This article is about electrical signal reception in the Mississippi River paddlefish, Polyodon spathula.  The paddlefish has a large structure called a rostrum protruding from its face that was not known to have electrosensory properties until relatively recently.  The authors give a lot of background information on the fish, and mention that earlier theories proposing the rostrum's function was to shovel through sand for food were unlikely because it is lined with sensitive skin that would be damaged and torn if used for digging. The paddle is lined with pores called ampullae of Lorenzini, the electroreceptive organ of the fish, except along a strip with no receptors down the midline. The pores are sensitive enough and provide enough spatial resolution to detect the paddlefish’s diet of plankton as they filter feed. Electrical signal reception is required to feed on the plankton because the water of the river they live in is muddy and brackish, so the fish cannot sense their prey using other senses.

The study consisted of juvenile paddlefish kept in artificial water channels that recirculate the water and provide a constant current. The authors say the younger fish have not yet developed the large raking structures for filter feeding in adults, so they detect single plankton as it floats in the water. The plankton used in this experiment were Daphnia spp. and the position of each relative to the center of the rostrum before consumption was recorded using cameras mounted in the recirculating flumes. The researchers found that most of the plankton was captured within 2 cm of the midline of the rostrum, indicating that the paddlefish were accurate in their feeding. After this, the authors experimented further by letting the fish feed in the dark and using IR cameras to record observations. They analyzed the data and found no statistical difference between food capture under the two conditions. The next step in the study was to add a high concentration of daphnia to the tanks in order to nullify any chemical signals given off that could alert the paddlefish to their location in the dark. They were then fed in turbulent water in order to remove possible detection of the plankton trail in calm water. The fish fed just as well in these cases as in the first portion of the experiment. In a further attempt to prove their claim, paddlefish were presented with various levels of electric stimulus using wires. They struck at the wires with low frequencies around 5-10 Hz because they match the electrical field dipole given off by plankton and the peak sensitivity of the receptor cells, but swam to avoid higher frequencies. The authors concluded that all of this is evidence that paddlefish feed by electrical reception using the rostrum rather than using any other sense modality.

This article relates to material covered in lecture because the authors discuss the ampullary organ of the paddlefish, including a description of the tubes connected to the receptors in their skin. They discuss the paddlefish’s method of electrolocation later. The fish determine the spatial location of prey based on distortion patterning in the skin just as discussed in lecture. For information about prey’s distance while they are swimming, however, they note an electrical field signature on each receptor as time goes on and compute a dominant frequency to determine how far away the plankton is. The authors also went on to say that the electrical signals were only detectable if they change in time, and that they performed Fourier analysis on the frequencies and compared them to paddlefish responses.

Wilkens, Lon A. & Hofmann, Michael H. (2007) The paddlefish rostrum as an electrosensory organ: A novel adaptation for plankton feeding. Bioscience, 57, 399-407.