Fish scale the surface, interacting only with the riblet tips, not causing any high-veloctiy flow in the valleys. Since this high velocity flow now only interacts with the riblet-tip, which is a very small surface area, the momentum transfer which causes drag is now much lower than before, thereby effectively reducing drag. Also, this reduces the cross-stream velocity fluctuations, which aids in momentum transfer too. The rough, sandpaper-like texture of shark and ray skin, coupled with its toughness, has led it to be valued as a source of rawhide leather, called shagreen. One of the many historical applications of shark shagreen was
Hide (skin) Hide (skin) A hide or skin is an animal skin treated for human use. The word "hide" is related to the German word "haut" which means skin. Common commercial hides include leather from cattle and other livestock animals, buckskin, alligator skin and snake skin. All are used for shoes, clothes and other fashion accessories. Leather is also used in upholstery, interior decorating, horse tack and harnesses. Such skins are sometimes still gathered from hunting and processed at a domestic or artisanal level but most leather making is now industrialized and large-scale. Various tannins are used for this purpose. The term