yellow swordtail
{{Short description|Species of fish}}
{{Speciesbox
| image =
| status = DD
| status_system = IUCN3.1
| taxon = Xiphophorus clemenciae
| authority = Álvarez, 1959
| synonyms =
}}
The yellow swordtail (Xiphophorus clemenciae) is a species of freshwater fish in the family Poeciliidae.{{FishBase | genus=Xiphophorus|species=clemenciae | year=2018 | month=September}} It is endemic to the upper Coatzacoalcos River basin in southern Mexico.{{cite journal | author1=Kang, J.H. | author2=M. Schartl | author3=R.B. Walter | author4=A. Meyer | year=2013 | title=Comprehensive phylogenetic analysis of all species of swordtails and platies (Pisces: Genus Xiphophorus) uncovers a hybrid origin of a swordtail fish, Xiphophorus monticolus, and demonstrates that the sexually selected sword originated in the ancestral lineage of the genus, but was lost again secondarily | journal=BMC Evolutionary Biology | volume=13 | issue=25 | pages=25 | doi=10.1186/1471-2148-13-25 | pmid=23360326 | pmc=3585855 | doi-access=free | bibcode=2013BMCEE..13...25K }} It is typically found in brooks and streams with slow current; it is less frequent in areas with strong current. The yellow swordtail is considered a threatened species by Mexican authorities.{{cite book | editor1=Ceballos, G. | editor2=E.D. Pardo | editor3=L.M Estévez | editor4=H.E. Pérez | year=2016 | title=Los peces dulceacuícolas de México en peligro de extinción | page=279 | publisher=Fondo de Cultura Económic | isbn=978-607-16-4087-1 }} It reaches up to {{cvt|5.1|cm}} in standard length.
Unusually, the yellow swordtail appears to be the result of hybrid speciation, and its ancestors a platy species and a swordtail species.{{cite journal | author1=Meyer, A. | author2=W. Salzburger | author3=M. Schartl | year=2006 | title=Hybrid origin of a swordtail species (Teleostei: Xiphophorus clemenciae) driven by sexual selection | journal=Molecular Ecology | volume=15 | issue=3 | pages=721–730 | doi=10.1111/j.1365-294X.2006.02810.x | pmid=16499697 | bibcode=2006MolEc..15..721M | s2cid=128413 | url=http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-34024 }} The southern mountain swordtail (X. monticolus), which is found further south in the Coatzacoalcos River basin than the yellow swordtail, is the result of a similar event.
The fish is named in honor of Álvarez' wife Clemencia, whose help and advice made it possible for him to devote himself to the pursuit of his scientific research.{{cite web | url = http://www.etyfish.org/cyprinodontiformes4/ | title = Order CYPRINODONTIFORMES: Families POECILIIDAE, ANABLEPIDAE, VALENCIIDAE, APHANIIDAE and PROCATOPODIDAE | access-date= 26 December 2021 | author1 = Christopher Scharpf | author2 = Kenneth J. Lazara | name-list-style = amp | work = The ETYFish Project Fish Name Etymology Database | publisher = Christopher Scharpf and Kenneth J. Lazara | date = 22 September 2018}}