Fish, Fish

Fish are abundant in the sea and in fresh water, with species being known from mountain streams (e.g., char and gudgeon) as well as in the deepest depths of the ocean (e.g., gulpers and anglerfish).

They are harvested either from wild fisheries (see fishing) or farmed in much the same way as cattle or chickens (see aquaculture). They are also exploited by recreational fishers and fishkeepers, and are exhibited in public aquaria. Fish have had a role in many cultures through the ages, ranging from deities and religious symbols to the subjects of books and popular movies.

Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 3, 1997, ISBN 0-86542-256-7 Tree of life web project - Chordates.

Tuna, swordfish, and some species of sharks show some warm-blooded adaptations, and are able to raise their body temperature significantly above that of the ambient water surrounding them. Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, pp 83-86, 1997, ISBN 0-86542-256-7 Streamlining and swimming performance varies from highly streamlined and rapid swimmers which are able to reach 1020 body-lengths per second (such as tuna, salmon, and jacks) through to slow but more maneuverable species such as eels and rays that reach no more than 0.5 body-lengths per second. Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 103, 1997, ISBN 0-86542-256-7 Many groups of freshwater fish extract oxygen from the air as well as from the water using a variety of different structures. Lungfish have paired lungs similar to those of tetrapods, gouramis have a structure called the labyrinth organ that performs a similar function, while many catfish, such as Corydoras extract oxygen via the intestine or stomach. Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, pp 53-57, 1997, ISBN 0-86542-256-7 Body shape and the arrangement of the fins is highly variable, covering such seemingly un-fishlike forms as seahorses, pufferfish, anglerfish, and gulpers. Similarly, the surface of the skin may be naked (as in moray eels), or covered with scales of a variety of different types usually defined as placoid (typical of sharks and rays), cosmoid (fossil lungfishes and coelacanths), ganoid (various fossil fishes but also living gars and bichirs, cycloid, and ctenoid (these last two are found on most bony fish. Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, pp 33-36, 1997, ISBN 0-86542-256-7 There are even fishes that spend most of their time out of water. Mudskippers feed and interact with one another on mudflats and are only underwater when hiding in their burrows.

In earlier times, even biologists did not make a distinction - sixteenth century natural historians classified also seal, whales, amphibians, crocodiles, even hippopotamuses, as well as a host of aquatic invertebrates, as fish. Jr.Cleveland P Hickman, Larry S. Roberts, Allan L. Larson: Integrated Principles of Zoology, McGraw-Hill Publishing Co, 2001, ISBN 0072909617 In some contexts, especially in aquaculture, the true fish are referred to as finfish (or fin fish ) to distinguish them from these other animals.

When it enters the stomach, the food is further broken down and, in many fish, further processed in finger-like pouches called pyloric caeca. The pyloric caeca secrete digestive enzymes and absorb nutrients from the digested food. Organs such as the liver and pancreas add enzymes and various digestive chemicals as the food moves through the digestive tract. The intestine completes the process of digestion and nutrient absorption.

Gills are made up of threadlike structures called filaments. Each filament contains a network of capillaries that allow a large surface area for the exchange of oxygen and carbon dioxide. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gill filaments. The blood in the capillaries flows in the opposite direction to the water, causing counter current exchange. They then push the oxygen-poor water out through openings in the sides of the pharynx. Some fishes, like sharks and lampreys, possess multiple gill openings. However, most fishes have a single gill opening on each side of the body. This opening is hidden beneath a protective bony cover called an operculum.

The skin of anguillid eels may be used to absorb oxygen. The buccal cavity of the electric eel may be used to breathe air. Catfishes of the families Loricariidae, Callichthyidae, and Scoloplacidae are able to absorb air through their digestive tracts.

Loaches, trahiras, and many catfish breathe by passing air through the gut. Mudskippers breathe by absorbing oxygen across the skin (similar to what frogs do). A number of fishes have evolved so-called accessory breathing organs that are used to extract oxygen from the air. Labyrinth fish (such as gouramis and bettas) have a labyrinth organ above the gills that performs this function. A few other fish have structures more or less resembling labyrinth organs in form and function, most notably snakeheads, pikeheads, and the Clariidae family of catfish.

At such times, fishes dependent solely on the oxygen in the water, such as perch and cichlids, will quickly suffocate, but air-breathing fish can survive for much longer, in some cases in water that is little more than wet mud. At the most extreme, some of these air-breathing fish are able to survive in damp burrows for weeks after the water has otherwise completely dried up, entering a state of aestivation until the water returns.

Fish can be divided into obligate air breathers and facultative air breathers . Obligate air breathers, such as the African lungfish, must breathe air periodically or they will suffocate. Facultative air breathers, such as the catfish Hypostomus plecostomus , will only breathe air if they need to and will otherwise rely solely on their gills for oxygen if conditions are favourable. Most air breathing fish are not obligate air breathers, as there is an energetic cost in rising to the surface and a fitness cost of being exposed to surface predators.

The atrium serves as a one-way compartment for blood to flow into the ventricle. The ventricle is a thick-walled, muscular chamber and it does the actual pumping for the heart. It pumps blood to a large tube called the bulbus arteriosus. At the front end, the bulbus arteriosus connects to a large blood vessel called the aorta, through which blood flows to the fish's gills.

Kidneys help fishes control the amount of ammonia in their bodies. Saltwater fish tend to lose water because of osmosis. In saltwater fish, the kidneys concentrate wastes and return as much water as possible back to the body. The reverse happens in freshwater fish: they tend to gain water continuously. The kidneys of freshwater fish are specially adapted to pump out large amounts of dilute urine. Some fish have specially adapted kidneys that change their function, allowing them to move from freshwater to saltwater.

Behind the olfactory lobes is the two-lobed telencephalon, the equivalent structure to the cerebrum in higher vertebrates. In fishes the telencephalon is concerned mostly with olfaction.

Many fish also have specialized cells known as chemoreceptors that are responsible for extraordinary senses of taste and smell. Although they have ears in their heads, many fish may not hear sounds very well. However, most fishes have sensitive receptors that form the lateral line system. The lateral line system allows for many fish to detect gentle currents and vibrations, as well as to sense the motion of other nearby fish and prey.

Source: Wikipedia > Fish