Here’s the next segment on our Shark Senses Series. If you missed the first article, click here. Shark vision has always seemed to be a bit of a mystery. We often hear about the mistaken identity argument related to shark-human interactions and that leads many to believe that sharks have poor eyesight. We also constantly hear about sharks amazing sense of smell, and it’s often thought that some other senses are sacrificed as a result. Is this true? Do sharks have poor vision?
A research group in Australia has been studying shark vision for years. In fact, they may be the worlds experts on vertebrate vision and have studied the eyes of literally hundreds of different species of elasmobranch (sharks, rays, skates and sawfish). Here, we’ll try to summarize their findings and explain what that means in practical terms.
Sharks lack color vision. That’s right. Sharks lack the necessary cells that process color vision and can only see in Black and White. This seems to be the case for all sharks as none of the tested animals possessed the necessary photoreceptor cells to see in color.So what’s all that talk about Yum Yum Yellow? We’ll get back to that. On a side note, rays are able to see in color.
Visual Acuity. Sharks are thought to be able to see very focused images. The fact that shark vision is monochromatic does not mean they lack visual acuity. In the human eye, we have muscles that control the shape of our lens and focuses light signals on the retina. By contrast, the lens in a sharks eye does not change shape. Rather, they have muscles that move the lens forward or backward to focus light. In both cases, the effect is the same and the retina receives a focused image. Sharks have great visual acuity and they absolutely rely on that vision for many of their behaviors. Obviously, water conditions will play a major effect on their ability to see and from what distance. In ideal conditions (the kind we get while shark diving in the Bahamas), sharks can see clearly from 10-15 meters or more. This means while their vision is good, it’s not the first sense that keys them into their prey and becomes more important as they get closer.
Light Sensitivity. The ability to see in low light conditions varies greatly between the shark species studied. Essentially, there are 2 types of photoreceptor cells located in the retina. The cones are active in bright light conditions and the rods are active in low light scenarios. Each species will have a different proportion of these cells. Not surprisingly, deep water sharks have large eyes with a much higher proportion of low light photoreceptor cells (rods) compared to cones. Shallow water sharks have cells in the opposite proportion. In addition, sharks possess a structure called the Tapetum lucidum. This is the reflective part of the eye that lies behind the retina and causes the shining eye you may have seen in a cat or a deer in headlights. This will actually reflect the light one more time back through the retina thereby making it available to the retina a second time and increasing low light vision.
In Practical Terms. Sharks have monochromatic vision. Sharks have good visual acuity. Sharks have vision suited to the environment they live in. Given their sharp focus and black and white view of the world, do we need to worry about the color of our dive gear? In short, Yes! Sharks do tend to be interested in high contrast areas. It’s the reason we don’t like very bright accent colors on dive gear as that may peak their interest. It’s also the reason we require divers to wear gloves for our shark dives. A pale fleshy hand sticking out of a black wetsuit sleeve may be a recipe for trouble.
If you’d like to learn more about shark vision, take a look at Vision in elasmobranchs and their relatives: 21st century advances.