The swordtail platyfish (Xiphophorus xiphidium) is a species of ray-finned fish in the family Poeciliidae. The species is endemic to the Soto la Marina River basin in Tamaulipas and Nuevo Leon. It lives in springs, streams, arroyos, ditches, marshes, and ponds in slow-flowing clear to muddy waters, often over mud, clay, gravel, and rock where vegetation is typically abundant. Males grow 3 cm (1.2 in) in length whereas females grow 4 cm (1.6 in) in length, however both sexes can reach 1 cm (0.4 in) over their usual length. Gestation is usually 24 to 35 days, where around 20 to 50 young are born.

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