A scute is an external bony plate on the surface of a fish. Scutes serve a protective function, acting as a body armor for fish against environmental abrasions and even predation. In some fishes, such as Alewife (Alosa pseudoharengus), scutes are a row of scales modified into sharp, protective plates. In other fishes, like Shovelnose Sturgeon (Scaphirhynchus platorynchus), scutes serve a homologous function but are derived from ossified deposits in the dermis (essentially bone over skin).
Osteichthyes are a taxonomic grouping of bony fishes. This group includes ray-finned fishes (class: Actinopterygii) and lobe-finned fishes (class: Sarcopterygii). This highly diverse group of fishes, which contains almost all fish species, is the most diverse group of vertebrates today. Osteichthyes differ from chondrichthyes by (in most cases) possessing a bony skeleton, a swim bladder, scales (ctenoid, cycloid, or ganoid scales), and external fertilization.
Placoid denticles are found on sharks, rays, and chimaeras. Not really a true “scale,” like ctenoid or cycloid scales, placoid denticles are actually modified teeth. They have an inner tissue component, which contains both blood vessels and nerves, that is covered by a layer of dentin and an outer enamel. They form a tough protective skin layer for sharks, rays, and chimaeras and also have shown to reduce friction and drag so that these fish can swim more efficiently through water.
Ganoid scales are dimond-shaped scales found in lower order fishes such as the bichirs (Polypteridae), Bowfin (Amia calva), paddlefishes (Polyodontidae), gars (Lepisosteidae), and sturgeons (Acipenseridae). Unlike ctenoid or cycloid scales, ganoid scales are comprised of bone. They have a bony basal layer, a layer of dentin (also found in human teeth), and an outer layer of ganoine which is the inorganic bone salt for which these scales are named. These scales interlock with peg-and-socket joints which make them quite inflexible, compared with ctenoid or cycloid scales, but very protective.
Ctenoid scales are scales with comb-like edge found in higher order teleost fishes, such as perch and sunfish. Cteni are the tiny teeth on the posterior margin of the scale. Similar to cycloid scales, they are overlapping which allows for greater flexibility in movement than other types of scales such as ganoid scales. The surface layer of the scale is comprised of calcium-based salts and the inner layer is predominately collagen. As a fish grows, its scales grow, adding concentric layers, similar to tree rings. For certain species, these rings can be counted to estimate the age of a fish.
Cycloid scales are smooth-edged scales predominately found in lower order teleost fishes, such as salmon, carp and other soft fin rayed fish. Similar to ctenoid scales, they are overlapping which allow for greater flexibility in movement than other types of scales such as ganoid scales. The surface layer of the scale is comprised of calcium-based salts and the inner layer is predominately collagen. As a fish grows, its scales grow, adding concentric layers, similar to tree rings. For certain species, these rings can be counted to estimate the age of a fish.