Cone monochromacy and visual pigment spectral tuning in wobbegong sharks

Abstract

Much is known regarding the evolution of colour vision in nearly every vertebrate class, with the notable exception of the elasmobranchs. While multiple spectrally distinct cone types are found in some rays, sharks appear to possess only a single class of cone and, therefore, may be colour blind. In this study, the visual opsin genes of two wobbegong species, Orectolobus maculatus and Orectolobus ornatus, were isolated to verify the molecular basis of their monochromacy. In both species, only two opsin genes are present, RH1 (rod) and LWS (cone), which provide further evidence to support the concept that sharks possess only a single cone type. Examination of the coding sequences revealed substitutions that account for interspecific variation in the photopigment absorbance spectra, which may reflect the difference in visual ecology between these species.

Figure 1.

(a) Normalized absorbance spectra for rod (black) and cone (grey) photopigments expressed in the retinae of two species of wobbegong shark, Orectolobus maculatus (solid line) and Orectolobus ornatus (dotted line) [15]. Each λ_max value (shaded and boxed) was determined by microspectrophotometry [15]. (b) Phylogenetic analysis of wobbegong LWS and RH1 retinal opsins. Confidence values are placed at each node. The scale bar shows the number of nucleotide substitutions per site.