Variations in the retinal designs of pulmonate snails (Mollusca, Gastropoda): squaring phylogenetic background and ecophysiological needs (I)

The eyes of aquatic pulmonates differ from those of terrestrial pulmonates; the latter, in species such as Cepaea nemoralis and Trichia hispida, possess conventional, cup-shaped retinas, but the aquatic species Lymnaea stagnalis, Radix peregra, Physa fontinalis, and Planorbarius corneus have retinas that are partitioned into dorsal and ventral depressions ("pits"). The pits are separated by an internal ridge, called the "crest", and on account of their pigmentation can be seen in vivo. The dominant cellular components of the retinae of terrestrial as well as aquatic snails are pigmented cells and microvillar photoreceptors, the latter occurring in two morphologically distinct types (I and II). Aquatic snails with preferences for shallow water possess eyes with both type I and type II photoreceptive cells, but Pl. corneus, an inhabitant of deeper water, only has type-I receptors, supporting an earlier finding that type I cells represent dim- and type II cells bright-light receptors. On the basis of histological and optical comparisons, we conclude that the eyes of L. stagnalis and R. peregra, species that are known to escape and seek temporary refuge above the water surface, are well adapted to function in water as well as air, but that the eyes of P. fontinalis and Pl. corneus are less modified from those of their terrestrial ancestors.