The ancient Mississippi River paddlefish, Polyodon spathula, has long been thought to use its oversized rostrum for excavation. Recent studies provide an entirely new interpretation for the function of the paddle, that of an electrical antenna for detecting the electric fields of plankton, P. spathula's primary food. Feeding experiments with juvenile fish demonstrate that paddlefish detect and capture individual daphnia when all sensory modalities except the electrosense have been blocked. The paddle provides space for an extravagant array of ampullary electroreceptors that are found in common with elasmobranchs and primitive bony fish. This exquisite electrosensory organ may also influence the migration of paddlefish in an environment replete with dams and other steel structures, sources of unnatural electric signals (corrosion potentials). In the laboratory, paddlefish are sensitive to and avoid metallic obstacles, even in the dark. Electrosensory processing in the brain involves physiological mechanisms for spatial imaging equivalent to planktivory based on passive electrosensitivity.