Xiphophorus pygmaeus

Xiphophorus pygmaeus  Hubbs & Gordon, 1943

Affiliation: northern swordtails

IUCN status: data deficient

Distribution: endemic to the Río Axtla system within the Río Pánuco drainage in San Luis Potosí, Mexico

Known locations (existing/former): multiple

Habitat description: clear, spring-fed streams, shaded and/or with undercut banks; the fish are often found near aquatic vegetation or submerged roots of trees, foraging on algae and detritus along the banks or large rocks 

Biotope reports:

• none yet

Description: Xiphophorus pygmaeus is one of the smallest species in the genus, with males reaching 2,5–3 cm and females 3,5–4 cm in total length. It has a slender, delicate body and subtle sexual dimorphism. Males display a short, translucent sword extension on the lower margin of the caudal fin, typically under 2 mm and never exceeding 8 mm, notably lacking the black edging seen in other swordtails. The dorsal fin is small and clear, with a faint black edge and a soft black band one-third up the fin. Wild individuals show a bold lateral line running from the eye to the tail; below it, the body is whitish with a bluish tint, while above, a pale blue stripe transitions into a brown back with darker scale edges creating a net-like appearance. A yellow male morph, fixed in the aquarium hobby, is also known and widely maintained by specialist breeders.

Variability: Apart from the yellow male morph, this species displays low morphological and colour variability across known populations. Genetic studies suggest a relatively stable and isolated evolutionary lineage with limited gene flow from sympatric congeners.

XWG status: not yet actively managed

Represented in aquaria: moderately

Known issues: like other northern swordtails needs clean, oxygenated water, otherwise is prone to diseases

Known collections or populations:

• “aquarium strain” (often with 100% yellow males)
• Río Huichihuayán
• Río Axtla
• El Nacimiento del Río Huichihuayán (Ivan Dibble, 1996)
• Río Axtla near Axtla de Terrazas (Meyer/Radda, 1980)
• …

(if you have more information, please contact us at info@xipho.org – thanks!)