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The Mexican tetra or blind cave fish (''Astyanax mexicanus'') is a freshwater fish of the family Characidae of the order Characiformes. The type species of its genus, it is native to the Nearctic ecozone, originating in the lower Rio Grande and the Neueces and Pecos Rivers in Texas, as well as the central and eastern parts of Mexico. Growing to a maximum overall length of , the Mexican tetra is of typical characin shape, with unremarkable, drab coloration. Its blind cave form, however, is notable for having no eyes and being albino, that is, completely devoid of pigmentation; it has a pinkish-white color to its body. This fish, especially the blind variant, is reasonably popular among aquarists. ''A. mexicanus'' is a peaceful species that spends most of its time in midlevel water above the rocky and sandy bottoms of pools and backwaters of creeks and rivers of its native environment. Coming from a subtropical climate, it prefers water with 6.0–7.8 pH, a hardness of up to 30 dGH, and a temperature range of . In the winter, it migrates to warmer waters. Its natural diet consists of crustaceans, insects, and annelids, although in captivity it is omnivorous. The Mexican tetra has been treated as a subspecies of ''A. fasciatus'', but this is not widely accepted.〔 ==Blind cave form== ''A. mexicanus'' is famous for its blind cave form, which is known by such names as blind cave tetra, blind tetra, and blind cavefish. Depending on the which population, cave forms can have degenerated sight or have total loss of sight and even their eyes. The fish in the Pachón caves have lost their eyes completely whilst the fish from the Micos cave only have limited sight. Cave fish and surface fish are able to produce fertile offspring. 〔 These fish can still, however, find their way around by means of their lateral lines, which are highly sensitive to fluctuating water pressure. Currently, 29 cave populations are known, dispersed over three geographically distinct areas in a karst region of northeastern Mexico. Recent studies suggest at least two distinct genetic lineages occur among the blind populations, and the current distribution of populations arose by at least five independent invasions.〔 The eyed and eyeless forms of ''A. mexicanus,'' being members of the same species, are closely related and can interbreed making this species an excellent model organism for examining convergent and parallel evolution, regressive evolution in cave animals, and the genetic basis of regressive traits. ''Astyanax jordani'', another blind cave fish, is sometimes confused with the cave form of ''A. mexicanus.'' 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「Mexican tetra」の詳細全文を読む スポンサード リンク
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