Fish are a paraphyletic group: that is, any clade containing all fish also contains the tetrapods, which are not fish. For this reason, groups such as the "Class Pisces" seen in older reference works are no longer used in formal classifications.
Fish are classified into the following major groups:
- Petromyzontidae (lampreys)
- Pteraspidomorphi (early jawless fish)
Cephalaspidomorphi (early jawless fish)
- Gnathostomata (jawed vertebrates)
Some palaeontologists consider that Conodonta are chordates, and so regard them as primitive fish.
For a fuller treatment of classification, see the vertebrate article.
The advent of jaws allowed fish eat a much wider variety of food, including plants and other organisms. In fish, food is ingested through the mouth and then broken down in the esophagus. When it enters the stomach, the food is further broken down and, in many fish, further processed in fingerlike pouches called pyloric ceca. The pyloric ceca 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.
Most fish exchange gases by using gills that are located on either side of the pharynx. 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. Some fishes, such as lungfish, have developed an adaptation known as a labyrinth that allows them to survive in oxygen-poor areas or places where bodies of water constantly dry up. These species of fish possess specialized organs that serve as lungs. A tube brings air containing oxygen to this organ by way of the fish's mouth. Some kinds of lungfish are so dependent on receiving oxygen from the air that they will suffocate if not allowed to reach the surface of the water.
Fish have a closed circulatory system with a heart that pumps the blood in a single loop throughout the body. The blood goes from the heart to gills, from the gills to the rest of the body, and then back to the heart. In most fishes, the heart consists of four parts: the sinus venosus, the atrium, the ventricle, and the bulbus arteriosus. Despite consisting of four parts, the fish heart is still a two-chambered heart. The sinus venosus is a thin-walled sac that collects blood from the fish's veins before allowing it to flow to the atrium, which is a large muscular chamber. 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.
As with many aquatic animals, most fishes release their nitrogenous wastes as ammonia. Some of the wastes diffuse through the gills into the surrounding water. Others are removed by the kidneys, excretory organs that filter wastes from the blood. 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.
Sensory and nervous system
Fish have well-developed nervous systems that organize around a central brain, that is divided into different parts. The most anterior, or front, end of the brain are the olfactory bulbs, which are involved in the fish's sense of smell. Unlike most vertebrates, the cerebrum of the fish primarily processes the sense of smell rather than being responsible for all voluntary actions. The optic lobes process information from the eyes. The cerebellum coordinates body movements while the medulla oblongata controls the functions of internal organs. Most fishes possess highly developed sense organs. Nearly all daylight fish have well-developed eyes that have color vision that is at least good as a human's. 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. In 2003, it was also found by Scottish scientists at Edinburgh University performing research on rainbow trout that fish experience pain. Some fishes, such as catfish and sharks, have organs that detect low levels electric current. Other fishes, like the electric eel, can produce their own electricity.
Most fish move by contracting paired sets of muscles on either side of the backbone alternately. These contractions form S-shaped curves that move down the body of the fish. As each curve reaches the back fin, backward force is created. This backward force, in conjunction with the fins, moves the fish forward. The fish's fins are used like an airplane's stabilizers. Fins also increase the surface area of the tail, allowing for an extra boost in speed. The streamlined body of the fish decreases the amount of friction as they move through water. Since body tissue is more dense than water, fish must compensate for the difference or they will sink. Many bony fishes have an internal organ called a swim bladder that adjust their buoyancy through manipulation of gases.
The eggs of fish are fertilized either externally or internally, depending on species. The female usually lays the eggs, and the embryos in the eggs develop and hatch outside her body. These kind of fish are called oviparous fish. Oviparous fish develop by obtaining food from the yolk in the egg. Salmon, for example, are oviparous.
Ovoviviparous fish keep the eggs inside of the mother's body after internal fertilization. Each embryo develops in its own egg. The young are "born alive" like most mammals.
Some species of fish, such as various sharks, are viviparous. Viviparous fish allow their embryos to stay in the mother's body like ovoviviparous fish. However, the embryos of viviparous fish obtain needed substances from the mother's body, not through material in the egg. The young of viviparous species are also "born alive".
Types of immune organs vary between different types of fish. In the jawless fish (lampreys and hagfishes), true lymphoid organs are absent. Instead, these fish rely on regions of lymphoid tissue within other organs to produce their immune cells. For example, erythrocytes, macrophages and plasma cells are produced in the anterior kidney (or pronephros) and some areas of the gut (where granulocytes mature) resemble primitive bone marrow in hagfish. Cartilaginous fish (sharks and rays) have a more advanced immune system than the jawless fish. They have three specialized organs that are unique to chondrichthyes; the epigonal organs (lymphoid tissue similar to bone marrow of mammals) that surround the gonads, the Leydig’s organ within the walls of their esophagus, and a spiral valve in their intestine. All these organs house typical immune cells (granulocytes, lymphocytes and plasma cells). They also possess an identifiable thymus and a well-developed spleen (their most important immune organ) where various lymphocytes, plasma cells and macrophages develop and are stored. Chondrostean fish (sturgeons, paddlefish and birchirs) possess a major site for the production of granulocytes within a mass that is associated with the meninges (membranes surrounding the central nervous system) and their heart is frequently covered with tissue that contains lymphocytes, reticular cells and a small number of macrophages. The chondrostean kidney is an important hemopoietic organ; where erythrocytes, granulocytes, lymphocytes and macrophages develop. Like chondrostean fish, the major immune tissues of bony fish (or teleostei) include the kidney (especially the anterior kidney), where many different immune cells are housed. In addition, teleost fish possess a thymus, spleen and scattered immune areas within mucosal tissues (e.g. in the skin, gills, gut and gonads). Much like the mammalian immune system, teleost erythrocytes, neutrophils and granulocytes are believed to reside in the spleen whereas lymphocytes are the major cell type found in the thymus. Recently, a lymphatic system similar to that described in mammals was described in one species of teleost fish, the zebrafish. Although not confirmed as yet, this system presumably will be where naive (unstimulated) T cells will accumulate while waiting to encounter an antigen. 
The early fossil record on fish is not very clear. It appears it was not a successful enough animal early in its evolution to leave many fossils. However, this would eventually change over time as it became a dominant form of sea life and eventually branching to include land vertebrates such as amphibians, reptiles, and mammals.
The formation of the hinged jaw appears to be what resulted in the later proliferation of fish because un-jawed fish left very few ancestors. Lampreys may be a rough representative of pre-jawed fish. The first jaws are found in Placodermi fossils. It is unclear if the advantage of a hinged jaw is greater biting force, respiratory-related, or a combination.
Some speculate that fish may have evolved from a creature similar to a coral-like Sea squirt, whose larvae resemble primitive fish in some key ways. The first ancestors of fish may have kept the larval form into adulthood (as some sea squirts do today, see Neoteny), although the reversal of this case is also possible. Candidates for early fish include Agnatha such as Haikouichthys, Myllokunmingia, and Pikaia.
Fish are susceptible to disease as any other organism.
Fish diseases can be refered to as etiology:
- Bacterial Disorders
- Fungal Disorders
- Parasitic Disorders
- Viral Disorders
- Metabolic Disorders
- Water conditions
or the organ system most affected
- Neurological Disorders
- Body Cavity
- Eye Disorders
- Fecal Disorders
- Fin Disorders
- Gill Disorders
- Intestinal Disorders
- Kidney Disorders
- Liver Disorders
- Locomotor Disorders
- Skin Disorders & Changes In Color
- Swim Bladder
- The Wikipedia Fish Category page which provides links to all aspects of the subject from icthyology to aquariums to sharks.
- Fish diseases Fish disease category
- Ichthyology (the study of fish)
- List of fish families
- List of fish common names
- Fish anatomy
- List of freshwater aquarium fish species
- Marine aquarium fish species
- Antimycin A piscicide
- Deep sea fish
- Walking fish
Note on usage: "fish" vs. "fishes"
"Fishes" is the proper English plural form of "fish" that biologists use when speaking about two or more fish species, as in "There are over 25,000 fishes in the world" (meaning that there are over 25,000 fish species in the world). When speaking of two or more individual fish organisms, then the word "fish" is used, as in "There are several million fish of the species Gadus morhua" (meaning that G. morhua comprises several million individuals). To see both in action, consider the statement "There are twelve fish in this aquarium, representing five fishes" (meaning that the aquarium contains twelve individuals, some of the same species and some of different species, for a total of five species). The usage of the two words is similar to that of the words "people" and "peoples". The collective noun for fish is shoal (or school).
- ^ A.G. Zapata, A. Chiba and A. Vara. Cells and tissues of the immune system of fish. In: The Fish Immune System: Organism, Pathogen and Environment. Fish Immunology Series. (eds. G. Iwama and T.Nakanishi,), New York, Academic Press, 1996, pages 1-55.
- ^ D.P. Anderson. Fish Immunology. (S.F. Snieszko and H.R. Axelrod, eds), Hong Kong: TFH Publications, Inc. Ltd., 1977.
- ^ S. Chilmonczyk. The thymus in fish: development and possible function in the immune response. Annual Review of Fish Diseases, Volume 2, 1992, pages 181-200.
- ^ J.D. Hansen and A.G. Zapata. Lymphocyte development in fish and amphibians. Immunological Reviews, Volume 166, 1998, pages 199-220.
- ^ Kucher et al.,. Development of the zebrafish lymphatic system requires VegFc signalling. Current Biology, Volume 16, 2006, pages 1244-1248.
- A World of Fish - Species profiles, fish humor, water conditions, definitions, setup info, equipment information, and much more.
- FishBase online - Comprehensive database with information on over 29,000 fish species
- Fishdatabase with german fishes - Informations about Freshwaterfishes from Germany
- University of Washington Libraries Digital Collections – Freshwater and Marine Image Bank -- Fish Species Part of an ongoing digital collection of images related to freshwater and marine topics.
- Join Ray Mullet in the Fish Olympics
- FINS Fish Guide - Site with a species database and guide to rearing fish.
Aquarium Fish Resources
- Age of Aquariums - Tropical Fish - A large aquarium hobbyist community that has been promoting responsible fishkeeping internationally since 1997.
- Aquatic Community - Site with one of the world's largest fish databases
- The Aquarium Fish Tank Friend - Site with FertFriend, Online Aquarium Fert Dosing Calculator
- The Aquarium Wiki - Discover detailed species profiles on how to care for fish, snails and frogs in an aquarium.
- Guide to Photographing Your Fishes - Tips to capture beautiful pictures of your pet fishes