Science Notes Fishes Fishes are one of the oldest groups of organisms on earth. They are vertebrates, having backbones, and they already have the foundation of the general body pattern of all vertebrates on our planet. Most fishes lay their eggs in the water, but a few, such as guppies, are live bearers. Some fish have swim bladders to help them with buoyancy.
Most fishes have body shapes that move easily in water.
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.