Body of the fish. The body of a fish is particularly adapted to aquatic life. The body is equipped with fins for the purpose of locomotion. Scales and mucus protect the body and keep it streamlined. The skeleton features a long backbone that can produce the side-to-side movements needed for forward propulsion in water. Since water is 800 times more dense than air, fish must be extremely strong to move in their environment. Fish respond to this condition by being mostly muscle. Thus, muscles make up 40 to 65 percent of a fish's body weight. Many fish have air or gas bladders (sometimes called swim bladders), which allow them to float at their desired depth. Fish also have gills, their underwater breathing apparatus, located in the head. Most fish have only one gill cover, although some, like sharks, have gill slits, some as many as seven. The gills are the most fragile part of the fish; anglers should avoid touching the gills on fish that they plan on releasing.

The limbs of fish come in the form of fins. A fin is a membrane that extends from the body of the fish and is supported by spines or rays. Because the number of rays is usually constant within a species, a ray count is often used by scientists to determine the species of a fish. Each of the fins on a fish has a name. Since these names are used in almost all descriptions of fish and are used in this book, it is worthwhile to become familiar with the different fin names.

Moving from the head toward the tail, the first fins are the pectoral fins. The pectoral fins are used for balance and maneuvering in many species and in a few are used for propulsion. Further down the underside of the fish are the pelvic fins, located beneath the belly and used for balance. On the back of the fish is the dorsal fin. Some fish have more than one dorsal fin; in this case the dorsal fins are numbered, with the fin closest to the head called the first dorsal fin. Behind the dorsal fin on the top part of the fish there is occasionally a smaller, fleshy fin called the adipose fin. Back on the underside of the fish, behind the pelvic fins and the anus, is the anal fin. The final fin, usually called the tail, is known scientifically as the caudal fin. The caudal fin is the most important fin for locomotion; by moving it from side to side, a fish is able to gather forward momentum.

The scales of a fish form the main protection for the body. Fish scales are kept for the entire life of a fish; as a fish grows, the scales get larger, rather than growing anew. Scales are divided into several types. Most fish have ctenoid or cycloid scales. Ctenoid scales are serrated on one edge and feel rough when rubbed the wrong way (largemouth bass have such scales). Cycloid scales are entirely smooth, like the scales of trout. More rare types of fish have different types of scales: Sharks have more primitive placoid scales, which are spiny; sturgeon have ganoid scales, which form armor ridges along parts of the body. Some species, like catfish, have no scales at all. Fish scales can be used to determine the age of a fish. A fish scale will develop rings showing annual growth, much like the rings of a tree.

Many fish also have a covering of mucus that gives them a slimy feel. This covering helps streamline their body and prevent infections. The mucus covering will rub off onto a person's hands (this is the slimy substance that you can feel on your hands after handling a fish). Since the loss of mucus is detrimental to the fish, it is better to wet your hands before handling a fish that will be released to minimize the amount of mucus removed, being careful not to harm a fish by holding it too tightly.

The skeletal and muscular systems of fish work together to maximize swimming power. The serially repeated vertebrae and the muscle structure work together to create the shimmering, undulating muscle movements that allow a fish to move forward quickly. This structure is particularly evident in a filleted fish, where the muscles show themselves in their interlocking pattern. The muscular nature of fish is the reason why fish make such good eating and is also a factor in making fish a high-yield food source.

Bony fish have developed an organ called an air bladder, which acts as a kind of flotation device. A fish's body is naturally a bit more dense than water, but the air bladder, filled with gas, increases a fish's ability to float. Fish can change the depth at which they float by varying the amount of gas in their air bladder. This allows a fish to float at any depth it desires without expending any effort. Fish that do not have air bladders, such as sharks, must continually move in order to prevent their sinking.

Like virtually all animals, fish need oxygen to survive. However, a fish can get all the oxygen it needs from water by the use of its gills. Water entering through the mouth of the fish is forced over the gills, and oxygen is removed from the water by the gills. In order to breathe, fish must constantly have water passing over their gills. However, in order to get enough oxygen, certain fish must either move continually or live in water with a strong current.

Although most fish are referred to as cold-blooded creatures, this is mostly but not entirely true. Some species are called warm-blooded, yet they cannot sustain a constant body temperature as humans do. Instead, the body temperature of fish approximates that of their surrounding medium— water. Certain types of fish, such as tuna, by their constant vigorous propulsion through the water, sustain high muscular flexion that creates heat associated with rapid metabolism. Through built-in heat-conservation measures, the fish is capable of maintaining a warmer body temperature than the medium that upholds it; for example, a bluefin tuna's fighting qualities are not impaired physically when it suddenly dives from surface waters where it was hooked down to the colder depths.

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