Paddlefish biology

The paddlefish Polyodon spathula, native to the entire Mississippi River drainage basin, is one of only two extant species (“living fossils”) in the family Polyodontidae, whose extinct representatives date back to the Upper Cretaceous. Although the elongated snout is a characteristic of the Polyodontidae, features of the cranium, gill arches, and jaws are distinctly adapted for filter feeding only in the genus Polyodon (P. spathula and the extinct P. tuberculata).
Paddlefish feed almost exclusively on plankton. Young animals swim constantly through the water in the search for small water fleas which are picked up individually. As paddlefish grow larger, they develop gill rakers and switch to filter feeding like whales do.

The electrosense

The electrosense appeared early in the evolutionary history of vertebrates as an ampullary sensory system with electroreceptors sensitive to weak, low-frequency electrical signals. This passive electrosensory system is present in lampreys, sharks and rays, lungfish, some amphibains, sturgeons and paddlefish, and in some groups of advanced bony fish (e.g. catfish).
Paddlefish are extremely sensitive and can detect tiny water fleas (Daphnia) with their electrosense alone. This is facilitated by an enormous number of electroreceptors. Up to 75,000 pores, each containing hundreds of receptor cells have been counted. Most of the electroreceptors are located on the characteristic rostrum which forms an electrosensory antenna to detect the Daphnia well ahead of the mouth.

Publications

Hofmann MH, Jung N, Chagnaud BP, Preissner M, Siebenaller U, Wilkens LA. (2008) Response properties of electrosensory units in the midbrain tectum of the paddlefish (Polyodon spathula). J Exp Biol (in press)

Wilkens LA, Hofmann MH. (2008) Electroreception. In: Senses on the Threshold, Function and Evolution of the Sense Organs in Secondarily Aquatic Vertebrates. J.G.M. Thewissen and Sirpa Nummela, (eds.). University of California Press.

Hofmann MH, Jung N, Wilkens LA. (2007) Resonant properties in the paddlefish electrosensory system caused by delayed feedback. Biological Cybernetics. DOI 10.1007/s00422-007-0181-1

Chagnaud BP, Wilkens LA, Hofmann MH. (2007) Response properties of electrosensory neurons in the lateral mesencephalic nucleus of the paddlefish. J Comp Physiol A. DOI 10.1007/s00359-007-0294-y

Wilkens LA, Hofmann MH. (2007) The paddlefish rostrum as an electrosensory organ: a novel adaptation for plankton feeding. Bioscience 57:399-407. PDF

Hofmann MH, Chagnaud B, Wilkens LA (2005) Response properties of electrosensory afferent fibers and secondary brain stem neurons in the paddlefish. Journal of experimental Biology 208:4213-4222. PDF

Hofmann MH, Wilkens LA (2005) Temporal analysis of moving d.c. electric fields in aquatic media. Physical Biology 2 (2005) 23–28. PDF

Wilkens LA, Hofmann MH (2005) The behavior of animals with a low-frequency, passive electrosensory system. In: Electroreception. Bullock TH, Hopkins CD, Popper, AN, Fay RR (eds.). Springer, New York, pp.229-263. PDF

Hofmann MH, Falk M, Wilkens LA (2004) Eectrosensory brain stem neurons compute the time derivative of electric fields in the paddlefish. Fluctuation and noise letters 4:L129-L138. PDF

Wilkens LA, Hofmann MH, Wojtenek W (2002) The electric sense of the paddlefish: a passive system for the detection and capture of zooplankton prey. J Physiol (Paris) 96:363-377. PDF

Hofmann MH, Woijtenek W, Wilkens LA (2002) Central organization of the electrosensory system in the paddlefish (Polyodon spathula). J Comp Neurol 446:25-36. PDF

Paddlefish striking at dipole electrodes

Recordings from an electrosensory hind brain neuron
(top) and simultaneously from different muscle groups.

The Mauthner cell of the paddlefish