All fish are vertebrates, which means they have a backbone, and are cold-blooded, which means their internal temperature changes to match the temperature of their surroundings. There are 25,000 known species of fish, which is more than all the mammals, birds, reptiles and amphibians put together.
The largest fish in the world is the whale shark, which can reach up to 50 feet long. The smallest fish in the world is the Phillipine goby, which is one-third of an inch long. To help them find their way in dark water, fish have a special sense organ called a lateral line, which works like a radar.