Those of you who regularly surf or swim in waters potentially inhabited by sharks might have been told to observe certain safety rules to prevent attacks by these predators. “Avoid swimming at dusk or at night” and “Don’t surf if you are bleeding” are common place. But you might also have heard that it is wise to leave your flashy yellow surf board and your bright orange swim wear at home.
This “survival tip” is based on studies done in the past decades, including one from the US navy done in the 1970s, that indicate sharks are attracted by bright colors. The US navy researchers noticed that blue sharks repeatedly attacked a child dummy wearing a yellow life vest, while strikes on dummies on red and black flotation devices were few.
Despite the evidence that sharks are attracted to yellow, a study published last month shows that they probably can’t tell what color surf gear or swim wear is. Sharks, it seems, are color-blind.
Together with colleagues, Dr Nathan Scott Hart from the University of Western Australia analyzed the retinas of sharks of 17 different species from eastern and western Australian waters. The list included species such as the tiger shark and the bull shark, which are dangerous to humans.
Using a technique known as microspectrophotometry, the research team scanned the shark’s retinas for pigments connected to the two main types of light-sensitive cells, rods and cones. Rod cells help distinguish between shades of brightness and are the light receptors our eyes use to see in the dark. Cones, on the other hand, function best in bright light and are the cells used to tell different colors apart.
The study found that rods were the most common type of light-sensitive cells found in all the 17 shark species analyzed, indicating that the predators’ eyes function at a wide range of light levels. Cones, however, were more rare. In fact, researchers were unable to find this type of cell in 10 of the 17 species, while a single type of cones — sensitive to green light — was found in the other 7.
Humans have three types of light-receptor cones in the retina — sensitive to blue, green and red — so we see a multicolored world. If sharks have none, or only one type of cone receptors, then they are likely to be color-blind.
So what about those yellow life vests? “Our study shows that contrast against the background, rather than color per se, may be more important for object detection by sharks,” Hart stated to National Geographic. Therefore, sharks are more attracted to bright swim gear not because they can tell it’s yellow or orange but because those are the colors that contrast the most with the blue-green water.
Because there are over 400 shark species in the world, it is too soon to infer that all or most sharks are color-blind. But the conclusions of the study could already prove useful to prevent shark attacks in Australian waters.
Talking to the New Zealand Herald, Hart mentioned that “certainly high contrast might be the wrong thing to wear. So you should wear perhaps more muted colors or colors that match the background in the water better.”
The author of this post is glad she has a black and aqua-green bikini.