Redtail Splitfin

The Redtail Splitfin is a colorful freshwater livebearer belonging to the family Goodeidae, native to highland rivers and lakes of west-central Mexico. Males display a steel-blue to gray body with a vivid orange-red band across the base of the tail, a striking contrast that makes the species one of the more popular goodeids in the aquarium hobby. Like other goodeids, it employs a distinctive reproductive strategy called matrotrophy, nourishing developing embryos internally via specialized structures called trophotaeniae rather than relying primarily on yolk reserves, resulting in fewer but larger, more developed offspring. While relatively hardy and still fairly widespread within its native range, the species faces ongoing pressure from habitat alteration and the introduction of non-native fish typical of many endemic Mexican freshwater fish.

• Deep-bodied, laterally compressed shape typical of goodeids, with a high-backed profile especially in mature males
• Base body coloration steel-blue to gray, often with iridescent sheen along the flanks in males
• Distinctive bright orange-red band across the base of the caudal fin, the species' key field mark
• Females are duller overall, with a smaller and less vivid version of the tail marking
• No gonopodium; goodeids use a different, less elongated modified anal fin structure for mating
• Look-alikes: closely resembles the Butterfly Splitfin, but the Redtail Splitfin's tail band is orange-red rather than yellow-orange

The Redtail Splitfin is native to the Ameca and Armería River drainages and associated lakes in the Mexican states of Jalisco, Colima, and Nayarit, on the western side of the Mexican Plateau. It inhabits clear to moderately turbid rivers, streams, and lake margins at moderate elevation, often among aquatic vegetation and near rocky or sandy substrates. Water temperatures in its native range tend to be moderate and relatively stable due to the region's highland spring influence. The species remains present across a fair portion of its historic range but, like many endemic Mexican freshwater fish, faces habitat degradation from agricultural runoff, water diversion, and competition from introduced species such as tilapia and livebearers.

Redtail Splitfins are active, schooling fish that graze on algae, small invertebrates, and organic detritus in vegetated shallows and slower river sections. As goodeids, females carry developing embryos for an extended gestation period, nourishing them through placenta-like trophotaeniae structures, resulting in relatively small broods of well-developed young compared to typical poeciliid livebearers. Males display their vivid tail coloration prominently during courtship, flaring fins and pursuing females within loose social groups. The species is generally peaceful, though males may show mild competitive posturing over females. Its algae-grazing habits make it an ecologically useful component of its native freshwater systems, helping to manage algal growth in the rivers and lakes it inhabits.