Semiconductor gel in shark sense organs?

Abstract

Sharks can sense bioelectric fields of prey and other animals in seawater using an extraordinary system of sense organs (ampullae of Lorenzini) [R.D. Fields, The shark's electric sense. Sci. Am. 297 (2007) 74-81]. A recent study reported that these sense organs also enable sharks to locate prey-rich thermal fronts using a novel mode of temperature reception without ion channels. The study reported that gel extracted from the organs operates as a thermoelectric semiconductor, generating electricity when it is heated or cooled [B.R. Brown, Neurophysiology: sensing temperature without ion channels, Nature 421 (2003) 495]. Here we report biophysical studies that call into question this mechanism of sensory transduction. Our experiments indicate that the material exhibits no unusual thermoelectric or electromechanical properties, and that the thermoelectric response is an artifact caused by temperature effects on the measurement electrodes. No response is seen when non-metallic electrodes (carbon or salt bridges) are used, and ordinary seawater produces the same effect as shark organ gel when silver wire electrodes are used. These data are consistent with the voltages arising from electrochemical electrode potentials rather generated intrinsically within the sample. This new evidence, together with the anatomy of the organs and behavioral studies in the literature, best support the conclusion that the biological function of these sense organs is to detect electric fields.

Temperature response in electrosensors and thermal voltages in electrolytes

“…The measurements of Fields et al, when viewed from the fundamental position of thermoelectric measurement practice, do not support the authors' position [36]. While salt bridges remain the reliable tool of choice for a great span of electrophysiology, they present an unintentionally complicated tool for thermoelectric measurements.…”

“…Fields et al reported gel thermoelectricity measurements, from skates, using salt bridge leads and carbon leads [36]. The authors noted artifact concerns with silver electrodes used by Brown in 2003.…”

“…However, seawater signals were nonlinear, hysteretic and of opposite sign from shark gels. For platinum, all listed temperature coefficients involve the oxidation of species that are formed with platinum by applying 0.8 V or more versus a standard electrode [36]. The platinum leads were never exposed to applied voltages, as the open-circuit technique is non-Faradaic [25].…”

“…Fields et al do not address the thermoelectric contribution of their leads [36]. They set the sample thermopower to the measured V/ T trace, instead of using the standard expression 4.e.…”

Electric Power Generation at Low Temperature Gradients

“…Notable TE sensitivity was reported [8], whereas further investigation [9] concluded that the TE response is an artifact caused by temperature effects on the measurement electrodes. Such a thermogalvanic (TG) effect is related to the thermally activated electrochemical reactions on electrodes [10]- [12].…”