Cynopoecilus is a genus of small annual killifish from the coastal lowlands of southern Brazil and Uruguay, part of the South American annual-killifish radiation often grouped with Austrolebias in the family Cynolebiidae (older literature places it in Rivulidae). At 1–2 in, these are compact fish built for a short, intense life: they inhabit temporary pools and marshes that dry out completely each year, and the adult population dies with the pool. Their eggs, buried in the mud bottom, enter a resting state and survive the dry months, hatching only when the rains refill the pool the following season.
This annual life history is exactly what draws specialist killifish keepers to Cynopoecilus — hobbyists raise them from eggs shipped in slightly moist peat, replicating the wild dry-season pause, and prize the males' bright breeding color for the brief months they are alive. None of the five species in the atlas carries an established common name beyond the Portuguese "peixe-anual" (annual fish) attached to Cynopoecilus intimus, underscoring how much of the interest in this genus sits with the annual-killifish specialist community rather than the wider trade. Conservation status leans toward concern: two species are Near Threatened and one Vulnerable, consistent with a genus tied to small, seasonal wetland habitat that is easily lost to drainage or land conversion.