Girardinichthys is a genus of splitfins in family Goodeidae, live-bearing fish endemic to the volcanic highlands of central Mexico and related to, but distinct from, the guppies and mollies of family Poeciliidae. Instead of a placenta, goodeid embryos are nourished through a trophotaenia, a ribbon-like structure that absorbs nutrients from the mother's follicular fluid - a form of live-bearing found nowhere else. The atlas' 3 species, including the Chapultepec splitfin (G. viviparus) and darkedged splitfin (G. multiradiatus), reach about 2–2.5 in and are all restricted to the Mesa Central lakes and springs in and around the Mexico City basin.
This is one of the most conservation-critical genera in the atlas: all three species carry threatened IUCN assessments, split between Critically Endangered and Endangered, and all three are flagged under the CARES conservation priority program. Habitat loss, water diversion for Mexico City's water supply, and introduced predatory and competing fish have hit these small highland lake and spring specialists hard, and some populations survive today mainly through captive-breeding efforts led by goodeid specialist hobbyists and conservation groups.