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Biology

The dark side of the…cephalopod eye?

Stubbs, A. L., & Stubbs, C. W. (2016). Spectral discrimination in colorblind animals via chromatic aberration and pupil shape. Proceedings of the National Academy of Sciences, 113(29), 8206-8211. DOI: 10.1073/pnas.1524578113

cuttle_display

Figure 1 – A cuttlefish donning a flamboyant display (courtesy of Prilfish via flickr.com)

Cephalopods are masters of camouflage, the chameleons of the ocean. Cuttlefish, octopus, and other members of the family can dynamically change the pattern of their skin. This ability allows them to hide from predators, ambush prey, and communicate with each other (fig. 1). They use their impressive range of displays to great effect (just check out these videos of cuttlefish signaling and an octopus hiding).

Scientists have studied cephalopods for decades. Despite all that research, there remain a number of vexing questions about these mysterious creatures. A big one is how cephalopods do such a good job changing their body color despite being colorblind. They can match color to the point that they disappear in kelp but only see in black and white? Huh??

Some experiments have suggested that cephalopods might sense light directly with their skin. But the father/son team of Alexander and Dr. Christopher Stubbs has a different theory. They maintain that there is not sufficient evidence that cephalopods have enough light sensitive protein in their skin to discern color. The Stubbs instead proposed that the shape of cephalopods’ pupils is uniquely adapted to separate light into different colors as it enters the eye (fig 2).

To test this idea, Stubbs and Stubbs wrote a computational model based on geometric optics, or ray tracing. The entire process is quite complex, but the underlying physics is taught in high schools across the country. The crux of their argument comes from the familiar Snell’s Law.

cephalopod_eyes_crop

Figure 2 – A selection of cephalopod eyes. [D] The cuttlefish Sepia bandensis. [E] The shallow water squid Sepioteuthis. [F] The shallow water octopus Octopus vulgaris. (adapted from Stubbs and Stubbs, 2016)

According to Snell’s Law, when a beam of light passes from one medium to another, it refracts or bends. The amount it bends depends on the materials it is traveling through and the wavelength, or color, of the light (check out this super cool interactive simulation to see the process in action). When different colors of light traverse the same interface at the same time and angle, they disperse. Dispersion is how physicists describe white light separating into a rainbow as it passes through a prism.

Stubbs and Stubbs demonstrate that the annular shape of cephalopod pupils actually maximizes the dispersion of light as it enters the eye. Figure 3 illustrates the principle in three cases. In all three eyes, different colors of light focus at different points on the retina. Notice how the light rays in 3b, the pupil with a narrow aperture, focus to a smaller cone. But the model cephalopod eye, shown in 3c, deflects each color at a higher angle. The resulting image would likely be a blurry blob.

From an evolutionary standpoint, the Stubbs’ theory suggests that cephalopods have sacrificed the ability to resolve a clear image in favor of a smeared, but chromatically separated, one. The authors propose that cephalopods could still identify an object, how far way it is, and its color by scanning their eyes. The animal would, in effect, create a composite image in its brain.

eye_ray_path

Figure 3 – Ray diagrams from Stubbs and Stubbs’ model. [A] is an illustration of a wide eye, open around the center. [B] Shows a narrow pupil, also centered on the middle axis. [C] Is the model of a cephalopod eye. Notice how the curved slit is off the optical axis. In all panels ‘CB’ stands for chromatic blur, referring to the apparent spread of the different colors of light on the back of the eye. The CB is largest for the cephalopod eye.

Such neural processing would be more computationally complex than simply resolving a crisp, gray scale image. Based on the human visual experience, that seems counterintuitive. Why bother evolving such a convoluted visual system? But the Stubbs say rather than being convoluted, the system actually allows cephalopods to differentiate color more rapidly and reliably; their strange eyes are, in fact, crucial to their life strategy.

Furthermore, there is physiological evidence supporting the Stubbs’ idea. For one, cephalopods developed these wacky eyes in the first place. Beyond that, many cephalopods have highly developed optic lobes that could conceivably process such complicated visual inputs. Both adaptations require significant energy to develop and use – energy that could have been devoted elsewhere.

For the time being, the Stubbs’ model of the cephalopod visual system is only supported by their computational results. Neither father nor son had the means to conduct lab tests on living cephalopods. But in the discussion section of their paper, they suggest a slew of experiments to test their theory. Should their model hold water, it might force us to reconsider what it means for animals to be “colorblind.”

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