strained biaxially is reminiscent of Pacific hagfish skins and contrasts taut fish skins, which are usually twice as stiff and strong in the circumferential axis. We propose that increased compliance in the circumferential axis of the skin does not inhibit the generation of torsion by the body core.
