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Sturgeon is the common name for the 27 species of fish belonging to the family Acipenseridae. Their evolution dates back to the Triassic some 245 to 208 million years ago.〔Birstein, V.J., R. Hanner, and R. DeSalle. 1997. Phylogeny of the Acipenseriformes: cytogenic and molecular approaches. Environmental Biology of Fishes 48: 127-155.〕 The family is grouped into four genera: ''Acipenser'', ''Huso'', ''Scaphirhynchus'' and ''Pseudoscaphirhynchus''. Four species may now be extinct. Two closely related species, ''Polyodon spathula'' (paddlefish) and ''Psephurus gladius'' (Chinese paddlefish, possibly extinct) are of the same order, Acipenseriformes, but are in the family Polyodontidae and are not considered to be "true" sturgeons. Both sturgeons and paddlefish have been referred to as "primitive fishes" because their morphological characteristics have remained relatively unchanged since the earliest fossil record.〔(【引用サイトリンク】url=http://www.dnr.state.mn.us/rsg/profile.html?action=elementDetail&selectedElement=AFCAA01020 )〕 Sturgeons are native to subtropical, temperate and sub-Arctic rivers, lakes and coastlines of Eurasia and North America.〔(【引用サイトリンク】publisher=University of Washington )〕 Sturgeons are long-lived, late-maturing fishes with distinctive characteristics, such as a heterocercal caudal fin similar to that of sharks, and an elongated spindle-like body that is smooth-skinned, scaleless and armored with 5 lateral rows of bony plates called scutes. Several species can grow quite large, typically ranging 7–12 feet (2-3½ m) in length. The largest sturgeon on record was a Beluga female captured in the Volga estuary in 1827, weighing 1,571 kg (3,463 lb) and 7.2 m (24 ft) long. Most sturgeons are anadromous bottom-feeders which migrate upstream to spawn but spend most of their lives feeding in river deltas and estuaries. Some species inhabit freshwater environments exclusively while others primarily inhabit marine environments near coastal areas, and are known to venture into open ocean. Several species of sturgeon are harvested for their roe which is processed into caviar — a luxury food and the reason why caviar producing sturgeons are among the most valuable of all wildlife resources.〔(【引用サイトリンク】 title=Sturgeons )〕 They are particularly vulnerable to overexploitation and other threats, including pollution and habitat fragmentation. Most species of sturgeon are considered to be at risk of extinction, making them more critically endangered than any other group of species.〔 ==Evolution== Acipenseriform fishes appeared in the fossil record some 245 to 208 million years ago presumably near the end of the Triassic, making them among the most ancient of actinopterygian fishes. True sturgeons appear in the fossil record during the Upper Cretaceous. In that time, sturgeons have undergone remarkably little morphological change, indicating their evolution has been exceptionally slow and earning them informal status as living fossils.〔B. G. Gardiner (1984) Sturgeons as living fossils. Pp. 148–152 in N. Eldredge and S.M. Stanley, eds. Living fossils. Springer-Verlag, New York.〕〔J. Krieger and P.A. Fuerst. (2002) Evidence for a Slowed Rate of Molecular Evolution in the Order Acipenseriformes ''Molecular Biology and Evolution'' 19:891-897.〕 This is explained in part by the long generation interval, tolerance for wide ranges of temperature and salinity, lack of predators due to size and bony plated armor, or scutes, and the abundance of prey items in the benthic environment. Although their evolution has been remarkably slow, they are a highly evolved living fossil, and do not closely resemble their ancestral chondrosteans. They do however still share several primitive characteristics, such as heterocercal tail, reduced squamation, more fin rays than supporting bony elements, and unique jaw suspension. Despite the existence of a fossil record, full classification and phylogeny of the sturgeon species has been difficult to determine, in part due to the high individual and ontogenic variation, including geographical clines in certain features, such as rostrum shape, number of scutes and body length. A further confounding factor is the peculiar ability of sturgeons to produce reproductively viable hybrids, even between species assigned to different genera. While ray-finned fishes have a long evolutionary history culminating in our most familiar fishes, past adaptive radiations have left only a few survivors, like sturgeons and garfish.〔(【引用サイトリンク】publisher=San Francisco State University )〕 The wide range of the acipenserids and their endangered status have made collection of systematic materials difficult. These factors have led researchers in the past to identify over 40 additional species that were rejected by later scientists.〔W. E. Bemis, E. K. Findeis, and L. Grande. (1997). An overview of Acipenseriformes. Environmental Biology of Fishes 48:25–71.〕 It is still unclear whether the species in the ''Acipenser'' and ''Huso'' genera are monophyletic (descended from one ancestor) or paraphyletic (descended from many ancestors)—though it is clear that the morphologically motivated division between these two genera is not supported by the genetic evidence. There is an ongoing effort to resolve the taxonomic confusion using a continuing synthesis of systematic data and molecular techniques.〔〔F. Fontana, J. Tagliavini, L. Congiu (2001) Sturgeon genetics and cytogenetics: recent advancements and perspectives. ''Genetica'' 111: 359–373〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「sturgeon」の詳細全文を読む スポンサード リンク
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