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.