Opsin gene evolution in aquatic, amphibious and terrestrial blennies (Blenniidae)

Evolutionary adaptations to life on land include changes to an animal's physiology, morphology and behaviour. The visual systems of amphibious fishes show pronounced morphological adaptations; however, whether molecular changes also occur remains largely unknown. Here, we investigated the molecular evolution of visual opsin genes in blennies (Blenniidae), with a primary focus on the amphibious and terrestrial Salariini. Using retinal transcriptomes and amino acid comparisons in nine species from fully aquatic to terrestrial, we found limited sequence changes and no correlation between habitat and cone opsin gene expression. The 'red-sensitive' lws, the 'green-sensitive' rh2a, and two 'blue-sensitive' sws2aα and sws2aβ paralogues were expressed in all species, with the latter two showing pronounced phylogenetic inertia. Long-wavelength-dominated vision is likely beneficial for feeding on algae and detritus, the primary food source of most study species, and may be co-adapted to perceive red-coloured displays in terrestrial blennies. Conversely, a lack of 'ultraviolet-sensitive' sws1 expression coincides with ultraviolet-absorbing lenses in blennies, which likely evolved to protect the retina from the damaging effects of short-wavelength radiation, independent of habitat. Our data suggest that, at the molecular level, the visual systems that evolved in aquatic blennies have been retained in species that have progressively transitioned onto land.