The skin of a fish must fulfill many functions. Helps maintain osmotic balance, provides physical protection for the body, is the site of coloration, contains sensory receptors and, in some fish, functions in respiration. The mucous glands, which help maintain water balance and offer protection against bacteria, are extremely numerous on the skin of fish, especially cyclostomes and teleosts. Since the mucous glands are present in modern lampreys, it is reasonable to assume that they were present in primitive fish, such as the ancient Silurian and Devonian agnathans. Protection against abrasion and predation is another function of the fish skin, and the dermal bone (skin) emerged early in the evolution of the fish in response to this need. The bone is believed to have evolved first in the skin and only later invaded the cartilaginous areas of the fish’s body, to provide additional support and protection. There is some argument as to which came first, cartilage or bone, and fossil evidence does not resolve the issue. In any case, dermal bone has played an important role in the evolution of fish and has different characteristics in different groups of fish. Several groups are characterized, at least in part, by the type of bone scales they possess.
The scales have played an important role in the evolution of fish. Primitive fish generally had thick bony plates or thick scales on various layers of bone, enamel, and related substances. Modern teleost fish have bone scales that, while protective, allow much more freedom of movement in the body. Some modern teleosts (some catfish, prickles, and others) have secondarily acquired bony plates on the skin. Modern and early sharks possessed placoid scales, a relatively primitive type of scale with a tooth-like structure, consisting of an outer layer of enameled substance (vitrodentin), an inner layer of dentin, and a pulp cavity containing nerves and vessels. blood. Primitive bone fish had thick scales of the ganoid or cosmoid type. Cosmoid scales have a hard outer layer, similar to enamel, an inner layer of cosmina (a form of dentin), and then a layer of vascular bone (isoptin). In ganoid flakes, the hard outer layer is chemically different and is called ganoin. Beneath this is a cosminel-like layer and then a vascular bone layer. The thin, translucent bone scales of modern fish, called cycloids and actinoids (the latter are distinguished by serrations on the edges), lack layers of enamel and dentin.
The skin has several other functions in fish. It is well supplied with nerve endings and presumably receives tactile, thermal, and pain stimuli. The skin is also well supplied with blood vessels. Some fish breathe in part through the skin, by exchanging oxygen and carbon dioxide between the surrounding water and numerous small blood vessels near the skin’s surface.
The skin serves as protection by controlling the coloration. Fish exhibit an almost limitless range of colors. Colors often blend closely with the environment, effectively hiding the animal. Many fish use bright colors for territorial advertising or as recognition marks for other members of their own species, or sometimes for members of other species. Many fish can change their color to a greater or lesser degree, by moving the pigment within the pigment cells (chromatophores). Black pigment cells (melanophores), almost universally occurring in fish, are often juxtaposed with other pigment cells. When placed under iridocytes or leucophores (which carry silver or white pigment guanine), melanophores produce structural colors of blue and green. These colors are often extremely intense because they are formed by refraction of light through the needle-shaped guanine crystals. The colors refracted in blue and green are often relatively pure, without the red and yellow rays, which have been absorbed by the black pigment (melanin) of the melanophores. The yellow, orange and red colors are produced by erythrophores, cells that contain the appropriate carotenoid pigments. Other colors are produced by combinations of melanophores, erythrophores, and iridocytes.
