Meeting Abstract
The whale shark (Rhincodon typus) is the largest of all extant elasmobranchs (sharks and rays). Similar to the mysticete whales, whale sharks are filter feeders and spend most of their foraging time in surface waters at depths less than 10m. Whale sharks are also capable of diving to depths in excess of 1500m with speculations that these dives are also associated with foraging. Sequence analyses of the recently published R. typus genome was used to describe the whale shark retinal opsins. Results show that whale sharks are rod monochromats and lack cone photoreceptors, a trait that is also found in the filter-feeding mysticete whales. The absorbance maximum of the single rod visual pigment is estimated to be 500nm based on conserved amino acids at rhodopsin positions 83, 292 and 299. Thus, whale shark rhodopsin is spectrally tuned and adapted to surface foraging in pelagic waters and is relatively less sensitive to light at depths greater than 200m. Because R. typus lacks cone photoreceptors, the retinal pigment melanopsin, expressed in intrinsically photosensitive ganglion cells (ipRGCs), is critical for controlling the pupil light reflex (PLR) in photopic light conditions to protect the rod-only retina from photobleaching. Previous studies have shown that the phosphorylation sites found in melanopsin’s carboxyl-tail are responsible for the activation/deactivation kinetics of ipRGCs expressing melanopsin, which control the rate of the PLR and pupil diameter under photopic conditions. Sequence analysis of the carboxyl tail from whale shark melanopsin reveals phosphorylation sites that are typical of a duplex retina (rods and cones) and are unlike other marine rod monochromats. This would result in a relatively fast PLR in whale sharks in stark contrast to the slow PLR described in rod monochromat whales.
