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Rhamphichthyidae (knifefish; subclass Actinopterygii, order Cypriniformes) A small family of freshwater fish that have a very long, tapering body lacking dorsal and pelvic fins, but with a very long anal fin. They have small eyes and a produced snout, with the upper jaw overlapping the lower. Knifefish are known to produce weak electrical discharges, which may assist in orientation or food location. There are several species, occurring in S. America.