Fish are vertebrates that live in the water. Fish are the most diverse group of vertebrates, with more species than any other vertebrate group. They use fins to help them swim, and they breathe underwater with gills. The gills are usually found in one or more slits in the skin behind their heads. Some fish lay eggs, while others give birth to live babies.
All fish are covered with a thin layer of slime on the outside of their bodies to help them move in the water, and most kinds of fish have scales on their skin under the slime. Most fish are "cold-blooded," which means that their body temperature is controlled by the temperature of the water around them. Fish can see, smell, and taste and almost all fish have a special sensory organ called the "lateral line system" that lets the fish feel vibrations in the water. Many fish can also detect the weak electricity given off by other animals in the water.
There are fish that eat just about every kind of food. Most of the fish included in the Critter Catalog are predators on smaller fish or invertebrates and a few eat plants. Freshwater fish are found in ponds, lakes, streams, rivers, or swamps. Marine fish are found in oceans, estuaries, tidal pools or salt marshes.