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Jan. 26, 2011

Are Cephalopods Really Colorblind?

by Noah Rodstein

Click to enlarge images
Strangely enough, even though cephalopods are masters at blending into the background, they are actually colorblind. In fact, there is only one species of cephalopod that is known to have color vision, the firefly squid (Watasenia scintillians), which has the ability to see three visual pigments just as we do. Other species tested have proven to be incapable of distinguishing the difference between colors.
 
Both electroretinogram experiments and behavioral studies -- such as training cephalopods to choose a black object over a white object, or a light gray over a dark gray -- have demonstrated that the cephalopod does, in fact, have monochromatic vision.

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You may be wondering, if cephalopods are in fact color blind, then how are they able to match the background so perfectly? How do they know what colors to change into?
 
Cephalopods actually have an eye structure that is much like the eyes of many vertebrates, but these similarities are merely superficial and are the result of convergent evolution.
 
The cuttlefish's eye looks nothing like a vertebrate’s eye. The cuttlefish has a specially designed “W” shaped eye. Unlike our eyes, which focus by changing the shape of the lens, the cuttlefish focuses through movement of the entire eye, actually pulling the pupil around and reshaping the eye itself.
 
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Like us, cephalopods (excluding the nautilus which is a quite primitive member of the cephalopod family), are endowed with camera-like eyes made up of a pupil that allows light to pass through, and a lens that focuses and projects the image to the photoreceptor cells of the retina. A big difference, however, between their eyes and our eyes is that, unlike us, they don’t have a blind spot in their vision. This is because vertebrate eyes receive light that is bounced back from the back of the eye, while cephalopod eyes receive light directly. (see figure below.) A vertebrate’s blind spot occurs because our optic nerve and our nerve fibers pass in front of our retina, whereas the cephalapod's optical nerve and nerve fibers lay beneath the retina. Also, vertebrate eyes are attached only to the brain and held in place by sockets. A cephalopod’s eyes are actually built into their body’s surface.
 
Cephalopods have the ability to sense differences in polarized light thanks to photoreceptors called rhabdomeres, which are like their version of our rod and cone receptors. These rhabdomeres allow cephalopods to see polarized and unpolarized light through the use of microvilli found in the rhabdomeres. Much like how wearing a pair of polarized lenses reduces the glare of the sun and allows us to see better, a cephalopod's ability to perceive differences in polarized light enhances their perception of contrast.

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In a document by Dr. James Wood and Kelsie Jackson, they explain that "Cephalopods can not only see differences in polarized light, they can create patterns using these differences on their bodies.” Cuttlefish even have a bottom layer of skin made up of cells known as leucophores, which are composed of colorless translucent protein reflectors that allow them to reflect any wavelengths of light approaching from any angle. In certain areas, the cuttlefish's skin has another layer of cells (on top of the leucophores) that is made up of iridophores, which further enhance the intensity of the reflection and greatly help the cuttlefish to blend into their environment.
About Noah Rodstein

The views expressed are those of the author and are not necessarily those of Science Friday.

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