A new study by Oxford University, published in the latest edition of Molecular Biology and Evolution, found that cartilaginous fish such as sharks and rays do not posses the SWS1 and SWS2 opsin genes, which can limit their range of visual sensitivity.
“We have characterized the molecular identity of the retinal visual pigments expressed in five species of shark and four species of ray, focusing on shallow-dwelling species known or likely to possess cone photoreceptors, and those filling particular phylogenetic gaps,” said corresponding author Nathan Hart.
The researchers looked at five species of shallow-dwelling shark and four species of ray to determine the molecular identity of the retinal visual pigments. While the eyes of both sharks and rays contain both rod and cone photoreceptors, sharks posses only one of the cone receptors needed to distinguish most colors while rays have two types.
“Furthermore, we provided measurements of the spectral characteristics of the visual pigments expressed in nine species of ray and two species of shark,” said Hart. “We can now confirm that all the shark species studied to date appear to be cone monochromats but report that in different species the single cone opsin may be of either the LWS or the RH2 class of opsins.”
Color discrimination may be useful for behaviors such as prey detection, predator avoidance, and mate choice. Since many ray species spend considerable periods of time resting on or partially buried in the substrate, color vision may instead aid in the detection of approaching overhead predators through either enhancement of visual contrast or elimination of achromatic flicker.
“Due to absorption, reflection, and scattering by the water itself and any dissolved or suspended substances, most aquatic habitats are characterized by low visual contrast,” Hart said. “Moreover, many sharks are active both night and day and must therefore operate under a wide range of light intensities. It is likely therefore that sharks are often operating close to the threshold of their visual capabilities where the disadvantages of color vision circuitry might be detrimental to survival.”