The retinal pigments of the whale shark (Rhincodon typus) and their role in visual foraging ecology

Widespread and Convergent Evolution of Cone Monochromacy in Galeomorph Sharks

Color vision is widespread in marine vertebrates but is notably lacking in whales, dolphins, seals, and apparently also sharks. All sharks studied to date possess only a single spectral class of cone and are thus potentially totally color blind. The reason why sharks lack color vision is unclear, but as the visual pigments of only a handful of this large and ecologically diverse taxon have been studied, more data are required to address this question. Here, we assembled the retinal transcriptomes of 9 species from 7 families and 3 orders within the superorder Galeomorphii to screen for visual opsin and phototransduction genes. We reveal that cone monochromacy is widespread in galeomorph sharks, but the type of cone opsin expressed varies, with lamniform and orectolobiform sharks expressing a long-wavelength-sensitive (LWS) opsin, and carcharhiniform and heterodontiform sharks expressing a rhodopsin-like 2 (RH2) opsin. Cone monochromacy has evolved from a dichromatic ancestral state at least 4 times, implying strong selection pressure to prioritize achromatic over chromatic vision. While all species express the GRK1A and GRK7 isoforms of G protein-coupled receptor kinase, only sharks with the LWS cone opsin express the GRK1B isoform, which suggests that nonspectral functions of photoreception may have influenced, or result from, the opsin complement in the shark retina. Finally, we show that the shark rod (RH1) opsin gene shows evidence of positive selection at sites known to influence pigment kinetics (i.e. metarhodopsin II stability) and that the rate of retinal release likely differs substantially between species in ways that reflect their physiology and ecology.

Knockout of sws2a and sws2b in Medaka (Oryzias latipes) Reveals Their Roles in Regulating Vision-Guided Behavior and Eye Development

The medaka (Oryzias latipes) is an excellent vertebrate model for studying the development of the retina. Its genome database is complete, and the number of opsin genes is relatively small compared to zebrafish. Short wavelength sensitive 2 (sws2), a G-protein-coupled receptor expressed in the retina, has been lost in mammals, but its role in eye development in fish is still poorly understood. In this study, we established a sws2a and sws2b knockout medaka model by CRISPR/Cas9 technology. We discovered that medaka sws2a and sws2b are mainly expressed in the eyes and may be regulated by growth differentiation factor 6a (gdf6a). Compared with the WT, sws2a^-/- and sws2b^-/- mutant larvae displayed an increase in swimming speed during the changes from light to dark. We also observed that sws2a^-/- and sws2b^-/- larvae both swam faster than WT in the first 10 s of the 2 min light period. The enhanced vision-guided behavior in sws2a^-/- and sws2b^-/- medaka larvae may be related to the upregulation of phototransduction-related genes. Additionally, we also found that sws2b affects the expression of eye development genes, while sws2a is unaffected. Together, these findings indicate that sws2a and sws2b knockouts increase vision-guided behavior and phototransduction, but on the other hand, sws2b plays an important role in regulating eye development genes. This study provides data for further understanding of the role of sws2a and sws2b in medaka retina development.

Immunohistochemical Characterisation of the Whale Retina

The eye of the largest adult mammal in the world, the whale, offers a unique opportunity to study the evolution of the visual system and its adaptation to aquatic environments. However, the difficulties in obtaining cetacean samples mean these animals have been poorly studied. Thus, the aim of this study was to characterise the different neurons and glial cells in the whale retina by immunohistochemistry using a range of molecular markers. The whale retinal neurons were analysed using different antibodies, labelling retinal ganglion cells (RGCs), photoreceptors, bipolar and amacrine cells. Finally, glial cells were also labelled, including astrocytes, Müller cells and microglia. Thioflavin S was also used to label oligomers and plaques of misfolded proteins. Molecular markers were used to label the specific structures in the whale retinas, as in terrestrial mammalian retinas. However, unlike the retina of most land mammals, whale cones do not express the cone markers used. It is important to highlight the large size of whale RGCs. All the neurofilament (NF) antibodies used labelled whale RGCs, but not all RGCs were labelled by all the NF antibodies used, as it occurs in the porcine and human retina. It is also noteworthy that intrinsically photosensitive RGCs, labelled with melanopsin, form an extraordinary network in the whale retina. The M1, M2, and M3 subtypes of melanopsin positive-cells were detected. Degenerative neurite beading was observed on RGC axons and dendrites when the retina was analysed 48 h post-mortem. In addition, there was a weak Thioflavin S labelling at the edges of some RGCs in a punctuate pattern that possibly reflects an early sign of neurodegeneration. In conclusion, the whale retina differs from that of terrestrial mammals. Their monochromatic rod vision due to the evolutionary loss of cone photoreceptors and the well-developed melanopsin-positive RGC network could, in part, explain the visual perception of these mammals in the deep sea.

Spectral tuning and deactivation kinetics of marine mammal melanopsins

In mammals, the photopigment melanopsin (Opn4) is found in a subset of retinal ganglion cells that serve light detection for circadian photoentrainment and pupil constriction (i.e., mydriasis). For a given species, the efficiency of photoentrainment and length of time that mydriasis occurs is determined by the spectral sensitivity and deactivation kinetics of melanopsin, respectively, and to date, neither of these properties have been described in marine mammals. Previous work has indicated that the absorbance maxima (λmax) of marine mammal rhodopsins (Rh1) have diversified to match the available light spectra at foraging depths. However, similar to the melanopsin λmax of terrestrial mammals (~480 nm), the melanopsins of marine mammals may be conserved, with λmax values tuned to the spectrum of solar irradiance at the water's surface. Here, we investigated the Opn4 pigments of 17 marine mammal species inhabiting diverse photic environments including the Infraorder Cetacea, as well as the Orders Sirenia and Carnivora. Both genomic and cDNA sequences were used to deduce amino acid sequences to identify substitutions most likely involved in spectral tuning and deactivation kinetics of the Opn4 pigments. Our results show that there appears to be no amino acid substitutions in marine mammal Opn4 opsins that would result in any significant change in λmax values relative to their terrestrial counterparts. We also found some marine mammal species to lack several phosphorylation sites in the carboxyl terminal domain of their Opn4 pigments that result in significantly slower deactivation kinetics, and thus longer mydriasis, compared to terrestrial controls. This finding was restricted to cetacean species previously found to lack cone photoreceptor opsins, a condition known as rod monochromacy. These results suggest that the rod monochromat whales rely on extended pupillary constriction to prevent photobleaching of the highly photosensitive all-rod retina when moving between photopic and scotopic conditions.

Not just shades of grey: life is full of colour for the ocellate river stingray (Potamotrygon motoro)

Previous studies have shown that marine stingrays have the anatomical and physiological basis for colour vision, with cone spectral sensitivity in the blue to green range of the visible spectrum. Behavioural studies on Glaucostegus typus also showed that blue and grey can be perceived and discriminated. The present study is the first to assess visual opsin genetics in the ocellate river stingray (Potamotrygon motoro) and test whether individuals perceive colour in two alternative forced choice experiments. Retinal transcriptome profiling using RNA-Seq and quantification demonstrated the presence of lws and rh2 cone opsin genes and a highly expressed single rod (rh1) opsin gene. Spectral tuning analysis predicted these vitamin A1-based visual photopigments to exhibit spectral absorbance maxima at 461 nm (rh2), 496 nm (rh1) and 555 nm (lws); suggesting the presence of dichromacy in this species. Indeed, P. motoro demonstrates the potential to be equally sensitive to wavelengths from 380 to 600 nm of the visible spectrum. Behavioural results showed that red and green plates, as well as blue and yellow plates, were readily discriminated based on colour; however, brightness differences also played a part in the discrimination of blue and yellow. Red hues of different brightness were distinguished significantly above chance level from one another. In conclusion, the genetic and behavioural results support prior data on marine stingrays. However, this study suggests that freshwater stingrays of the family Potamotrygonidae may have a visual colour system that has ecologically adapted to a riverine habitat.

Vision in sharks and rays: Opsin diversity and colour vision

The visual sense of elasmobranch fishes is poorly studied compared to their bony cousins, the teleosts. Nevertheless, the elasmobranch eye features numerous specialisations that have no doubt facilitated the diversification and evolutionary success of this fascinating taxon. In this review, I highlight recent discoveries on the nature and phylogenetic distribution of visual pigments in sharks and rays. Whereas most rays appear to be cone dichromats, all sharks studied to date are cone monochromats and, as a group, have likely abandoned colour vision on multiple occasions. This situation in sharks mirrors that seen in other large marine predators, the pinnipeds and cetaceans, which leads us to reassess the costs and benefits of multiple cone pigments and wavelength discrimination in the marine environment.