Phalloceros is a genus of small live-bearing fish in the family Poeciliidae, native to coastal and inland drainages of southeastern Brazil and adjoining parts of Argentina, Uruguay, and Paraguay. Species are tiny, most well under 2.5 in, with the extended anal fin (gonopodium) of male poeciliids used to fertilize females internally, and broods are born free-swimming rather than laid as eggs. The atlas covers 25 species; what was long treated as a single widespread species, Phalloceros caudimaculatus, was eventually recognized as a complex of many narrow-range, look-alike species distinguished mainly by fine differences in male genital structure, which is why the genus now holds so many similar, geographically restricted names.
The dusky millions fish (P. caudimaculatus) itself has a long history in the aquarium and pond trade, valued outside its native range as a hardy livebearer and mosquito-control fish, but most of its recently split relatives are local Brazilian coastal endemics with little to no aquarium presence. Conservation status for the genus is comparatively reassuring; most assessed species rank least concern, though the atlas flags one species for active CARES conservation-breeding attention, and a handful remain data deficient or unassessed.