They are found in all marine, fresh, and brackish waters throughout the world and at all depths. Most are confined either to saltwater or to freshwater, but some are physiologically adapted to moving from one to the other. Fish may be carnivorous, herbivorous, or omnivorous. Some fish are scavengers on lake or ocean bottoms. Most fish propel themselves through the water by weaving movements of their bodies and control their direction by means of the fins .
All fishes have a lateral line system of sensory organs for detecting pressure changes in the water.
In all but the most primitive class, the gill passages are supported by skeletal structures called gill arches. 4 Reproduction Methods of reproduction are varied. Sharks have internal fertilization, and most give birth to live young. Those that lay eggs produce large ones with tough shells. Since embryonic development is well-protected in these fish, they produce a relatively small number of young, only seven or eight at a time in some species.