Species of fish
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Xiphophorus nigrensis, the Panuco swordtail, is a species of fish in the family Poeciliidae that is endemic to a small part of the Pánuco River basin in Mexico.
Taxonomy.
Being a small swordtail, "Xiphophorus nigrensis" was originally considered a subspecies of "X. pygmaeus". Today it is recognized, along with "X. multilineatus", as the closest related species to "X. pygmaeus". The three species form a clade within the larger clade of northern swordtails. The similarity of the large males of "X. nigrensis" to the small males of "X. cortezi", another northern swordtail, points to a close evolutionary relationship between these two species.
Description.
The body of "Xiphophorus nigrensis" is larger and deeper than that of "X. pygmaeus". The males possess on their caudal fin a moderately to well-developed sword set off by a black line, from which the specific epithet "nigrensis" ("niger" meaning black and "ensis" sword in Latin) is derived. Pregnant females exhibit a dusky gravid spot on their bellies. Larger individuals are deeper-bodied and develop brilliant iridescent blue on their sides and flanks, which may somewhat conceal their midlateral black stripe.
Populations regularly contain both large (up to 40 mm) and small adult males (down to 18 mm). Large males possess swords, while small males do not. Small males are slender and of solid gold color.
Distribution and habitat.
"Xiphophorus nigrensis" is found in Rio Choy. The species inhabits clear, deep, fast-flowing waters along undercut slopes of river and stream beds. The depth can be as great as . 
Reproduction.
"Xiphophorus nigrensis" readily hybridizes with "X. pygmaeus"; in fact, females of the latter species prefer "X. nigrensis" males to those of their own species because large "X. nigrensis" males court females while "X. pygmaeus" males merely sneak up and chase them instead. Small "X. nigrensis" males also exhibit sneak-chase behavior.
References.
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