For the last couple of years I have been studying the retinal circuits of mice. While it is amazing how similar visual circuitry is among many species, I am always fascinated by surprising unique strategies that have developed in this system. The human visual system (from the retina to visual cortex) is a remarkable network that can see colors, adapt to a wide range of light intensities, perceive depth and distance, and much more. It is perfectly put-together such that each part contributes to a specific function: the lens focuses the image on the retina, different photoreceptors allow for color detection, our two frontal eyes allow for depth perception through parallax. The visual system of some animals has found other strategies to achieve the same functions, sometimes even using the same tools in new ways!
Cephalopods are marine vertebrates that can adapt their skin-tone to their surrounding, and don colorful patterns to attract individuals of the opposite sex. Amazingly, cephalopods only have one photoreceptor type. How can they discriminate colors in their environment or on the coats of their conspecifics? This problem has puzzled researchers for decades. Recently, Alexander and Christopher Stubbs (2016) proposed a model that could explain this phenomenon. According to the Stubbs, cephalopods have a mechanism for detecting different colors unlike any other animals. Their theory is based on the observation that the shape of the pupil of these animals maximizes chromatic blurring through their lens. Light of different colors has different wavelengths and will converge at a different point on the retina when going through a lens. Humans and many other animals do not suffer from this because the shape of our lens minimizes chromatic blurring onto our retina. You can see this clearly in the figure: while the mouse has the same small circular on-axis pupil that humans have, cephalopods have a wave-like off-axis pupil. This shape allows light to come in from many angles, increasing chromatic aberration. By bringing into focus through the lens light of different wavelengths one at a time, Alexander and Christopher predict the cephalopod is able to detect the color composition of the visual scene!
Lenses can be used for other interesting tricks, as was shown by Nagata and colleagues in 2012. This team studies the visual system of jumping spiders, a species known for its remarkable ability to execute precise targeted jumps during hunting. Equipped with four shiny beady eyes on the front of its head, Hasarius Adansoni seems to have the perfect tools for this task. The Japanese researchers were surprised to find that not only were the central two eyes sufficient for depth assessment, but also that the spiders seemed unchallenged after losing one of these principal eyes. Unlike animals that use binocularity to assess distance, H. Adansoni used a unique trick for this job. Once again, researchers found that convergence of light through the lens in the eye of the animal was being used in a new and creative way.
Nagata and colleagues found that the retina of jumping spiders was composed of four layers of photoreceptors and that two of these expressed the same green-light sensitive visual pigment. However, the lens only focuses light on the deeper layer. The amount of defocus on the other layer informs the spider of the distance of the object! The distance of an object to the lens is inversely proportional to the amount of defocus. The presence of green light is critical for this to work. The same amount of defocus is generated by a red-light source that is farther to the lens than a green-light source. When jumping spiders had to attempt prey capture in red-lit environment they consistently underestimated the distance.
As we can see from the jumping spider and cephalopods, there is a great deal to be learned from the variety of unique and creative strategies that different species use for common problems. Barbara Natterson-Horowitz and editor Kathryn Bowers celebrate the diversity of species in their 2013 book Zoobuiqity, in which they explore the amazing yet often overseen parallelisms between human and animal health. Veterinary and human medicine, they wrote, are usually practiced separately and the two fields rarely take advantage of what they could teach each other. Many diseases occur in other species just as they do in us, but in some cases we can find species that are more protected or susceptible; both these scenarios could be very instructive as to the etiology and mechanisms of disease. Their critique could be extended to both clinical and fundamental research. Here we study almost any biological process in just a handful of species. There are many very good reasons for this choice: highly advanced genetic access, short reproductive cycle, extended accumulated knowledge, and simple behaviors are just a few. Nevertheless, let’s take the unique visual tricks of cephalopods and jumping spiders as a reminder to study and learn from biological diversity.
- T Nagata, M Koyanagi, H Tsukamoto, S Saeki, K Isono, Y Shichida, F Tokunaga, M Kinoshita, K Arikawa, A Terakita (2012) Depth perception from image defocus in a jumping spider, Science, 335, p. 469-471.
- AL Stubbs and CW Stubbs (2016) Spectral discrimination in color blind animals via chromatic aberration and pupil shape, PNAS, 113(29), p. 8206-8211.