Cnesterodon is a genus of small livebearing fish in the family Poeciliidae, native to the Paraná-Paraguay-Uruguay river system of southern South America — the atlas's countries are Brazil, Argentina, Uruguay, and Paraguay. These are diminutive fish, the atlas's species running roughly 1–2 in, inhabiting shallow marshes, ditches, and slow-flowing streams. As livebearers, females carry and give birth to free-swimming young rather than laying eggs, and males are typically smaller and more colorful, with a gonopodium modified from the anal fin for mating.
The ten-spotted live-bearer (Cnesterodon decemmaculatus) is the genus's best-known aquarium representative, a hardy, easy-to-breed small livebearer popular with hobbyists who favor wild-type poeciliids over fancy-bred guppy and platy strains. Most other Cnesterodon species are narrow-range endemics, each confined to a small drainage or wetland system, and that narrow distribution shows clearly in the atlas's conservation numbers: three species carry a Critically Endangered rating and two more are Endangered, making Cnesterodon one of the more conservation-urgent livebearer genera the atlas covers. Seven of the genus's ten species are tracked under CARES.