The deepest parts of the ocean are dark. For marine animals living one thousand feet below sea level and lower, the absence of light makes it challenging to find food, attract a mate, and identify predators.
Some animals make their own light through a process called bioluminescence. Others have adapted in ways that help them detect light in an environment beyond the reach of the sun’s rays.
In the first stereological study of the eyes of deep sea sharks, scientists in Queensland, Australia quantified photoreceptor cell populations and mapped their topography in the retina of five different species of deep sea sharks.
The sharks, including the Borneo catshark, the longsnout dogfish, the prickly dogfish, the beige catshark, and McMillan’s catshark, were caught in the nets of deep-sea fishermen off the coast of New Zealand. Each type of shark featured large, round pupils and a tapetum lucidum, a reflective structure at the back of the eye – two common adaptations deep-sea animals use to enhance sensitivity in environments where bioluminescence is the only available light source, according to the paper.
Delving deeper into the microscopic layers of the shark eye, the scientists used Stereo Investigator to outline the photoreceptor layer in the retina of each shark species. They then used the optical fractionator probe to quantify cell population and topographic distribution in this region. Their analysis revealed that all five shark species had only one type of photoreceptor cell (rod), and these cells formed dense groups in distinct areas of the retina, which varied by species.
“The disparity of areas of increased sensitivity seen between species is thought to reflect distinctive predator avoidance and prey capture strategies,” the authors say in their paper. “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.”
Newman, A. S., Marshall, J. N., & Collin, S. P. (2013). Visual Eyes: A Quantitative Analysis of the Photoreceptor Layer in Deep-Sea Sharks. Brain, Behavior and Evolution, 82(4), 237-249. (U of Queensland) http://www.karger.com/Article/FullText/355370