Adaptative Radiation of the Retina in Galaxiidae (Salmoniformes)

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Abstract
The retinas of seven galaxiid species collected from different habitats in Tasmania were investigated by light microscopy. There are remarkable differences between Galaxias and Paragalaxias species. The retinas of Galaxias species are characterised by a less dense retinal epithelial pigment (REP), large numbers of short and long rods, the presence of a tapetum, and stable (non-responsive) single cones in dark-adapted specimens. On the other hand, the common retinal features in Paragalaxias species include few large rods and many cones, the presence of a tapetum, lower summation and a pattern of retinomotor responses not unlike that of most other teleosts. There is no striking retinal feature that could be related to differences in their clear or turbid habitats. In spite of its probably being nocturnal, the retinal structure of P. dissimilis is basically similar to that of other diurnal Paragalaxias species. Some of the ecological constraints placed upon these species are also discussed.

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