OSTARIOPHYSI

'Ostariophysi' is the second-largest superorder of fish. Members of this superorder are called 'ostariophysans'. This diverse group contains almost 8,000 species, about 28% of known fish species in the world and 68% of freshwater species, and are present on almost all major continents except Antarctica. They have a number of common characteristics such as an alarm substance and a 'Weberian apparatus'. Members of this group include fish important to people for food, sport, the aquarium industry, and research.

Contents

Taxonomy

Evolution

Diversity

Physical characteristics

Weberian apparatus

References

Taxonomy

Currently, this superorder is divided into two series, 'Anotophysi' and 'Otophysi'. However, in older literature, Ostariophysi was restricted only to the fish that are currently classified under Otophysi. Otophysi was coined in 1970 by Rosen and Greenwood to separate the traditional Ostariophysans from the added Gonorynchiformes.^[1]
The superorder is classified below:

★ Series 'Anotophysi'

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★ Gonorynchiformes - about 37 species

★ Series 'Otophysi' (Euostariophysi)

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★ Cypriniformes (minnows and allies) - about 3,268 species (contains Cyprinidae, largest family of freshwater fishes)

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★ Characiformes (characins and allies) - at least 1,674 species

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★ Siluriformes (catfishes) - about 1,727 species

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★ Gymnotiformes (electric eels, American knifefishes) - at least 173 species (sometimes grouped under Siluriformes)
The monophyly of Ostariophysi has come into question with molecular evidence. Gonorynchiformes is more closely related to Clupeiformes than Otophysi. It is possible that the Gonorynchiformes and Clupeiformes form a monophyletic group. Mitochondrial Genomics of Ostariophysan Fishes: Perspectives on Phylogeny and Biogeography, , Kenji, Saitoh, Journal of Molecular Evolution, 2003 There is evidence for a sister group relationship between Ostariophysi and Clupeomorpha (the taxon 'Ostarioclupeomorpha', also known as 'Otocephala', was coined to describe this possibly monophyletic group).

Evolution

Ostariophysan fossils, both anotophysan and otophysan, are known as early as the early Cretaceous. They are found worldwide on all continents with the exception of Australia.
Some sources place the origin of otophysans to be Pangean, about 250 million years ago. However, due to the lack of presence of otophysans in Madagascar, the time of origin of otophysans is likely between about 115 and 160 million years ago. The centre of origin of otophysans has been discussed a great deal. Recent evidence points to South America as their origin, where the most advanced groups exist. Though anotophysi has a primitive form of the otophysan Weberian apparatus, otophysans did not evolve from anotophysi; Gonorynchiformes are shown to be more closely related to Clupeiformes.
The order of divergence of the four otophysan orders has been debated; the lack of clarity of divergence is possible because splitting of the groups occurred within a short period of time. In the past, it was once believed that Cypriniformes were the most advanced group; however, this has since changed and they are now generally recognized as the most primitive and diverged earliest. The relationships of the characiphysans, Characiformes, Siluriformes, and Gymnotiformes, has been debated. Gymnotiformes is often recognized as the most advanced order of otophysi; however, its relationship to the other groups has been debated. Traditionally, Characiformes is the second-most primitive and Siluriformes is sister to Gymnotiformes, based on anatomical studies. However, some molecular evidence shows Siluriformes diverging from the sister groups Characiformes and Gymnotiformes. It is concluded by some that morphological characters are less homoplastic than the molecular traits.
Cypriniformes and Siluriformes both spread into Asia. This likely occurred during the late Jurassic, during which connections through Europe to Asia allowed them to spread. The Characiformes had not extended into Africa in time to reach Asia in this way. Characiformes and Siluriformes are both represented in Africa, however, in relatively primitive states. Before the South American and African land masses separated 115 million years ago during the Albian Stage of the Cretaceous, it is likely that a barrier formed from incursions of seawater followed by desiccation, which would separate species on either side. This would would mean the evolutionary separation of African and South American species was earlier than when the continents had actually separated. As only the primitive forms of catfishes and characins managed to cross to Africa, they was reduced competition between these fish and other, more ancient families such as Denticipitidae, Pantodontidae, Mormyridae, Gymnarchidae, Phractolaemidae and Kneriidae. These other families could survive in Africa, while being outcompeted in South America. Subsequently, Cypriniformes and Siluriformes would have spread to North America through the Bering land bridge.
By the late Cretaceous, many of the modern forms of fish were beginning to diverge. By the late Miocene about 10 million years ago, otophysans had essentially become modern, with representatives of modern genera and even species being present. It seems that the ice ages had little effect in the diversity of otophysans.

Diversity

Ostariophysi is the second largest teleost superorder. Ostariophysans includes fiver major lineages and is a very diverse group. As of 2006 (Nelson), the five orders contain 1,075 genera and about 7,931 species, which is about 28% of all known fish species. The four largest families in this group (Cyprinidae, Characidae, Loricariidae, and Balitoridae) include 4,656 species, over half (59%) of ostariophysan species. Cyprinidae itself is the largest fresh water fish family and the largest family of vertebrates after Gobiidae. Ostariophysians account for about 68% of all freshwater species; in fact, there are only about 123 marine species (Chanidae, Gonorynchidae, most Ariidae, about half of Plotosidae). They are present on all continents and major land masses except Antarctica, Greenland, and New Zealand.
This group includes a wide variety of different fishes. It contains the largest freshwater fish ever caught, the Mekong giant catfish, which can weigh up to about 300 kg. Grizzly Bear-Size Catfish Caught in Thailand It also contains the smallest species of freshwater fish, ''Danionella translucida'' at only 12 mm in length. Some of these fish are able to breathe atmospheric oxygen (Clariidae) or even live out of water (''Phreatobius cisternarum'').^[2] Some Ostariphysans have the ability to produce electricity (Malapteruridae, Gymnotiformes).

Physical characteristics

Most species possess a swim bladder (except in ''Gonorynchus''). The swim bladder is usually divided into two chambers. A smaller anterior chamber is partially or completely covered by a silvery peritoneal tunic. A larger posterior chamber may be reduced or absent in some groups. Minute, unicellular, horny projections known as "unculi" are commonly present on various body parts and are only known from ostariophysans.
Many Ostariophysans have the characteristic of an alarm substance that is part of a fright reaction. This is a pheromone produced in epidermal club cells and is similar or identical in all ostariophysians. When the fish is injured, this pheromone is released; other fish of the same species or similar species can smell this pheromone, causing a fright reaction. However, some fish possess the alarm substance without the fright reaction or lack both the alarm substance and the fright reaction to the alarm substance.

Weberian apparatus

In Otophysans, one of the main characteristics is the Weberian apparatus. Apart from this structure, there is no other trait that could explain the success of Otophysans. The biogeography of otophysan fishes (Ostariophysi: Otophysi): a new appraisal, , John C., Briggs, Journal of Biogeography, 2005 It is made up of a set of bones known as ''Weberian ossicles'', a chain of small bones that connect the auditory system to the gas bladder of fishes. The ossicles connect the gas bladder wall with Y-shaped lymph sinus that abuts the lymph-filled transverse canal joining the sacculi of the right and left ears. This allows the transmission of vibrations to the inner ear.
In anotophysans, the three first vertebrae are specialized and associated with one or more cephalic ribs (a primitive Weberian apparatus). In the otophysians a distinct modification of the anterior four or five first vertebrae is found as well as the Weberian ossiciles.
The fully functioning Weberian apparatus consists of the swim bladder, the Weberian ossicles, a portion of the anterior vertebral column, and some muscles and ligaments. It is named after the German anatomist and physiologist Ernst Heinrich Weber who first described the Weberian ossicles.

References

1. Ostariophysi
2. Species Summary: ''Phreatobius cisternarum'' Froese, R. and D. Pauly. Editors.