Most vertebrates belong to the clade Osteichtheys, this name means “bony fish”. Nearly all of the osteichthyans have an ossified bony endoskeleton, with a matrix made of calcium phosphate. If you have not already guessed, the osteichthyans are commonly known as the many fishes of the sea. Terminology in the biology world is quite long and complex …. So we will try to keep it a little simpler.
Most fish breathe by drawing the water in over their four or five gills. The water that is drawn in goes through the mouth then through the pharynx and then out through the gills again, this is all done through the contracting muscles of the gill chambers. As far as buoyancy (the upward force that keeps things afloat) goes, fish control their buoyancy by use of an air sac known as a swim bladder. The moving of gases from the blood to the swim bladder will increase the animals buoyancy, making it rise. The opposite process, as the gas moves back to the blood, makes the fish sink.
In nearly all fishes, bony scales cover the skin. The scales are covered by a slimy mucus that is secreted by the glands in the skin. This will reduce their drag under the water, and if you have ever gone fishing, or picked up a fish, you know just how slimy and slippery the fish really are. Most fish reproduce by external fertilization in which the female will lay many small eggs, and the male will go around to fertilize them. On the other hand, some fish do reproduce internally.
Most of the fish that we are familiar with are ray-finned fishes. They are so called because their fins are mainly supported by bony rays. Moreover, their rays may be modified for defense and some other functions. The ray finned fishes probably originated in the fresh waters and then spread out to the seas. Some of these ray finned fishes are still in the fresh waters, and you probably know many of them, like salmon and trout. Some of these fish even go back and forth from saltwater to freshwater. You probably also remember the ray-finned fishes as a source of food that’s on your dinner plate. For humans, these fishes are a major source of protein. In fact, we have over-fished many of these ray finned fishes. Cod for instance, was during the early 90’s restricted due to the over-fishing. In fact, Cod levels have yet to return to normal.