The scales of bony fish grow out of the dermis and are embedded in the dermis at the base of the scale. They overlap like shingles on a roof, says The Earth Life Web. Most bony-fish scales have two layers: a calcified layer on the bottom and a fibrous layer on the top. Fish don't shed their scales; the scales continue to grow with the fish.
Cartilaginous fish, such as sharks, have very different scales. Their scales resemble teeth in structure, with a hard enamel outer layer, a middle layer of dentine and a soft inner core of pulp. Unlike the scales of bony fish, the scales of sharks don't grow larger. Instead, as the shark grows larger, new scales form to fill in the gaps between the old scales, says The Earth Life Web. The tooth-like scales grow out of a thick dermis layer.
Fish scales exist primarily to protect the fish. Light, small scales provide greater ease of movement, and large, heavy scales provide more protection. Fish that need protection more than speed, such as some catfish, sometimes have scales that are basically a coat of armor. Very fast-moving fish, such as trout, have small, light scales, reports The Earth Life Web.