Being cephalopod: Changing color in a color-blind world

Author: Ada Hagan

Editors: Bryan Moyers, Kevin Boehnke, Shweta Ramdas

 

Just a couple of weeks ago in “Camouflaged: Finding cephalopods” MiSciWriters blogger Irene Park told us about how cephalopods (octopuses, cuttlefish, and squids) alter their skin color, and texture to blend into their surroundings. But based on what scientists know about cephalopods’ eyes, they should be color-blind. So how can they mimic colors with such incredible accuracy?

This question has stumped scientists for decades. But in a recent journal article, researchers reported that they may have stumbled on the answer – cephalopods may be able focus their eyes between different colors.

The human eye

Imagine it’s a gorgeous, warm, sunny day and you’re reading by the community pool. Just as you’re engrossed in a plot twist, a mighty splash of water soaks your feet and legs, narrowly missing your book. Quickly, you glance up, notice it was your neighbor’s 8-year-old son, and send a grouchy glare his way before returning to your book.

Shifting what our eyes focus on, from near to far to near again, is something we do all the time. Human eyes function by focusing a single source of light through our cornea to a single point on the retina. When light hits our eyes, the cornea first alters the angle of the light, focusing it to pass through pupil and then the lens. The lens, in turn, further focuses the light to the retina at the back of the eye. Photoreceptor cells in the retina interpret color (cones) and contrast (rods), and direct the signal to the brain. To switch focus between objects at different distances, e.g., from your book to the face of your neighbor’s son, small muscles in the eye contract, altering the curve of the lens to correct for distance.

The cephalopod eye

Like human eyes, cephalopod eyes are also “camera-like” with a lens and a retina. Unlike the human eye which has a small pupil with multiple types of photoreceptor cells, nearly all cephalopods have large pupils with only one type of photoreceptor cell. The presence of only a single photoreceptor cell type is why scientists thought that cephalopods could not distinguish between colors. However, these larger pupils might allow cephalopods to perform a process called chromatic blurring, making them far from color-blind.

Chromatic blur

As Isaac Newton demonstrated, different colors of light are refracted at different angles by a lens or prism, such as with a rainbow. This effect, called chromatic aberration, is a source of frustration for scientists because it limits the use of microscopy: different colors of light from a single object come into focus at different distances from a lens, causing scientists to see a blurry image. But cephalopods may have turned this property of light to their advantage. Instead of using their lens to focus at different distances, as humans do, cephalopods might shift focus to distinguish between different wavelengths of light, and their larger pupil size may enhance this process. In other words, cephalopods may take advantage of a physical property of light, chromatic aberration, through a process termed chromatic blurring in order to see different colors.

To test whether this was possible, the authors built a computer model that took into account many variables including features of cephalopod eyes, pupil types, and even how seawater affects light. When given information about light coming from theoretical fish (color, distance, intensity) the program “modeled” a cephalopod eye and brain, reporting what it might see. The authors demonstrated that they could determine a fish’s color based on when its image went in and out of focus.

Now, because this experiment didn’t include live cephalopods, it doesn’t definitively prove that this is how cephalopod eyes work. However, the authors noted that their results fit well with previous studies using live cephalopods. In their words, this “scenario may force us to rethink what it means to be a color-blind animal”.

And that’s how scientists think “color-blind” cephalopods can see the colors they so adeptly mimic.

About the author

Ada Hagan is a doctoral student here at the University of Michigan in the department of Microbiology and Immunology. She does recon on the sneaky ways bacteria find nutrients (like iron!) when they are invading our bodies. Originally hailing from the mountains of East Tennessee, Ada earned both her B.S. and M.S. in Microbiology from East Tennessee State University. In her spare time, Ada spends time with her pets and husband, cooking, fishing & the occasional Netflix binge. Follow her on Twitter (@adahagan) and see more of her posts on LinkedIn.

Read more posts by Ada here.

Image Credit:

Anatomy of an eye credit: By Matticus78 at the English language Wikipedia, CC BY-SA 3.0, https:\commons.wikimedia.org\w\index.php?curid=2748615

Octopus eye credit: By Octopusv.JPG: Gronkderivative work: Mgiganteus (talk) – Octopusv.JPG, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11101549

Squid eye credit: By wildxplorer – Flickr, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=9867460

Cuttlefish eye credit: By Alexander Vasenin – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=25106323

Chromatic aberration credit: By DrBob at the English language Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=46592651