Encyclopedia
A
fish is a
water-dwelling
vertebrate with
gills, that remains so throughout its life. Most are
cold-blooded, though some, such as some species of
tuna and
shark, are
warm-blooded. There are over 29,000 species of fish, making them the most diverse group of vertebrates. Taxonomically, fish are a
paraphyletic group whose exact relationships are much debated; a common division is into the
jawless fish , the
cartilaginous fish , with the remainder classed as
bony fish .
Fish come in different sizes, from the 16 m
whale shark to a 8 mm long stout infantfish. Many types of aquatic animals named "fish" are not true fish, and in the case of animals such as
jellyfish and
cuttlefish, are not even vertebrates. Other sea dwelling creatures, like
dolphins are actually mammals.
Although most fish are exclusively aquatic and cold-blooded, there are exceptions to both cases. Fish from a number of different groups have evolved the capacity to live out of the water for extended periods of time. Of these
amphibious fish some such as the
mudskipper can live and move about on land for up to several days. Also, certain species of fish maintain elevated body temperatures to varying degrees. Endothermic
teleosts are all in the suborder Scombroidei and include the billfishes, tunas, and one species of "primitive" mackerel . All sharks in the family
Lamnidae – shortfin mako, long fin mako, white, porbeagle, and salmon shark – are known to have the capacity for endothermy, and evidence suggests the trait exists in family
Alopiidae . The degree of endothermy varies from the billfish, which warm only their eyes and brain, to bluefin tuna and porbeagle sharks who maintain body temperatures elevated in excess of 20 °C above ambient water temperatures.
See also gigantothermy. Endothermy, though metabolically costly, is thought to provide advantages such as increased contractile force of muscles, higher rates of central nervous system processing, and higher rates of
digestion.
Fish are an important source of food in many cultures. Other water-dwelling animals such as
mollusks,
crustaceans, and
shellfish are often called "fish" when used as food. For more details, see
Fish .
Classification
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:
...
Some palaeontologists consider that
Conodonta are
chordates, and so regard them as primitive fish.
For a fuller treatment of classification, see the
vertebrate article.
Fish anatomy
Digestive system
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.
Respiratory system
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.
Circulatory system
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.
Excretory system
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.
Muscular system
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.
Reproductive system
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".
Immune system
Types of immune organs vary between different types of fish.
In the
jawless fish , 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 and some areas of the gut resemble primitive
bone marrow in hagfish.
Cartilaginous fish have a more advanced immune system than the jawless fish. They have three specialized organs that are unique to chondrichthyes; the epigonal organs 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 . They also possess an identifiable
thymus and a well-developed
spleen where various
lymphocytes, plasma cells and macrophages develop and are stored.
Chondrostean fish possess a major site for the production of granulocytes within a mass that is associated with the meninges 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 include the kidney , where many different immune cells are housed. In addition, teleost fish possess a thymus, spleen and scattered immune areas within mucosal tissues . 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.
[Refs 1-4]
Evolution
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.
Fish are the second-most successful group of animals in terms of number of species. Only
arthropods are more numerous, partly due to the number of insect species.
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, although the reversal of this case is also possible. Candidates for early fish include
Agnatha such as
Haikouichthys and Pikaia.
Fish disease
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
- Malnutrition
or the organ system most affected
- Neurological Disorders
- Body Cavity
- Eye Disorders
- Fecal Disorders
- Fin Disorders
- Gallbladder
- Gill Disorders
- Intestinal Disorders
- Kidney Disorders
- Liver Disorders
- Locomotor Disorders
- Skin Disorders & Changes In Color
- Swim Bladder
See also
which provides links to all aspects of the subject from icthyology to aquariums to sharks.
Fish disease category
- Ichthyology
- List of fish families
- List of fish common names
- Fish anatomy
- List of freshwater aquarium fish species
- Marine aquarium fish species
- Antimycin A piscicide
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" . 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" . To see both in action, consider the statement "There are twelve fish in this aquarium, representing five fishes" . The usage of the two words is similar to that of the words "people" and "peoples". The collective noun for fish is
shoal .
External links
- - Comprehensive database with information on over 29,000 fish species
- The Freshwater and Marine Image Bank is an ongoing digital collection of images related to freshwater and marine topics. From the University of Washington Libraries Digital Images Collection
- Join Ray Mullet in the
- - Site with a species database and guide to rearing fish.
Aquarium Fish Resources
- - A large aquarium hobbyist community that has been promoting responsible fishkeeping internationally since 1997.
- - Site with one of the world's largest fish databases
- - Site with FertFriend, Online Aquarium Fert Dosing Calculator
- - Information and help on keeping tropical fish.
- - Discover detailed species profiles on how to care for fish, snails and frogs in an aquarium.
- - Aquarium guide with access to database of aquatic fish, plants, biotopes and articles.
- - Tips to capture beautiful pictures of your pet fishes
References
- Zapata, A.G., Chiba, A. and Vara, A. Cells and tissues of the immune system of fish. In: The Fish Immune System: Organism, Pathogen and Environment. Fish Immunology Series. New York: Academic Press pp 1-55.
- Hansen, J.D. and Zapata, A.G. Lymphocyte development in fish and amphibians. Immunological Reviews 166: 199-220.
- Anderson, D.P. Fish Immunology. , Hong Kong: TFH Publications, Inc. Ltd.
- Chilmonczyk, S. The thymus in fish: development and possible function in the immune response. Ann. Rev. Fish Dis. 2: 181-200.
