UC San Diego

Elasmobranch fishes use their electroreceptive organs, the Ampullae of Lorenzini, to sense DC and low-frequency electric fields in the ocean. The natural environment offers a wide range of electric field strengths, from typically less than 5 nV/cm in the rapidly falling fields a meter away from hidden prey to 500 nV/cm in the wind- driven currents of the North Atlantic. In contrast, electrophysiological recordings from the electroreceptor's afferent nerves show a limited dynamic range in response to low-frequency sinusoidal stimuli, with non-linearity starting at amplitudes only eight times the organ's sensitivity threshold. We employ in vivo recordings from the hyomandibular nerve of the thornback ray, Platyrhinoidis triseriata, to explore the physiological mechanisms exercised by elasmobranch fishes to extend their electroreceptor's sensory dynamic range in widely ranging ambient electric field strengths. We measure the organ's nerve activity in response to DC fields, to low- frequency sinusoidal electric stimuli in the presence of various DC fields and to low-frequency sinusoidal electric stimuli of different relative orientation. When possible, we record and analyze the simultaneous activity of two nerve fibers during the experiment. We propose that the animal may extend its electroreceptor's linear dynamic range of operation by using the organ's adaptation to DC fields and directional response in combination with previously described behavioral mechanisms that modulate, and sometimes produce, the effective stimulus