Nervous system Diet Reproduction Habitat Economic
Fish can be divided into three goups. The largest group of fish, bony fish, have skeletons.
Cartiliganous fish, such as sharks and rays, have cartilage rather than bones.
The third group jawless fish, are fish with suction cups instead of jaws.
Fish are also cold blooded animals. This means that their blood temperature changes with the temperature of the surrounding water. Most fish live close to the shore and are found in water that is less than 180 metres or 600 ft deep.
In the deepest parts of the ocean , the light is very dim. Many of the fish that live at these great depths of 2000 feet or deeper glow in the dark.
The word fish is normally used to refer to an individual animal or to a species. A fish is an aquatic vertebrate animal, usually possessing gills and limbs, in the forms of fins; this also means that they are animals with backbones. Fish are the only completely aquatic vertebrate animal and spend all of their lives in the water.
In general, fish are normally torpedo-shaped; their bodies are normally flat to round at the sides and taper more markedly at the tail than at the head.
The main feature is the repeated set of vertebrae and segmented muscles, which enable the fish to propel itself by moving its body from side to side.
The body of a fish normally carries a number of fins, which are membranes supported by rays or spines that function in the propulsion or orientation. One or more dorsal fins may be located along the centerline of the back. A fin called the caudal fin lies at the end of the tail and is the main organ for generating thrust in most species. One or more anal fins are situated on the ventral midline between the vent and the tail.
The body has two pairs of pectoral fins; these are usually located on the fish’s sides just behind the gill openings, and the pelvic fins, which lie on the belly between the head and the tail.
The various species of fish also range in extreme variations of size. Small blennies around 50mm in length found in rock pools up to the monstrous whale shark that can reach 15 meters (50 ft) in length and an estimated weight of 68,000 kg (150,000 lb). Fish vary greatly in coloring as well, and display a wide range of colors and patterns. Generally the coloration is lighter on the belly than on the sides and back. A large number of tropical fish such as the butterfly fish, clown fish etc are brilliantly colored and patterned. And some fish have the ability to change their colors to enable them to blend in better with their surroundings.
Some species have specialized shapes and organs that aid them in hunting and feeding. Among such fishes are the lantern fish and other deep-water species, which have luminous organs to reveal or attract their prey. The anglerfish lies on the ocean bottom and presents a small, worm-like knob of flesh on the end of a long spine as bait to other fishes.
The bodies of most fish are covered with a layer of scales, These scales are bony or horny plates arranged in overlapping rows, with the free rear end of one scale overlapping the attached front end of the scale behind it.
The scaly skin covering the body of a fish comprises the dermal skeleton. The inner bony framework of most modern fish consists of a skull, containing jaws equipped with teeth; a vertebral column; ribs; a pectoral arch; and a series of interspinal bones that support the fins.
The digestive system of a fish usually consists of a mouth with rows of sharp, crushing, or brush-like teeth, a pharynx, an oesophagus, a stomach a pancreas and a liver and an intestine ending in the anal vent.
The respiratory apparatus of jawed fishes consists of a series of slits, the gill clefts, which open and close. These chambers open to the water outside.
Fish cannot breath air, but they have to breath oxygen from the water. When the fish takes in water and expels it through the gills, dissolved oxygen passes across the thin gill membranes into the blood, and carbon dioxide waste passes out of the blood into the water.
Most bony fish have an organ used in buoyancy control. Called the swim bladder, this precursor of the lung is a chamber that opens off the alimentary canal and fills with oxygen and nitrogen taken from the blood. The chief function of this organ is to adjust the fish to the varying water pressures at different depths so that the animal will have neither positive nor negative buoyancy. Thus the fish may maintain its depth without effort.
The circulatory system in most fishes is simple, consisting of a two-chambered heart that forces blood forwards through the gills, then to the head, and from the head back to the rest of the body through a major artery situated beneath the spine. The rate of circulation is slower in fishes than in other vertebrates.
The chief muscles in the body of a fish are arranged along the sides of the trunk and tail. The larger mass runs along the back on each side of the spinal cord, and the smaller mass is located below it. Each muscle mass is composed of a series of interlocking segments. In ordinary swimming the successive contraction of the muscle segments from front to back on each side alternately gives the caudal fin a wavy motion. Small muscles control the motions of mouth, gills, fins, and eyes. A few types of fishes, such as the eel, swim by sinuous motions of their bodies. Certain others, among them the trunkfish, propel themselves by the action of their fins without much body motion.
The central nervous system of most fishes consists of a spinal cord and a brain that has a large cerebellum, a pair of optic lobes, a small cerebrum, and a medulla oblongata. The form and size of various parts of the brain vary markedly in different species. The eyes have almost spherical lenses with a flattened cornea. The entire lens of the eye is moved towards or away from the retina in order to focus at different distances. The eyes of some cave fishes that live in complete darkness are rudimentary or absent. Fishes smell by means of a pair of double nostrils leading into an olfactory pit, which is part of the smell organ. Many fishes detect chemical stimuli through sense organs or tentacles (barbels) around the mouth or on other parts of the body.
Fish are able to hear by sensing sound vibrations, these are transmitted through the bones of the skull to an internal ear containing three semicircular canals. This inner ear acts as an organ of equilibrium as well as an auditory organ. Fishes are also equipped with unique sensory organs called lateral lines. These organs consist of canals that run along the sides of the head and body and connect with the outside surface of the fish through small pores. The chief function of the lateral line is to sense extremely low-frequency vibrations, but in some species it can also detect weak electrical fields.
Most fish are carnivorous and eat mainly other fish but can also eat worms, shellfish, and other kinds of water animals.
The herbivorous fish have a diet of algae and other water plants, although they probably also eat fish.
Some of the larger fishes, such as the whale shark, and also the flying fish and herring feed on plankton. Those fish that feed on plankton are known as filter feeders as they have comb-like structures, gill rakers that strain plankton from the water that is pumped through the gills. Others are scavengers and live on waste products or the dead bodies of other animals found on the seabed.
Fish have a number of body organs that have been specially adapted for the capture of food. The razor-sharp teeth of barracudas, and sharks, are used to tear the flesh of their victims. Electric fish, such as the electric eel, stun their prey with electric shocks, using their electricity-producing organs.
Fishes have various ways of producing young.
OVIPAROUS fish are those that lay eggs that are fertilized outside the female's body. The female lays her eggs in the water and the male then covers them in sperm. In such species, development of the young is also external. Species that scatter eggs in open water often produce eggs in prodigious numbers. A single fish for example, may produce up to 7 million eggs. Other egg-layers, such as the Pacific salmon, may undertake remarkable homing migrations associated with spawning activity. Parental care after hatching may be absent, or it may be elaborate, often involving the defence of a nest or territory. In the bowfin and some African species of cichlid, the young enter the mouth of a parent for protection when predators threaten.
VIVIPAROUS fish have internal fertilization and give birth to young in an advanced state of development. These fish mate in pairs and copulate, fertilization of the eggs taking place inside the body of the female. Various mechanisms exist whereby nutrients are provided to the embryos, which may greatly increase in size before birth.
Fish occupy almost every conceivable aquatic habitat. From the smallest rock pool to the great depths of the ocean (7,000 metres/22,960 ft). In temperatures as high as 45° C to as cold as the Antarctic at about -2° C. The water does not freeze at this temperature because of its high salt content, and the fish does not freeze because its blood contains a form of biological antifreeze. Some fish live in almost pure fresh water, while other fish are able to tolerate salinity as high as four times that of the sea. Cavefish may pass their lives in complete darkness, while fish in desert marshes experience record levels of solar radiation.
The greatest number of marine species are found in tropical waters, particularly around coral reefs.
Fish are one of the most important sources of animal protein for humans, and many fish are used as food. Other uses of fish and fish products include the manufacture of fertilizers from fish and fish scraps, the extraction of fish-liver oils as one of the sources of vitamin D, and the manufacture of pet food. Fish scales are sometimes used in making artificial pearls. Isinglass, a form of gelatin, is prepared from the swim bladders of certain species, and glue can be made from fish offal.