Unlike mammals, fish are cold-blooded (ectothermic). This means that they do not maintain a constant internal body temperature; instead, their temperature is greatly influenced by their environment. True fish have a backbone and fins. Most also breathe with gills and have scales that cover their bodies. It is currently believed that fish began to evolve about 480 million years ago. There are about 22,000 known species of fish.
A fishâ??s fins are used for balance and to help propel and steer through the water. Most fish have 2 types of fins: single fins that are found along the centerline (top and bottom) of the fish, and paired fins. The caudal fin, or tail fin, is the main fin used to move the fish forward in the water, while the dorsal and anal fins (on the top and bottom, respectively) help the fish balance and keep it from rolling over. The paired fins help with steering and hovering.
