Comparative analysis of the structure and crystallin composition of the lenses of freshwater fish and gastropods with respect to their vision.

The structural organisations of the lenses of the pikeperch Sander lucioperca and the whitefish Coregonus lavaretus as well as of the pond snail Lymnaea stagnalis and the apple gold snail Pomacea canaliculata were studied by phase contrast microscopy, scanning electron microscopy and Raman spectroscopy. All microscopical examinations were based on 70 um thick lens sections, whereas spectral measurements were performed on whole crystalline lens. The lenses of S. lucioperca and C. lavaretus are characterised by a well-defined dense central region and layer-by-layer arrangement of substance in the form of concentric rings. The serrated shape of the membranes of their anucleate fibre cells suggests the formation of ball-and-socket contacts between them. The substance of the acellular lens of L. stagnalis and P. canaliculata appears to be homogeneous, but shows signs of a layer-by-layer formation. The lens of L. stagnalis exhibits weakly expressed peripheral ring structures, while optical heterogeneity in P. canaliculata is represented by a centrally located, indistinctly shaped nucleus. The Raman spectra of light scattering by the lens material of studied hydrobionts are fundamentally similar. Differences in the position and intensity of individual peaks may be explained by variations in the secondary structure of crystallin molecules, as well as the physical and chemical properties of the lens crystallins identified by polymerase chain reaction (PCR). PCR-based search for crystallin gene transcripts identified three lens-specific γ-crystallins (XM_031316242.2, XM_031292083.2, XM_031293803.2) in S. lucioperca. For the first time, the presence of Alpha-crystallin A chain-like and S-crystallin 4-like gene transcripts in the eyestalk of P. canaliculata was demonstrated.