Scales have evolved over time and are of major importance in classifying fishes. Most scales are deeply buried in the fish's epidermis, or outer skin layer, with only part of them showing. Below the pictures of scales are examples of how the scales would look on the fish's body.
There are four common types of scales in fishes. They are shown in the figure below. Below each scale type is an illustration of what they would look like on the fish.
| Ganoid | Placoid | Cycloid | Ctenoid |
|---|---|---|---|
|
|
|
|
| This "primitive" kind of scale is an evolutionary reminder of the time when fishes used armor plating to protect themselves. Ganoid scales are hard and smooth, and may take the form of only a few scales (or scutes, as in the sturgeon and stickleback), partial plating, or overall body plating. Sturgeons, gars, and sticklebacks have ganoid scales (or scutes). | Sharks have placoid (PLAK-oyd) scales: tiny, tooth-like structures that are partially embedded in the skin. These tiny, pointed scales, made of the same materials as their (and our) teeth, make their skin feel like sandpaper. | Many fishes with which we are most familiar have cycloid scales, which are the thin, round, almost transparent scales that we find when we are cleaning trout, salmon, or herring. Minnows also have cycloid scales. These scales are mostly buried in the epidermis, allowing only the small posterior margin to show. | Highly evolved fishes often have ctenoid (TEEN-oyd) scales, which are much like cycloid scales except that they have tiny, comb-like projections (ctenii) on their posterior edges (the edges that show, and are not buried in skin). The colors of brightly colored fishes also show on these posterior edges. |
Besides the above types, there are also cosmoid scales, as well as scaleless fishes (sculpins, many catfish, some eels, and swordfish), and fishes which have scales so deeply buried that they look scaleless (many tunas and anguillid eels).