The smallest and most delicate of the nominal hatchetfishes. The rounded keel is shallower than related species, giving this fish a distinctive elongate profile.

The family Gasteropelecidae is separated from other Characiformes by the following combination of characters: front…

Described from the Río Ampiyacu, a small tributary draining into the main Amazon channel in Loreto Department, northeastern Peru, with additional records from the Yavarí (Javari), Napo, Nanay, and lower Ucayali drainage basins.

All of these are Amazon tributaries, and C. ambiacus appears to occur in most or all affluents of the main Amazon channel between th…