The visual system of the blind shark (Brachaelurus waddi)

Sharks have evolved over a period of approximately 400 million years. This extensive time frame has resulted in a divergence of shark species that occupy a wide range of environments and exhibit considerable diversity in morphology and behavior. It is likely that this diversity is reflected in the anatomy and physiology of shark visual systems. However, only a very small proportion of extant shark species has been studied in detail and much remains to be discovered about interspecific variation in their visual system. Furthermore, there have been no studies to date that have undertaken a comprehensive assessment of the visual abilities of a single shark species. Blind sharks (Brachaelurus waddi) are a benthic species found in abundance along Australia’s coast and adapt well in captivity, which makes them amenable to an in-depth assessment of their visual system. In this study, the anatomy and physiology of the blind shark’s eye was investigated and the results interpreted within the context of their environment and behavior. Electrophysiological techniques were used to measure the temporal resolution and contrast sensitivity of the eye. The spectral absorption properties of the photoreceptor visual pigments that subserve vision were measured in situ using single cell microspectrophotometry. Spatial resolving power was assessed by measuring the topographic distribution of ganglion cells in wholemounted retinas. When presented with a sinusoidally modulated light stimulus, blind sharks have a maximum critical flicker fusion rate of 22 Hz and a contrast sensitivity threshold of 177.6 (0.56%) at the brightest light intensity. Rods were the only type of photoreceptor identified in the retina. The rods contained a medium-wavelength (‘green’) sensitive visual pigment with a mean max at 501 nm. The topographic distribution of ganglion cells in B. waddi showed a weakly defined, elongated horizontal band or visual streak of higher density in the central-dorsal region of the retina. Based on the mean peak density of ganglion cells in the retina, the estimated spatial resolving power of the eye was 2.36 cycles per degree (cpd).