There are over thirty thousand different species of fish, and they are the most numerous vertebrates. Vertebrates are animals that have a bony spine that contains a nerve (spinal) chord. Vertebrates usually have an internal skeleton that provides support and protection for internal organs. This spine and skeleton allow vertebrates to move quickly and to have great strength.

Fish usually live surrounded entirely by water. They all have gills for breathing and fins for swimming. Most fish are ectotherms, which means that their bodies are nearly the same temperature as the water in which they live. About 60% of all fish live entirely in saltwater, while the rest live in freshwater or both freshwater and saltwater.

One of the most remarkable things about fish is their diversity. Fish can be very large, like the whale sharks that can reach 90,000 pounds (41,000 kilograms) or very small, like gobies that...