Breeding male topminnows darken to jet black and develop bright yellow fins and golden tints along their midsections. Although the lifespan of the Gila topminnow is only about one year, the species is a prolific breeder. Gestation varies from 24 to 28 days for the Gila topminnow and 12 to 14 days for the Yaqui subspecies. Unlike most other fish, the topminnow gives birth to live young, as opposed to laying eggs. The onset of breeding is affected by water temperature, daylight, and food availability.
Topminnows are able to exist in a broad range of habitats.
They make their homes in permanent and intermittent streams, marshes, and river banks, where they seek dense mats of algae and debris with sandy substrates for their preferred living environment. Historically, this topminnow was abundant throughout the Gila River system and Rio Yaqui drainage in Arizona, New Mexico, and northern Sonora and Chihuahua, Mexico. The current severe reductions of population and suitable habitat are reasons for grave concern for the survival of the this species. The Gila Topminnow still occurs in natural populations at only eight isolated locations in the Santa Cruz River System. It is believed that extensive groundwater pumping and diversion of water for the irrigation of agriculture in Mexico have annihilated the species from that country.