In search of magnetic compass in the retina of marsh frog, Pelophylax ridibundus.

Abstract

The magnetic compass has been shown in several classes of vertebrates, including anuran amphibians. Frogs obtain information from the magnetic field (MF) to follow migratory directions during their movements between wintering and breeding sites. In birds that are most studied in this respect, the magnetic compass sensor is thought to be located in the retina, and we explored this possibility in anurans. We previously reported the effect of the MF on the avian retina by electrophysiological methods; in the present study, we performed similar research in amphibians. Marsh frogs were captured in southern Russia in early autumn, and subsequently induced into a breeding migratory state through exposure to a spring-like photoperiod and temperature. Electroretinograms (ERGs) were recorded from isolated retinas that were carefully oriented in the sample holder to ensure consistent application of the MF. ERGs in response to red, blue, and ultraviolet light flashes were recorded under different MF directions: baseline (field passing through the eye during eastward migration, the predominant migratory direction for this frog population) and inverted inclination. Data were also examined by MF orientation relative to the retinal preparation (near‑perpendicular/near‑parallel). Our analysis revealed no significant effect of MF direction on the amplitudes of the a‑ or b‑waves of the ERG in response to light of any wavelength. This suggests that the sensory mechanism of the magnetic compass in frogs is either not located in the retina or that the reaction is so subtle that it cannot be recorded using the total ERG from the entire retina.