Polyodon spathula feeds on zooplankton, particularly Daphnia, in rivers where visibility can be a few centimetres. Its rostrum carries dense fields of ampullary electroreceptors, with counts running into the tens of thousands over the snout and head, tuned to the low-frequency fields that swimming crustaceans generate.
Experiments led by Lon Wilkens and colleagues at the University of Missouri-St. Louis showed that juvenile paddlefish strike accurately at electrical dipoles that mimic plankton in the dark, and continue to detect and capture individual Daphnia at ranges of several centimetres when every sense but the electrosense is blocked. Adults filter with long gill rakers while swimming forward, mouth agape; the electroreception guides them to plankton-rich patches rather than picking prey one by one.
An older idea, that the rostrum is used to dig in the substrate, does not hold up. What remains uncertain is how heavily adults depend on electroreception once they have shifted to bulk filtering, since the fine-scale detection work has largely been done on juveniles.