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
Bicast leather Bicast leather Bicast leather (also spelled as bi-cast leather or bycast leather) is a material made with a split leather backing covered with an embossed layer of polyurethane or vinyl. Bicast leather was originally made for the apparel industry for glossy shoes, and was later adopted by the furniture industry. The leather used in the making of bicast is usually a portion of the fibrous, lower grade of leather that remains when the higher-grade grain layer is split off. It is coated with a thin polyurethane or vinyl layer, giving it a uniform shine and a long-lasting "like new" appearance.