Fishes are vertebrates that live in water, and breathe using special organs called gills. They can be found in nearly all aquatic environments, from high mountain streams to the abyssal and even hadal depths of the deepest oceans. There are over 30.000 known species of fish and they can form schools containing millions of them. They use their eyes and something called a lateral line to hold their places in the school. The lateral line is a row of pores running along the fish’s sides from head to tail. Special hairs in the pores sense changes in water pressure from the movements of other fish or predators.