''Xiphophorus nigrensis'', the Panuco swordtail, is a species of fish in the

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

Poeciliidae The Poeciliidae are a family of freshwater fishes of the order Cyprinodontiformes, the tooth-carps, and include well-known live-bearing aquarium fish, such as the guppy, molly, platy, and swordtail. The original distribution of the family was t ...

that is

endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...

to a small part of the

Pánuco River The Pánuco River ( es, Río Pánuco, ), also known as the ''Río de Canoas'', is a river in Mexico fed by several tributaries including the Moctezuma River and emptying into the Gulf of Mexico. The river is approximately long and passes throug ...

basin in Mexico.

Taxonomy

Being a small

swordtail ''Xiphophorus'' is a genus of euryhaline and freshwater fishes in the family Poeciliidae of order Cyprinodontiformes, native to Mexico and northern Central America. The many ''Xiphophorus'' species are all known as platyfish (or platies) and s ...

, ''Xiphophorus nigrensis'' was originally considered a

subspecies In biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics (morphology), but that can successfully interbreed. Not all species ...

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 A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

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 In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, bot ...

''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

{{Taxonbar, from=Q3753597 nigrensis Freshwater fish of Mexico Endemic fish of Mexico Taxa named by Donn Eric Rosen Fish described in 1960