Fish Evolution Homeothermy Diseases

Fish are aquatic vertebrates that are cold-blooded, covered with scales, and equipped with two sets of paired fins and several unpaired fins. 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 of tremendous importance as food for people around the world, either collected from the wild (see fishing) or farmed in much the same way as cattle or chickens (see aquaculture). Fish are also exploited for recreation, through angling and fishkeeping, and fish are commonly exhibited in public aquaria. Fish have an important role in many cultures through the ages, ranging as widely as deities and religious symbols to subjects of books and popular movies.

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.

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, Pikaia, and Conodonts.

Homeothermy
A 3 to 4 m great white shark off Isla Guadalupe

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 (bony fishes) are all in the suborder Scombroidei and include the billfishes, tunas, and one species of "primitive" mackerel (Gasterochisma melampus). 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 (thresher sharks). 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.

Diseases

Like other animals, fish can suffer from a wide variety of diseases and parasites. To prevent disease they have a variety of non-specific defences and specific defences. Non-specific defences include the skin and scales, as well as the mucus layer secreted by the epidermis that traps microorganisms and inhibits their growth. Should pathogens breach these defences, fish can develop an inflammatory response that increases the flow of blood to the infected region and delivers the white blood cells that will attempt to destroy the pathogens. Specific defences are specialised responses to particular pathogens recognised by the fish's body, in other words, an immune response.  In recent years, vaccines have become widely used in aquaculture and also with ornamental fish, for example the vaccines for furunculosis in farmed salmon and koi herpes virus in koi.

Some fish will also take advantage of cleaner fish for removal of external parasites. The best known of these are the cleaner wrasses of the genus Labroides found on coral reefs in the Indian Ocean and Pacific Ocean. These small fish maintain so-called "cleaning stations" where other fish, known as hosts, will congregate and perform specific movements to attract the attention of the cleaner fish.  Cleaning behaviours have been observed in a number of other fish groups, including an interesting case between two cichlids of the same genus, Etroplus maculatus, the cleaner fish, and the much larger Etroplus suratensis, the host.