Adaptations to aquatic life include various modes of feeding, specialized structures for
gas exchange, and paired fins for loco motion.
Feeding
There are herbivores, carnivores, parasites, filter feeders, and detritus feeders. In fact, a single fish may exhibit several modes of feeding, depending on what type of food happens to be available.
From the fish's mouth, food passes through a short tube called the esphagus to the stomach, where it is partially broken down. In many fishes, the food is further processed in finger like pouches called pyloric ceca. The pyloric ceca secrete digestive enzymes and absorb nutrients from the digested food. Any undigested material is eliminated through the anus.
Respiration
Most fishes exchange gases using gills located on either side of the pharynx. The gills are made up of feathery, thread-like structures called filaments. Fishes that exchange gases using gills do so by pulling oxygen-rich water in through their mouths, pumping over their gill filaments, and then pushing oxygen-poor water out through openings in the sides of the pharynx. Sharks and lampreys have several gill openings but most fish have a single gill opening on each side of the body through which water is pumped out. This opening is hidden beneath a protective bony cover of the operculum.
Circulation
Fishes have closed circulatory systems with a heart that pumps blood around the body in a single loop-from the heart to the gills, from the gills to the rest of the body, and back to the heart. In most fishes the heart consists of four parts: the sinus venosus, atrium, ventricle, and the bulbus arteriosus. The sinus venosus is a thin walled sac that collects blood from the fishes veins before it flows to the atrium, a large muscular chamber that serves as a one-way compartment for blood that is about to enter the ventricle. The ventricle is a thick walled muscular chamber that is the actual pumping portion of the heart. It pumps blood to a large, muscular tube called the bulbus arteriosus. At its front end, the bulbus arteriosus connects to a large blood vessel called the aorta, through which blood moves to the fish's gills.
Excretion
Fishes rid themselves of nitrogenous wastes in the form of amnonia. Kidneys help fishes control the amount of water in their bodies. A great deal of water continually enters the bodies of freshwater fishes. The kidneys of freshwater fishes pump out plenty of dillut urine. Some fishes are able to move from fresh to saltwater by adjusting their kidney function.
Response
Fishes have well developed nervous systems organized around a brain. The most anterior parts of a fishes brain are the olfactory bulbs, which are involved with the sense of smell. They are connected to the two lobes of the cerebrum. In most vertebrates, the cerebrum is responsible for all voluntary activities of the body. In fishes, however, the cerebrum processes the sense of smell. the optic lobes process information from the eyes. The cerebellum coordinates body movements. The medulla oblongata controls the function of many internal organs. Most fishes can detect gentle currents and vibrations in the water with sensitive receptors that form the lateral line system. They use this system to sense the motion of other fishes or prey swimming nearby.
Movement
Most fishes move by alternately contracting paired sets of muscles on either side of the backbone. This creates a series of S-Shaped curves that move down the fishes body. As each curve travels from the head toward the tail fin, it creates backwards force on the surrounding water. This force along with the action of the fins, propels the fish forward. The fins of fishes are used to keep them on course and adjust direction. Because their body issues are more dense then the water they swim in, sinking is an issue for fishes. Many bony fishes have an internal, gas-filled organ called a swim bladder that adjusts their buoyancy.
Reproduction
The eggs of fishes are fertilized either externally or internally, depending on the species. In many fish species, the female lays the eggs and the embryos and the eggs develop and hatch outside her body. Fishes whose eggs hatch outside the mothers are oviparous. As the embryos of oviparous fishes develop, they obtain food from the yolk in the egg. A few fish species, including several sharks, are viviparous. In viviparous animals, the embryos stay in the mothers body after internal fertilization as they do in ovoviviparous species.