Prey detection in selective plankton feeding by the paddlefish: Is the electric sense sufficient?

The long rostrum of the paddlefish Polyodon spathula supports an extensive array of ampullary electroreceptors and has been proposed to function as an antenna for detecting planktonic prey. Evidence in support of this hypothe sis is presented in experiments that preclude the use of other sensory mech anisms for plankton detection. Paddlefish swimming in a recirculating obser vation chamber are shown to feed normally in the dark when prey-related che mical and hydrodynamic sensory cues are masked or attenuated. Specifically, we demonstrate that the spatial distribution of plankton captured by paddl efish is little changed when the plankton are individually encapsulated in agarose, when a high background concentration of plankton extract is added to the chamber, when the nares are plugged and under turbulent water how co nditions. Paddlefish also discriminate between encapsulated plankton and 'e mpty' agarose particles of the same size. Although capture distributions di ffered somewhat under certain conditions, the general pattern and effective ness of prey capture were not disrupted by these procedures. These results support the conclusion that paddlefish, as zooplanktivores, rely on their p assive electric sense for prey detection.