Click to Enlarge Scales form growth rings just like those in trees. You'll need a magnifying glass to see the rings and find out a fish's age.   Fish From The Outside In by Barbara Baird illustrations by Mark Raithel From nose to tail, fish are adapted to life in their watery world. We use our legs, but fish move about with the help of fins. Fish have many fins. If you stand a fish up like a human, it has a pair of fins (pectoral fins) about where our shoulders are and another pair about where our legs would branch off from our bodies (pelvic fins). A fish maneuvers these fins to help it glide, slow down or angle upward or downward. The four fins move independently, giving the fish the ability to move quickly in any direction. Coordinating all these fins seems as difficult to us as flying a plane, but the ability comes naturally to fish. The dorsal fin, located along the back of a fish, works like the keel of a boat. It helps keep the fish upright and stable. Some fish have split dorsal fins. Some species of fish, such as sunfish, have sharp spines in their fins to discourage other fish from eating them. Trout, salmon and a few other groups of fish have fleshy adipose fins behind the dorsal fin, near the tail. These fins don't seem to perform any particular function. Anal fins are located on a fish's underside, between the vent and the tail. They help a fish propel itself and may be useful for choosing nest locations and other spawning activity. Fish rely on their tail, or caudal, fin for most of their forward propulsion. When fish contract the muscles in their bodies-first on one side and then on the other-they can whip, or gently move, their tails from side to side. Making sense of fish Fish see prey, danger, food and obstacles. Many fish have large eyes to help them see in dim light. Fish eyes are located on the side of the head, allowing them a wide range of view. Some species of fish have developed what looks like eyes on their tails. The extra "eyes" likely confuse predators and increase the chances of a fish surviving an attack. Some fish rely on smell more than vision to find food or alert them to danger, especially at night or in murky water. Scientists say trout have a keener sense of smell than humans. Fish have ear bones, called otoliths, that vibrate. Fish can hear very well. They sometimes flee from noisy motors or may be attracted to a noisy lure. Scientists believe at least some species of fish are able to communicate by making grunts, pops and squeaks. The otoliths also help a fish remain upright and level. A fish's tongue is full of taste buds, but many species also have taste buds on other parts of their bodies, including on the fins and face and near the tail. They can taste food before taking it into their mouths. Catfish have taste buds from head to tail. Many of them are packed into a catfish's barbels, which look like whiskers. They drag these barbels along the lake or river bottom to find food. Fish can orient themselves to the bottom and "feel" the substrate with their fins, but they also have a special sense receptors called a lateral line. Up close, the line is a series of pores, or holes, located along each side of the fish, from head to tail. Each pore contains hair-like nerve endings that vibrate to pressure changes or movement within the water. Scaly and Slimy The skin of many fish is covered with scales that protect a fish like roof shingles protect a house. You could also compare the covering of scales to a suit of armor. A fish only develops scales during its first year of life. As fish grow, their scales get bigger. Fish growth, and scale growth, slows in the winter and speeds up in the summer. The alternating periods of growth form rings on a scale just as they do in tree trunks. It's possible to examine these rings under a magnifying lens to see how old a fish is. All fish have body slime. The species with less well developed scales tend to have more slime. The slime reduces drag to help a fish move faster in the water. It also provides protection against parasites, covers wounds and helps a fish maintain the proper balance of salt in its body. Fish Guts What's left over after you clean a fish is pretty much bones, skin and guts. A fish has many of the same internal organs as we have. They have a heart to pump blood, intestines and stomach to digest food, a kidney, a liver, a gall bladder and a spleen. Their gills serve as lungs. Fish breathe like we do, but not in the same way. Fish absorb dissolved oxygen from water passing over their gills. The gills contain capillaries (fine blood vessels) that absorb up to 85 percent of the oxygen available in the water. The oxygen is used in metabolism. The harder a fish works, the more oxygen it needs. Imagine yourself breathing hard after exercise. If you were a fish, your gill covers (opercula) would move in and out, pumping more water through your mouth and over your gills. Fish would all sink to the bottom if they didn't have a special organ called a swim bladder. The bladder is actually a sac that holds gasses-mostly oxygen. Missouri darters don't have swim bladders. They sink if they stop swimming. The swim bladder works a little like a hot air balloon. The more oxygen it contains, the higher a fish will suspend. A fish can swim deeper or shallower, but the swim bladder takes a little time to adjust to the new depth. Fish brought up quickly from deep water by anglers experience a quick decrease in pressure that can swell their swim bladder to the point that it protrudes through their mouth and bulges their eyes. Some fish are able to vibrate their swim bladder, or they may squeeze it to make squeaks or grunts. Catfish make grunting sounds by pressing a flat bone against their swim bladder. The air bladder also acts as a resonator, amplifying sounds. Not So Fishy Facts! Missouri's smallest fish is the Banded Pygmy Sunfish. Its maximum length is 1.5 inches. Missouri's largest fish is the Lake Sturgeon. In 1991 a Lake Sturgeon was caught and released that was almost 8 feet long and weighed almost 200 pounds. It was caught from the Missouri River near Hartsburg. There are more than 20,000 living species of fish. Nearly 200 different species live in Missouri. The pin-like teeth found in a fish's mouth are used to capture and hold food, not chew it. Some fish have teeth in their throats to mash food. Fish don't need eyelids. The water they swim in keeps their eyes moist. Most sharks have transparent eyelids, called nictitating membranes. They likely serve to protect a shark's eyes from damage during vicious feeding frenzies.