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 hypothesis is presented in experiments that preclude the use of other sensory mechanisms for plankton detection. Paddlefish swimming in a recirculating observation chamber are shown to feed normally in the dark when prey-related chemical and hydrodynamic sensory cues are masked or attenuated. Specifically, we demonstrate that the spatial distribution of plankton captured by paddlefish 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 flow conditions. Paddlefish also discriminate between encapsulated plankton and 'empty' agarose particles of the same size. Although capture distributions differed somewhat under certain conditions, the general pattern and effectiveness of prey capture were not disrupted by these procedures. These results support the conclusion that paddlefish, as zooplanktivores, rely on their passive electric sense for prey detection.

Paddlefish strike at artificial dipoles simulating the weak electric fields of planktonic prey.

The freshwater paddlefish Polyodon spathula (Polyodontidae) feeds primarily on the water flea (Daphnia sp.), and previous studies suggest that these fish detect their planktonic prey using their rostral electrosensory system. Zooplankton produce direct-current and oscillating alternating-current electric fields containing multiple frequencies and amplitudes. We asked whether an inanimate electric field is sufficient to elicit paddlefish strikes equivalent to their feeding behavior. Juvenile paddlefish respond to artificial dipole stimuli by investigating the electric field and striking at the dipole electrode tips. These behavioral responses, scored as strikes, exhibit a bandpass characteristic with a maximum response between 5 and 15 Hz. Responses were less frequent at higher (20, 30, 40, 50 Hz) and lower (0.1, 0.5, 1 Hz) test frequencies, with a steep drop-off below 5 Hz. Strike rates also varied with stimulus intensity. Response frequency was greatest at 0.25 microA peak-to-peak amplitude, with reduced responses at lower and higher amplitudes (0.125 and 1.25 microA). Striking behavior was also influenced by water conductivity: strike rate was reduced at higher water conductivity. Dipole-elicited strikes exhibit behavioral plasticity. Fish habituate to repetitive dipole stimuli that are not reinforced by prey capture, and they dishabituate after food reinforcement. These experiments characterize paddlefish feeding strikes towards dipole electrodes at signal frequencies and intensities simulating the electric fields of zooplankton, their natural prey, and demonstrate that electric fields are sufficient to elicit feeding behavior. The results support the conclusion that paddlefish use their passive electrosensory system for planktivorous feeding.

The puzzle of hydrodynamic information processing: how are complex water motions analyzed by the lateral line?

We studied the discharges of neurons in the ascending lateral line pathway in response to the complex water motions generated by a moving object. The wave stimulus generated by the object was monitored with a hot-wire anemometer and with a custom-built particle imaging system. Responses of central lateral line neurons differ from those of primary afferent fibers in aspects like temporal discharge patterns and directional sensitivity. The data are consistent with the hypothesis that central lateral line neurons integrate input from many afferents innervating neuromasts distributed across large portions of the body surface.