The fins help to keep the body right side up. Fish need to weigh the same as, or a little less than, the water that would fill the space occupied by their bodies. Fishes and Salinity Life began in the early oceans, and the cells developed to work best with a certain amount of salt in their protoplasm. However, as time went on, salt continued to build up in the oceans, and so ocean fishes have less salt in their tissues than the water that surrounds them. They drink a lot of water, and have developed ways to get rid of the extra salt in their bodies.
Some fishes live in rivers and lakes.
Their body tissues are more salty than the water around them. They drink only a little of the water to conserve their salt. Fishes and Temperature Fishes are cold blooded, which means that their bodies are at the same temperature as the surrounding water. Each species of fish has a temperature range in which it functions well. If the water gets too cold, the fish almost stops eating and just waits.