There are more than 24,000 species of fish living today, and it’s believed that many more species, living at extreme depths, have yet to be discovered. The three main groups are: the cartilaginous fish (with skeletons of cartilage rather than bone) which include sharks, rays and skates, the jawless fish, including lamprey and hagfish; and the largest grouping — the bony fish, including sturgeon, perch, and thousands of others.
One thing that all fish have in common: they breathe with gills. As water flows past the gills, oxygen in the water passes through the gill membranes and into the fish’s blood.
All fish also have fins, which help them swim forward, turn, and stop, as well as keep upright.
Most fish also have scales and most hatch from eggs.
Under the Sea
Fish can be found in oceans, rivers and wetlands all over the world, including saltwater and freshwater. These underwater residents come in an incredible variety of shapes and colors. They range in size from the pygmy goby, about a half-inch in length, to the tuna and marlin, which may be as much as 15 feet long, and the ocean sunfish, which may weigh as much as a ton.
Fish have adapted to their watery world with amazing features-some fish can puff up to look much bigger, some are flat as a pancake for a life on the ocean floor, and some even glow in the dark.