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
Shagreen ray comes from an instrument used by fullers to smooth cloth, referring to its rough and spiny back. The pectoral fin disc of the shagreen ray is somewhat wider than it is long. The forward margin concave and undulated, and about one-third longer than the rear margin. The snout is cone-shaped and pointed. The tail is shorter than the disc, with two small, equally sized dorsal fins and a rudimentary caudal fin. The eyes and mouth are large; the teeth are pointed and number about 64 in the upper jaw and 56 in the lower. The upper surface is rough in