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Fish are largely distinguished from other animals by their adaptation to their environment, which is of course water. Fish physiology differs in some key ways from the physiology of animals that inhabit land. The major differences lie in how fish breathe, or obtain oxygen, and how they move through water. Fish have also made other remarkable physiological adjustments to their environment.
Like animals on land, fish need oxygen to survive. The problem is that water has only two percent of the amount of oxygen contained in air. In addition, oxygen levels decrease as water gets warmer, and polluted or stagnant water has less oxygen as well.
Fish get oxygen mainly through their gills. By opening and closing their mouths, fish move water over their gills, which are filled with thousands of tiny blood vessels that absorb oxygen and send it into to the bloodstream. A few fish can take in oxygen in other ways. For example, a tarpon — a large saltwater fish, can swim to the surface and take in a little extra oxygen when needed. The lungfish has gills but gets a lot of its oxygen by swallowing air that fills up a sac that is somewhat like a lung, thus its name.
How fish move through water is a pretty remarkable element of fish physiology as well. Since water is dense fish must be very strong in order to move through it. Fish have a lot of muscles that enable them to swim. Fins also allow fish to move forward and backward, and their strong tailfins help propel them through the water. The majority of fish have seven fins, but some have six or eight.
The swim bladder, or air bladder, is another important element of fish physiology. Fish remain buoyant and move up and down in water by decreasing or increasing the amount of air in their swim bladders. Some fish also use the swim bladder to intensify sound.
Fish physiology has adapted to the environment in other important ways. Most fish have a protective covering of scales made out of calcium, which protect fish them injury and sickness. Another important adaptation has to do with staying hydrated and maintaining a proper salt balance, which is a special challenge for saltwater fish. Ingesting too much salt is not good, so saltwater fish drink water and excrete salt through their urine and gills. Freshwater fish take in water through their gills and skin rather than drinking it.