How a Shark's Vision Works | When Sharks Attack

Elvin is part of a series of events puzzling investigators: nine shark attacks along the southeast Florida coast in 2017, more than double the average. With leads coming up short, some local authorities come up with their own theories for the spike. Vero Beach Ocean Rescue's Eric Tomsu explains, "In my experience, it's based on a few different issues. People in the water, number one. Number two, how clear the water is."

Usually, there's murky water when we've had an incident. Murky water may seem like an innocuous detail, but witness Jessica Veach echoes the idea that it's important. "There's a lot of sand being churned, which makes it hard for the shark to see what is actually fighting." Could the shark's ability to see in murky water be the one thing that ties all the attacks together?

According to Dr. Huber, sharks are adept at navigating murky water, relying on other senses. "In murky water, sharks can lean more heavily upon the lateral line and the electro-sensory systems in order to locate their prey." However, he does think there is a unique limit to a shark's vision that could have played a role in some of the attacks, and it has to do with available light.

"We're bringing out a spiny dogfish to investigate." Dr. Huber performs an eye dissection using this common shark found in waters all over the world. "Getting towards the back of the eyeball, we'll be able to free it up and remove it from the skull. This is the back half of the eyeball."

One of the really interesting things about sharks' eyes is that they have a layer of reflective crystals that coat the back of the eye, called a tapetum lucidum. "What this does is it actually causes night shine. You may have seen night shine on your cat or your dog, on a variety of different animals. In humans, light passes through the eye once, but for sharks in dark waters, the tapetum lucidum bounces the light back, letting the eye process images a second time."

In layman's terms, this basically means that sharks have night vision. But as the sun begins to rise, a shark's eye must react to the increasing light. "You can think of when you first open your eyes in the morning and things seem a little bit too bright. After a few seconds, your eyes accommodate to the amount of light in the room, and then you can see things as you normally would."

In sharks, this process can take anywhere from a half an hour to potentially a couple of hours. "It's a process called visual accommodation. During these periods of changing light, like dusk or dawn, a shark's vision may be compromised," potentially leaving swimmers vulnerable during these times. "Sharks are very, very active looking for their prey, which would increase the probability of a shark attack if people were in the water at those same times."