What Fish Can Dog Eat

[Angelique Syria]

Afish (pl.: fish or fishes) is an aquatic, anamniotic, gill-bearing vertebrate animal with swimming fins and a hard skull, but lacking limbs with digits. Fish can be grouped into the more basal jawless fish and the more common jawed fish, the latter including all living cartilaginous and bony fish, as well as the extinct placoderms and acanthodians. Most fish are cold-blooded, their body temperature varying with the surrounding water, though some large active swimmers like white shark and tuna can hold a higher core temperature. Many fish can communicate acoustically with each other, such as during courtship displays.

The earliest fish appeared during the Cambrian as small filter feeders; they continued to evolve through the Paleozoic, diversifying into many forms. The earliest fish with dedicated respiratory gills and paired fins, the ostracoderms, had heavy bony plates that served as protective exoskeletons against invertebrate predators. The first fish with jaws, the placoderms, appeared in the Silurian and greatly diversified during the Devonian, the "Age of Fishes".

Bony fish, distinguished by the presence of swim bladders and later ossified endoskeletons, emerged as the dominant group of fish after the end-Devonian extinction wiped out the apex placoderms. Bony fish are further divided into the lobe-finned and ray-finned fish. About 96% of all living fish species today are teleosts, a crown group of ray-finned fish that can protrude their jaws. The tetrapods, a mostly terrestrial clade of vertebrates that have dominated the top trophic levels in both aquatic and terrestrial ecosystems since the Late Paleozoic, evolved from lobe-finned fish during the Carboniferous, developing air-breathing lungs homologous to swim bladders. Despite the cladistic lineage, tetrapods are usually not considered to be fish, making "fish" a paraphyletic group.

Fish have been an important natural resource for humans since prehistoric times, especially as food. Commercial and subsistence fishers harvest fish in wild fisheries or farm them in ponds or in breeding cages in the ocean. Fish are caught for recreation, or raised by fishkeepers as ornaments for private and public exhibition in aquaria and garden ponds. Fish have had a role in human culture through the ages, serving as deities, religious symbols, and as the subjects of art, books and movies.

The word fish is inherited from Proto-Germanic, and is related to German Fisch, the Latin piscis and Old Irish īasc, though the exact root is unknown; some authorities reconstruct a Proto-Indo-European root *peysk-, attested only in Italic, Celtic, and Germanic.[1][2][3][4]

About 530 million years ago during the Cambrian explosion, fishlike animals with a notochord and eyes at the front of the body, such as Haikouichthys, appear in the fossil record.[5] During the late Cambrian, other jawless forms such as conodonts appear.[6][7]

Jawed vertebrates appear in the Silurian, with giant armoured placoderms such as Dunkleosteus.[8] Jawed fish, too, appeared during the Silurian:[9] the cartilaginous Chondrichthyes[10][11] and the bony Osteichthyes.[12]

Fishes (without tetrapods) are a paraphyletic group and for this reason, the class Pisces seen in older reference works is no longer used in formal classifications. Traditional classification divides fish into three extant classes (Agnatha, Chondrichthyes, and Osteichthyes), and with extinct forms sometimes classified within those groups, sometimes as their own classes.[21]

Fish account for more than half of vertebrate species. As of 2016, there are over 32,000 described species of bony fish, over 1,100 species of cartilaginous fish, and over 100 hagfish and lampreys. A third of these fall within the nine largest families; from largest to smallest, these are Cyprinidae, Gobiidae, Cichlidae, Characidae, Loricariidae, Balitoridae, Serranidae, Labridae, and Scorpaenidae. About 64 families are monotypic, containing only one species.[16]

Fish range in size from the huge 16-metre (52 ft) whale shark[22] to some tiny teleosts only 8-millimetre (0.3 in) long, such as the cyprinid Paedocypris progenetica[23] and the stout infantfish.[24]

A typical fish is cold-blooded, has a streamlined body for rapid swimming, extracts oxygen from water using gills, has two sets of paired fins, one or two dorsal fins, an anal fin and a tail fin, jaws, skin covered with scales, and lays eggs. Each criterion has exceptions, creating a wide diversity in body shape and way of life. For example, some fast-swimming fish are warm-blooded, while some slow-swimming fish have abandoned streamlining in favour of other body shapes.[26]

Fish species are roughly divided equally between freshwater and marine (oceanic) ecosystems; there are some 15,200 freshwater species and around 14,800 marine species.[27] Coral reefs in the Indo-Pacific constitute the center of diversity for marine fishes,[28] whereas continental freshwater fishes are most diverse in large river basins of tropical rainforests, especially the Amazon, Congo, and Mekong basins.[29] More than 5,600 fish species inhabit Neotropical freshwaters alone, such that Neotropical fishes represent about 10% of all vertebrate species on the Earth.[30]

Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams (e.g., char and gudgeon) to the abyssal and even hadal depths of the deepest oceans (e.g., cusk-eels and snailfish), although none have been found in the deepest 25% of the ocean.[31] The deepest living fish in the ocean so far found is a cusk-eel, Abyssobrotula galatheae, recorded at the bottom of the Puerto Rico trench at 8,370 m (27,460 ft).[32]

A few fish live mostly on land or lay their eggs on land near water.[37] Mudskippers feed and interact with one another on mudflats and go underwater to hide in their burrows.[38] A single undescribed species of Phreatobius has been called a true "land fish" as this worm-like catfish strictly lives among waterlogged leaf litter.[39][40] Cavefish of multiple families live in underground lakes, underground rivers or aquifers.[41]

Like other animals, fish suffer from parasitism. Some species use cleaner fish to remove external parasites. The best known of these are the bluestreak cleaner wrasses of coral reefs in the Indian and Pacific oceans. These small fish maintain cleaning stations where other fish congregate and perform specific movements to attract the attention of the cleaners.[42] Cleaning behaviors have been observed in a number of fish groups, including an interesting case between two cichlids of the same genus, Etroplus maculatus, the cleaner, and the much larger E. suratensis.[43]

Fish occupy many trophic levels in freshwater and marine food webs. Fish at the higher levels are predatory, and a substantial part of their prey consists of other fish.[44] In addition, mammals such as dolphins and seals feed on fish, alongside birds such as gannets and cormorants.[45]

The body of a typical fish is adapted for efficient swimming by alternately contracting paired sets of muscles on either side of the backbone. These contractions form S-shaped curves that move down the body. As each curve reaches the tail fin, force is applied to the water, moving the fish forward. The other fins act as control surfaces like an aircraft's flaps, enabling the fish to steer in any direction.[46]

Since body tissue is denser than water, fish must compensate for the difference or they will sink. Many bony fish have an internal organ called a swim bladder that allows them to adjust their buoyancy by increasing or decreasing the amount of gas it contains.[47]

The scales of fish provide protection from predators at the cost of adding stiffness and weight.[48] Fish scales are often highly reflective; this silvering provides camouflage in the open ocean. Because the water all around is the same colour, reflecting an image of the water offers near-invisibility.[49]

Fish have a closed-loop circulatory system. The heart pumps the blood in a single loop throughout the body; for comparison, the mammal heart has two loops, one for the lungs to pick up oxygen, one for the body to deliver the oxygen. In fish, the heart pumps blood through the gills. Oxygen-rich blood then flows without further pumping, unlike in mammals, to the body tissues. Finally, oxygen-depleted blood returns to the heart.[50]

Fish exchange gases using gills on either side of the pharynx. Gills consist of comblike structures called filaments. Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. Capillary blood in the gills flows in the opposite direction to the water, resulting in efficient countercurrent exchange. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Cartilaginous fish have multiple gill openings: sharks usually have five, sometimes six or seven pairs; they often have to swim to oxygenate their gills. Bony fish have a single gill opening on each side, hidden beneath a protective bony cover or operculum. They are able to oxygenate their gills using muscles in the head.[51]

Some 400 species of fish in 50 families can breathe air, enabling them to live in oxygen-poor water or to emerge on to land.[52] The ability of fish to do this is potentially limited by their single-loop circulation, as oxygenated blood from their air-breathing organ will mix with deoxygenated blood returning to the heart from the rest of the body. Lungfish, bichirs, ropefish, bowfins, snakefish, and the African knifefish have evolved to reduce such mixing, and to reduce oxygen loss from the gills to oxygen-poor water. Bichirs and lungfish have tetrapod-like paired lungs, requiring them to surface to gulp air, and making them obligate air breathers. Many other fish, including inhabitants of rock pools and the intertidal zone, are facultative air breathers, able to breathe air when out of water, as may occur daily at low tide, and to use their gills when in water. Some coastal fish like rockskippers and mudskippers choose to leave the water to feed in habitats temporarily exposed to the air.[52] Some catfish absorb air through their digestive tracts.[53]

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