Gymnorhamphichthys is a genus of South American sand knifefish in the weakly electric knifefish order Gymnotiformes, found across major river basins from Brazil and Venezuela to Argentina and Bolivia. Species in the atlas run from about 6–9 in and share an elongated, tapering body with a long tubular snout and a weak electric organ used for orientation and communication in the dark. The genus's defining habit is fossorial: by day, Gymnorhamphichthys buries itself in fine river-bottom sand, emerging at night to forage over the substrate — a lifestyle shared by only a handful of gymnotiform genera.
Gymnorhamphichthys is essentially unknown in the aquarium hobby; its burrowing, nocturnal habits and need for very fine sand substrate make it impractical and unrewarding for display tanks, and none of the five species in the atlas has an established common name in wide use beyond generic labels such as "sand knifefish" and "mousetail knifefish." It remains almost entirely a subject for researchers studying electric-fish diversity and behavior. All five species in the atlas are currently ranked Least Concern, a comparatively reassuring picture for a genus that is otherwise very poorly known to the wider public.