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. A few of the bony fishes, including some aquarium species, are live bearers, but most lay small, unprotected eggs that are fertilized after deposition in water. In most marine species the eggs float freely in the currents, where they are eaten by other animals. The eggs of most marine fishes contain oil droplets that buoy them up, while those of most freshwater fishes are heavy, with sticky surfaces that adhere to objects in the water.