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Delta smelt, ''Hypomesus transpacificus'', is an endangered〔〔 slender-bodied smelt, about long, in the Osmeridae family. Endemic to the upper Sacramento-San Joaquin Estuary of California, it mainly inhabits the freshwater-saltwater mixing zone of the estuary, except during its spawning season, when it migrates upstream to freshwater following winter "first flush" flow events (around March to May).〔Sommer, T. et al. 2011. The spawning migration of delta smelt in the Upper San Francisco Estuary. San Francisco Estuary and Watershed Science: 9(2).〕 It functions as an indicator species for the overall health of the Delta's ecosystem.〔(【引用サイトリンク】url=http://www.biologicaldiversity.org/species/fish/Delta_smelt/ )〕 Because of its one-year lifecycle and relatively low fecundity, it is very susceptible to changes in the environmental conditions of its native habitat.〔Moyle, PB. 2002. Inland fishes of California. University of California Press, Berkeley.〕 Efforts to protect the endangered fish from further decline have focused on limiting or modifying the large-scale pumping activities of state and federal water projects at the southern end of the estuary. However, these efforts have not prevented the species from becoming functionally extinct in the wild.〔 ==Taxonomy and evolution== |1= }} }} }} The delta smelt is one of five currently recognized species within the ''Hypomesus'' genus, which is part of the larger Osmeridae family of smelts. The genus has been subject to many revisions since it was first classified by Gill in 1863.〔 The first major revision occurred in 1963, when the Osmeridae family was reexamined by Canadian ichthyologist Donald Evan McAllister. Expanding on Japanese researcher Hamada's earlier determination that ''H. olidus'' was not a monolithic widespread species, but rather one of three distinct species of ''Hypomesus'', McAllister assigned them new names, and further delineated what he believed were four subspecies. This was the first description of ''H. transpacificus''; named for its supposed occurrence on both sides of the Pacific, and also "to the friendship of Japanese and Canadian ichthyologists." He separated these geographically isolated populations into separate subspecies: ''H. t. transpacificus'' and ''H. t. nipponensis''.〔 Modern analysis of the genus would elevate all of McAllister's subspecies to full species status, based on fin ray counts and the number of chromatophores between their mandibles, a change which genetic analysis has supported.〔〔 In fact, genetic analysis would conclude that despite their morphological similarities, ''H. nipponensis'' and ''H. transpacificus'' are actually members of different phylogenetic clades.〔 The abbreviated distribution of ''Hypomesus'' species along both the east and west sides of the Pacific ocean suggests that their common ancestor had a range that would have crossed the Pacific. Researchers have hypothesized that climatic changes may have reduced the range of the ancestral species during cooling periods, which would have created a reproductive barrier, allowing speciation to occur.〔 Although the low number of species in the genus and high levels of homoplasy have frustrated attempts to determine whether the northern Pacific ''H. olidus'' or ''H. nipponensis'' are the basal species of ''Hypomesus'',〔 it is known that the most recent speciation event in ''Hypomesus'' was between the two native east Pacific species, ''H. pretiosus'' and ''H. transpacificus''. This is plausibly due to a geographic isolation of a widespread eastern Pacific ancestor, of which some members were isolated in a freshwater basin in western California, possibly in the lakes that would have been located in the southern San Joaquin Valley during the Pleistocene epoch.〔 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「delta smelt」の詳細全文を読む スポンサード リンク
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