Xiphophorus nigrensis
species of fish
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).
“Xiphophorus nigrensis” enters the record as species of fish. Crown Archives preserves that source wording while asking what Xiphophorus, nigrensis and species can confirm, complicate or overturn.
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“Xiphophorus nigrensis” is worth following because a concise public description often conceals a longer documentary argument. Here, Xiphophorus, nigrensis and species provides the most credible route into that argument.
Datasets, specimens, observations and peer-reviewed methods provide the appropriate test for the technical claims summarized here. The source revision retrieved here is dated May 21, 2026. The linked authority identifier is Q3753597. None of the 0 selected statements returned an explicit reference.
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This entry incorporates text from “Xiphophorus nigrensis” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.