Xiphophorus is the genus of the swordtails and platies, freshwater livebearers in the family Poeciliidae native to Mexico, Guatemala, Belize, and Honduras. The atlas covers 28 species, ranging in adult size from about 1.5 in up to 5.5 in. Males in many species carry the elongated lower caudal-fin extension — the 'sword' — that gives swordtails their name, used in courtship display, while platyfish species lack it and tend toward a more compact, rounded body. The genus lives in habitats from clear highland streams to lowland rivers and springs, and because closely related species readily hybridize, Xiphophorus has long been one of the most heavily studied genera in vertebrate genetics, including classic work on pigment-cell biology and hybrid melanoma.
Three Xiphophorus species — the Green Swordtail (X. hellerii), the Southern Platyfish (X. maculatus), and the Variable Platyfish (X. variatus) — are among the most widely kept freshwater aquarium fish in the world, the wild-type ancestors behind decades of domesticated color and fin varieties sold as swordtails and platies. Away from those few workhorse species, though, the genus tells a starker conservation story: ten Xiphophorus species appear in the atlas's CARES conservation program, and the IUCN tally includes two species classified as Extinct in the Wild, among them the Monterrey Platyfish (X. couchianus), a spring-dwelling endemic that survives only through captive populations after its native habitat was lost. A further thirteen species are Data Deficient, underscoring how narrowly many wild Xiphophorus are restricted to single springs or short stream reaches in Mexico.