Are not octopuses colorblind?

Wide pupils could allow color perception despite the lack of photoreceptors

Can octopuses recognize colors? © Roy Caldwell / UC Berkeley
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Sophisticated patent: Octopuses may even recognize colors - despite the lack of photoreceptors. This could be made possible by the particularly wide pupils of the animals. Through them, light can come in from many different directions and split into its different wavelengths. This could help cephalopods to distinguish colors based on their chromatic spectrum.

Octopuses have unique abilities: they can change the color, pattern, and even texture of their skin at lightning speed. In this way, the animals adapt perfectly to their surroundings to camouflage themselves. However, the intelligent cephalopods also communicate with each other about these changes of appearance and use their impressive play of colors, for example, in the mating dance.

All the more surprising seems that the animals can not see colors. Her eyes have - unlike humans - only one type of light receptor. Therefore, they are likely to perceive the world only in black and white. For Alexander Stubbs of the University of California at Berkeley, the lack of color perception of the octopi makes no sense. He thinks: Octopuses see the environment around them in all their splendor of color - just unlike other animals.

Due to their pupil shape, cephalopods may not only see the world in black and white (from above: Sepia bandensis, Sepioteuthis, Octopus vulgaris). © Roy Caldwell / Klaus Boots / Alexander Stubbs

Pupil shape as a key

Together with his father Christopher Stubbs from Harvard University, biology has developed a theory that cephalopods, despite their color blindness in the traditional sense, could still recognize colors: the key, they suspect, lies in the unusual pupil shape of the animals.

Because of the U-, W- or dumbbell-shaped pupils of octopus, octopus and their relatives, light can fall into the eyes from many different directions. This creates an effect that many probably know from the ophthalmologist: Extend our eyes through eye drops, for example, not only does everything look blurry. There are also color spaces in the field of view. display

This is because light of different colors is refracted differently by a lens blue light is stronger than red. In the eye, the wavelengths therefore strike at different points. Experts speak of the so-called chromatic aberration. The larger the pupil area through which the light can enter, the stronger this effect becomes.

Error becomes function

Most animal eyes have evolved to minimize chromatic aberration. Even manufacturers of photo cameras try to reduce the effect with the help of technical tricks. "The wide pupils of the Kopff er, on the other hand, emphasize the spectral splitting and make the error a special function, " says Alexander Stubbs.

Chromatic aberration of a condenser lens public domain

Using a computer simulation, the scientists found out how the animals could use the chromatic aberration for color perception. They conclude: The heads adjust the focus of their eyes by changing the distance between the lens and the retina and in doing so can focus on the wavelengths that strike at different points focus and distinguish colors based on their chromatic signature.

Blurriness as a distinguishing feature

According to the researchers, the animals would observe in this back-and-forth focusing how the blurriness of the image changes and where it seems the most. Accordingly, they differentiate colors according to how focused they appear: "White objects, which reflect all wavelengths of light, must be extremely blurry for octopuses. Purer colors such as yellow or blue, on the other hand, should enable them to focus clearly, "says Stubbs.

However, this trick works only if the colors in the environment have a sufficiently high contrast. "Only then can changes in chromatic blurriness be clearly recognized, " the scientists write.

"Elegant mechanism"

In order for them to be able to distinguish colors, the octopuses, however, are impaired by visual acuity. Nevertheless, the Stubbs are impressed with the principle: "We believe we have discovered an elegant mechanism that can compensate for the lack of different types of photoreceptors, " they say. "This principle is fundamentally different from what we know from humans and many other animals." They hope that their theory can be confirmed in the future by behavioral tests with heads. (Proceedings of the National Academy of Sciences, 2016; doi: 10.1073 / pnas.1524578113)

(University of California - Berkeley, 07.07.2016 - DAL)