There are more than 24,000 species of fish living today. The three main groups are: the jawless fish, including lamprey and hagfish; the cartilaginous fish (with a skeleton made of cartilage rather than bone), including sharks and rays; and the largest grouping -- the bony fish, including sturgeon, perch, and thousands of others.
There are some things 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. Fins come in various shapes and sizes.
Most fish also have scales and most hatch from eggs.
Wet and Wild
Fish can be found in oceans, rivers and wetlands all over the world, including saltwater and freshwater. They come in an incredible variety of shapes, sizes and colors. 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.