Most fish breath using gills. They take water in through their mouth and push it out through their gills. As the water enters the fishes mouth, it is rich in oxygen. When it passes through the gills, the oxygen is absorbed from the water as it flows through the capillary-rich gill filaments. At the same time, the waste product of respiration--carbon dioxide--is released into the water before it is then released from the gills through openings on either side of the pharynx.
Most fish are cold-blooded animals that have a streamlined body that is adapted for efficient movement in water. There are exceptions to both of these rules though. Tuna, swordfish and a few shark species are warm-blooded, not cold-blooded. Rays are flat-bodied fish that not streamlined. They move through the water at a slower pace by undulating their broad pectoral fins.
The development of jaws in the evolutionary history of fish represents an important step. Jaws enabled fish to catch and eat a wide variety of food including marine plants and animals. Food is broken down in the esophagus and digested in the stomach and digestive tract. Waste is excreted in the form of ammonia.
Most fish move using pairs of muscles on either side of their backbone. These muscle pairs contract in sequence such that the fish moves in S-shaped wave through the water. Fish have a streamlined body that enables them to move efficiently through the water. Many species have a swim bladder, a organ that contains air and enables the fish to control its buoyancy in the water.
There are many animals whose name includes the term fish but that are not true fish. Such organisms include crayfish (a group of crustaceans), cuttlefish (a group of cephalopod), starfish (a group of echinoderm), and jellyfish (a group of invertebrates).
A group of fish that is loosely organized, with each individual fish foraging on its own independently is called a shoal. A tightly organized group of fish that moves and feeds as one coherent unit is called a school.
The largest living fish is the whale shark which grows to 60 feet in length and over 20 tons. The smallest known living fish is Paedocypris progenetica, a species that belongs to the carp family and is native to the Indonesian island of Sumatra. It is less than 8 mm in length.
Fish do not represent a true taxonomic class and consequently describing any classification of fishes is fraught with conditions and exceptions. But for the purposes of this site, we can make some simplifications and generalizations and say that fishes are divided into four subgroups: bony fishes, cartilaginous fishes, hagfish and lampreys.
It should be noted that hagfish and lampreys are particularly unique and their classification as fish is debatable. Hagfish and lampreys may not even belong to the chordates.
The first fish were the ostracoderms, a now-extinct group of jawless fishes that appeared in the Cambrian Period, about 510 million years ago. Ostracoderms were small, armored fish that lacked jaws. They were followed by the conodonts and the agnanthans. Fish are the most diverse group of vertebrates alive today. The ray-finned fishes alone number in the region of 24,000 species.