Notes from Down Under 2: Color-blind predators

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.

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A grey reef shark in Ningaloo Reef, Western Australia (photo: Andy Malone, source: Flickr).
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3 thoughts on “Notes from Down Under 2: Color-blind predators

  1. Akin Olagoke Ogunleye

    Thank you Barbara for this post . I think it is better to play safe. If there is a probability of a predator’s presence in a water or river, we need not swim or play in such waters.

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  2. Laura Wheeler

    I do find sharks fascinating and there are so many interesting facts about them you could go on forever for instance I particularly like the fact that sharks very rarely get cancer so scientists study their cartilage in the hopes of finding a cure for the disease!
    I actually didn’t know sharks were colour blind, but I have read that some species of sharks have a movable, transparent nictitating membrane that covers and protects their eyes when they are attacking and eating. SO I guess shark eyes are interesting things to study. Greenland sharks(Somniosus microcephalus) sometimes have small parasites on their eyes that glow in dark water. Scientists think these glowing parasites attract prey into the sharks’ mouths.
    With regards to catching their prey, I know hearing is a sharks best sense. For example, some sharks can hear prey in the water from 3,000 feet away. They are also better at detecting low frequency sounds so dolphins are safe.  Then they have their sense of smell, some sharks can smell one part of blood in 100 million parts of water. They can also tell which direction that smell is coming from. Because of this sharks have been called swimming noses!
    Therefore overall we have learnt that to prevent being attacked by a shark, make high pitched noises, try not to bleed, and wear dull colours that match the background in the water. Or…just swim for your life…

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  3. Barbara Ferreira

    Thanks for so many interesting shark facts Laura. I did see a shark while snorkelling in Ningaloo and I did exactly that: swim to the boat as fast as possible. Even though the Aussies kept saying it was just a grey reef shark that doesn’t attack humans unless it feels threatened, the Europeans that did see it didn’t want to take any chances!

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