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
Porcupine ray over its disc and tail. The diet of the porcupine ray consists mainly of benthic invertebrates and bony fishes, which it digs up from the sea floor. It is aplacental viviparous, in which the developing embryos are nourished by histotroph ("uterine milk") produced by the mother. The porcupine ray has long been valued for its rough and durable skin, which was made into a shagreen leather once used for various utilitarian and ornamental purposes, such as to cover sword hilts and shields. It is caught incidentally by coastal fisheries. Because it must be handled carefully due to its thorns, its