On the Responsiveness of Elasmobranch Fishes to Weak Electric Fields

Abstract

Determination of the threshold field strength that produces effects in cell membranes of living organisms exposed to electromagnetic (EM) fields is of prime importance in the establishment of safe levels of exposure to EM fields. For most organisms the transduction modalities, properties of receptors and signal processing pathways which mediate responses to EM fields are not well known. An exception to this predicament is the electroreceptive organ in elasmobranch fishes. The organ known as the ampulla of Lorenzini in these fishes has been demonstrated to be an exquisitely sensitive organ that provides these animals with an electromagnetic sense for orientation in the earth’s magnetic field and provides detection of other marine animals (Kalmijn, 1974). Based on the behavioral response threshold of these fishes in controlled experiments, a single ampulla of Lorenzini was estimated to be able to respond to an external electric field intensity of 5 nV/cm (Kalmijn, 1982). Previously, the most sensitive known cellular response to electric fields was that of the crayfish stretch-receptor neuron (Terzuolo and Bullock, 1956), which is more than six orders less sensitive (10 mV/cm) than the threshold response of elasmobranch fishes. Because of the body of literature (see Bullock and Heiligenberg, 1986) that exists on these organs of electroreception with respect to their function, anatomical features and receptor membrane electrophysiology, it seems appropriate to ask the following fundamental question: Is the threshold response of elasmobranch fishes to weak electric fields a consequence of an unknown, sensitive ion-conduction mechanism in the membrane of the receptor cells of the ampulla of Lorenzini or is the electric field sensitivity due to processing of the multiple receptor cell responses in each ampulla?