Light and Sharks' Sense of Sight
It's possible that light attracts sharks -- at the very least, it'll catch their eyes. We know that most species of sharks have keen eyesight, and in some circumstances, they may mistake light reflected off an object for prey. Not all sharks can see well, however. Bull sharks are nearly blind. They use their sense of smell to find prey. Once they discover another animal, bull sharks bump it with their heads, then bite. By using this hunting technique, bull sharks have adapted to life even with their poor eyesight.
Most sharks would make an optometrist giddy with joy, though. In his doctoral thesis, shark researcher Dr. Samuel Gruber tested sharks' ability to see in low light. He found that some sharks see as well as 10 times better in dim environs [source: PBS]. This is due to many millions of rod cells found in sharks' eyes, the same cells humans have that help us see light. But sharks' eyes have an added feature that our eyes don't: the tapetum lucidum. This is a membrane in the back of a shark's eye that reflects light back into the eye. It increases sharks' sensitivity to light so they can see better in murky water [source: Sea World].
This is good for sharks, but potentially bad for you. It means that a shark will see you before you see it. Any list of shark attack prevention tips nearly always advises against wearing shiny jewelry [source: The Dallas Morning News]. The light reflected off shiny metal can be mistaken by sharks as fish scales -- and they may want to investigate a little too close for your comfort.
Because they have so many rod cells in their eyes, sharks are able to distinguish between light and dark tones [source: Schultz]. So if you're wearing a dark wetsuit while exploring a white coral outcropping, you'll stand out in a shark's field of vision. There are some sharks -- found in well-lit environments, such as shallow, clear water -- that have developed cones [source: SharkTrust]. These are the same cells you have that allow you to distinguish colors. It's possible that some sharks can tell the difference between colors, especially light and dark ones. This gives a bit of credence to the unproven but persistent rumors among divers and surfers that sharks are attracted to "yummy yellow" [source: The Times].
There's possibly another, more roundabout way that light attracts sharks: The idea that light attracts other fish, which, in turn, attract sharks. Fishermen have long believed that lights draw fish. Exactly why remains something of a biological mystery [source: Popper and Carlson]. One explanation is that a light (especially ones on the blue-green wavelength) submerged underwater creates a feeding chain reaction. The light attracts phytoplankton, which in turn attracts small fish like minnows. These smaller fish will ostensibly attract larger fish and so on until sharks turn up to see what the big deal is.
Again, this is just a theory. But that hasn't stopped plenty of companies from producing fish lights. The products are actually legitimized, ironically, by a particular shark -- the cookiecutter shark. This species emits a greenish glow through tiny organs called photophores. The light attracts large fish, which the cookiecutter shark bites into, removing a plug of flesh and leaving a round hole [source: University of Florida]. With predators like the cookiecutter shark around, perhaps young fish learn to be afraid of the light instead of the dark.
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