Oligosarcus is a genus of predatory characins from the rivers of southeastern South America — Brazil, Argentina, Uruguay, Paraguay and Bolivia — known locally as "peixe-cachorro" (dogfish) and, in English, dogtooth or toothy tetras. Unlike the small, peaceable tetras most aquarists picture, Oligosarcus are elongated, streamlined hunters with prominent canine-like teeth built for seizing other fish, and the genus spans a wide size range, from modestly built stream species up to large-river predators recorded at 12.5 in in the atlas. They typically patrol open water in loose groups, ambushing smaller fish and insects, and several species are important components of local food fisheries as well as the aquatic food web.
Because of their size and predatory habits, Oligosarcus turn up far more often in ichthyological and fisheries literature than in home aquariums — this is a genus of mostly scientific rather than hobby interest, and the atlas does not record any species with a CARES conservation-breeding priority. The IUCN picture is mixed: about half of the assessed species are Least Concern, but two are listed Endangered, a reminder that even a genus spread across a whole subcontinent can carry pockets of real vulnerability in individual, narrowly distributed species.