Visual eyes: a quantitative analysis of the photoreceptor layer in deep-sea sharks.

The marine environment presents unique visual challenges for a range of organisms, particularly those dwelling at great depths, where sunlight may either be absent or drop to very low levels. Under these environmental conditions, the visual system must maximise light absorption in order to enhance the detection of prey, predators and potential mates. Using stereological analysis of retinal wholemounts, the distribution and number of photoreceptors (rods) was determined for 5 deep-sea shark species from a range of depths (46-1,500 m). All species possessed areas of increased photoreceptor density (with peaks between 41,000 and 82,000 rods/mm(2)) within discrete regions of the retina. The total number of rods in the photoreceptor layer also varied between 17 × 10(6) and 63 × 10(6). It is evident that increasing sensitivity of the retina is an important adaptation to life in the deep sea. The location of discrete areas of high cell density within the photoreceptor layer of the retina corresponds to discrete areas of the visual field that are sampled at a higher intensity, hence increasing sensitivity. The location of these areas of increased sensitivity differed between the species of this study. The disparity of areas of increased sensitivity seen between species is thought to reflect distinctive predator avoidance and prey capture strategies. This study reveals that the visual demands of deep-sea sharks vary interspecifically and that sampling of each species' visual field is not solely determined by its habitat.