A typical fish's body is covered with thin scales that overlap each other like the shingles of a roof. They are prominent outgrowths of skin, or epidermis, in which numerous glands secrete a protective coating of slime, often referred to as mucus. The slime is a barrier to the entry of parasites, fungi, and disease organisms that might infest the fish, and it seals in the fish's body fluids so that they are not diluted by the watery surroundings. The slime also reduces friction so that the fish slides through the water with a minimum of resistance; it also makes the fish slippery when predators, including the human variety, try to grab hold. Some fish, such as lampreys and hagfish, give off copious amounts of slime.
As a fish grows, its scales increase in size but not in number. Lost scales may be replaced, however. The ridges and spaces on some types of scales become records of age and growth rate. These can be read or counted like the annual rings in the trunk of a tree to determine a fish's age—the fish's
growth slowing or stopping during winter when food is scarce and becoming much more rapid during the warm months when food is plentiful. Experts in reading scales can tell when a fish first spawned and each spawning period thereafter. They can determine times of migration, periods of food scarcity, illness, and similar facts about the fish's life. The number of scales in a row along the lateral line can be used to identify closely related species, particularly the young. Growth rings occur also in the vertebrae and in other bones of the body, but to study these requires killing the fish. A few scales can be removed without harm to the fish.
Most bony fish have tough, shinglelike scales with a comblike or serrated edge (ctenoid) along their rear margins or with smooth rear margins (cycloid). The scales of garfish are hard and almost bony, fitting one against the other like the bricks on a wall. These are called ganoid scales. Sturgeon also have ganoid scales, some of which form ridges of armor along portions of their sides and backs.
Sharks have placoid scales, which are the most primitive type. These scales are toothlike, each with a central spine coated on the outside with enamel and with an intermediate layer of dentine over a central pulp cavity. The skin of sharks, with the scales still attached, is the shagreen of commerce, widely used in the past and still used today in primitive areas as an abrasive, like sandpaper, or to make nonslip handles for knives and tools.
Cycloid scales have smooth rear margins, whereas ctenoid scales have comblike margins; placoid scales, found on sharks, are toothlike. Scales generally are layered, overlapping in rows like roof tiles.
The scales may be variously modified on different species. Some fish do not have scales at all. Most species of catfish, for example, are "naked" or smooth-skinned. Their skin is very slippery, however, and some of the rays in their fins are modified as sharp spines. Paddlefish and sculpin have only a few scales. The scales of mackerel are minute. Trout also have tiny scales. Those of eels are widely separated and buried deep in the skin.
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