Why should we celebrate shark week at eyegotcha?
Well we can learn a lot about human eyes by studying shark eyes and how they compare. Shark eyes are similar to humans in that the eyeball has a cornea, iris, pupil, lens and retina. In the late 1960s researchers discovered that shark eyes have duplex retinas or retinas containing rod and cone cells. Rods enable the shark to see light and darkness, while cones allow for the detection of color.
One very important feature sharks have is the tapetum lucidum. Tapetum lucidum is Latin and means “shining layer”. The tapetum improves vision in low light conditions. Tapetum occur in many vertebrates, particularly nocturnal animals with good night vision such as cats and dogs. This feature is the cause of “red-eye” effect causing eyes in flash photography to appear to “glow”.
The tapetum is located behind the retina and it is made up of mirrored crystals. What happens when light goes through the retina and hits the crystals, the light is reflected back onto the retina instead of being absorbed.
The size and shape of the eye varies for different sharks. The shark’s eye and pupil size varied depending on the layer of the ocean in which they predominately reside. This was found in a 2007 study. Many sharks that stay near the surface have evolved to hunt in the sunlight and rely on their vision more than other senses, so have large eyes. Some deep-sea sharks also have big eyes to pick up faint traces of light down in the darkness—but their eyes are loaded with light-sensing rods and have fewer color-sensing cones. Researchers also have found that bioluminescent deep-sea sharks have a higher density of rods in their eyes than their non-bioluminescent counterparts, allowing them to see more details in the dark water when bioluminescence is present. Sharks that live in shallow water on the seafloor often have the smallest eyes because floating sediment kicked up from the bottom blocks their vision. These animals instead rely on senses like smell and electroreception over vision. Lastly, sharks that hunt fast-moving prey like fish and squids have bigger eyes (and presumably better eyesight) than those that eat non-moving prey.
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Photo:
By Hermanus Backpackers (Great White Shark Cage Diving) [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons)