Eigenmannia, the glass knifefish, is a genus of weakly electric fish in the family Sternopygidae, related to the bluntnose knifefish of Brachyhypopomus within the neotropical order Gymnotiformes. The atlas covers 29 species, distributed across Brazil, Colombia, Venezuela, Guyana, Argentina, Paraguay, Peru, and Bolivia. Most species are slender, laterally compressed, and translucent enough to see some internal structure, giving the genus its common name, and they inhabit slow water, submerged vegetation, and root tangles in lowland rivers and streams, generating a continuous weak electric field to navigate and communicate in low light or murky conditions. One species, E. vicentespelaea, is notable for living in cave systems, a habitat where troglobitic fish more generally tend to evolve reduced eyes and pigmentation.
Eigenmannia virescens is the species most familiar outside specialist circles, both as an occasional aquarium fish sold simply as 'glass knifefish' and as a long-standing subject in neuroscience research, where its jamming avoidance response — adjusting its electric discharge frequency to avoid interference from a neighboring fish's field — has been studied for decades. In the aquarium, glass knifefish are usually kept in groups in a heavily planted, dimly lit tank, since they are social, somewhat shy, and sensitive to strong electrical interference from equipment. Conservation status across the genus is mixed: most assessed species are Least Concern, but a substantial number are Data Deficient, and E. vicentespelaea's cave habitat makes it an obvious candidate for concern even without a full assessment.