There are more fish than tetrapods (land vertebrates): there are over 33,000 described species of fish. Fish are usually covered with scales. They have two sets of paired fins and several unpaired fins. Most fish are cold-blooded (poikilotherm). A fish takes in the oxygen from the water using gills. There are many different kinds of fish. They live in fresh water in lakes and rivers, and in salt water in the ocean. Some fish are less than one centimeter long. The largest fish is the whale shark, which can be almost 15 meters long and weigh 15 tons. Most fish live in the water. A group of fish called the lungfish have developed lungs because they live in rivers and pools which dry up in certain parts of the year. They burrow into mud and aestivate until the water returns.