Detection and processing of electromagnetic and near-field acoustic signals in elasmobranch fishes

The acoustic near field of quietly moving underwater objects and the bio-el ectric field of aquatic animals exhibit great similarity, as both are predo minantly governed by Laplace's equation. The acoustic and electrical sensor y modalities thus may, in directing fishes to their prey employ analogous p rocessing algorithms, suggesting a common evolutionary design, founded on t he salient physical features shared by the respective stimulus fields. Sharks and rays are capable of orientating to the earth's magnetic field an d, hence, have a magnetic sense. The electromagnetic theory of orientation offers strong arguments for the animals using the electric fields induced b y ocean currents and by their own motions in the earths magnetic field. In the animal's frame of reference, in which the sense organs are at rest, the classical concept of motional electricity must be interpreted in relativis tic terms. In the ampullae of Lorenzini, weak electric fields cause the ciliated apica l receptor-cell membranes to produce graded, negative receptor currents opp osite in direction to the fields applied. The observed currents form Dart o f a positive-feedback mechanism, supporting the generation of receptor pote ntials much larger than the input signal. Acting across the basal cell memb ranes, the receptor potentials control the process of synaptic transmission .