Visual pigments and spectral sensitivity in Rana temporaria and other European tadpoles

Abstract

Retinal extracts were prepared from Rana temporaria tadpoles reared from spawn collected in England and Finland, and from Bufo bufo tadpoles. The latter was found to have visual pigments based on vitamin A 1, irrespective of the age of the tadpoles. Rana temporaria tadpoles, however, had between 50 and 70% vitamin A 2 based pigments when young (Saxén's stages 6–8). The amount of A 2 pigment then decreased at the same stage of development as the emergence of the fore-limbs. No “green-rod” pigment could be detected in any of the tadpoles before metamorphosis. Photopic spectral sensitivity curves were also obtained, using the electroretinogram (ERG), from Rana temporaria tadpoles of English and Finnish origin, from Bufo bufo tadpoles, and from Rana esculenta tadpoles. Bufo bufo tadpoles showed photopic sensitivity maximal at about 560 nm, and Rana esculenta tadpoles photopic sensitivity maximal at about 620 nm. This suggests that photopic vision is mediated by vitamin A 1-based pigments, in the former and by vitamin A 2-based pigments in the latter. The photopic sensitivity of Rana temporaria tadpoles of English origin was identical to that of Bufo bufo tadpoles and adult frogs, and thus had the characteristics expected if the visual pigments were based on vitamin A 1. Since the extractions showed that more than half the visual pigments from the rods are vitamin A 2 based, it is suggested that the cones, which are known to develop faster than the rods, have a “preference” for vitamin A 1. Previous results, in which ERG determinations of both scotopic and photopic spectral sensitivity of Rana temporaria tadpoles were made, can also be interpreted as supporting this view. The photopic spectral sensitivity of Rana temporaria tadpoles of Finnish origin differed from that of tadpoles of English origin, and indicated that in this case vitamin A 2 pigments were present in the cones as well as the rods. Rana temporaria forms separate subspecies in England and Finland, which differ in several aspects of development.