Learning From Visual Systems Across Species

Aiden Salk | March 28th, 2024

The last time you went swimming, you likely struggled to keep your eyes open while underwater. What differs between a human and a fish eye that allows fish to see underwater? What optic adaptations allow semi-aquatic animals such as crocodiles to expertly hunt both underwater and on land? Dr. Gianni Castiglione’s lab at Vanderbilt University explores these questions by studying a protein in the rods of the eye, rhodopsin.

How does vision work?

Phototransduction is the process by which a visual stimulus is turned into an electrical signal for reception by the brain. Vision begins with light–made of small particles called photons–hitting the retina in the back of the eye. The retina contains rods and cones, which are both specialized types of neurons (nerve cells) called photoreceptors. Cones respond to different wavelengths of light, allowing us to see in color. Rods are sensitive to dim light; this sensitivity allows us to make out objects even in the dark or at night. Combined, rods and cones convert a wide variety of light information into electrical signals that the brain can interpret.

What is the role of rhodopsin?

Rhodopsin is a protein forming part of each rod. When part of rhodopsin absorbs light at a specific wavelength, it undergoes a conformational change from 11-cis-retianl (11-CR) into all-trans-retinal (ATR). This change in shape causes the rhodopsin to let go of the ATR. ATR’s release activates other proteins, ultimately causing the neuron to send an electrical signal to the brain. When ATR is floating freely, it is also toxic. As a result, it cannot remain in the eye and must be sequestered. The rhodopsin with the 11-cis-retinal confirmation must be regenerated. 

What is unique about rhodopsin in icefish?

Antarctic icefish (Crynotothenioidea) live in extremely cold temperatures under ice. Castiglione analyzed the rhodopsins of icefish to determine if they had any unique adaptations. One difference he discovered was that rhodopsin in icefish absorbs light at a slightly longer wavelength than in most other animals. This makes icefish rhodopsin red-shifted. Castiglione suggests that this is an adaptation that allows icefish to see more clearly, even under ice caps that reflect much of the incoming light. Interestingly, water is known to blue-shift light, so there may be a tradeoff between the red-shifting of the icefish’s rhodopsin and the blue-shifting of light by the water that allows icefish to see optimally. Furthermore, Castiglione says that “although increasing rod: cone ratios and elongated rod outer segments likely confer increased photosensitivity in some deep-dwelling [icefish], the retina of other [icefish] species appear highly photosensitive, despite displaying no obvious anatomical adaptations that would indicate enhanced dim-light sensitivity.

What adaptations do other aquatic animals have for underwater vision? 

Crocodiles are semi-aquatic animals that can see both on land and underwater without their eyes becoming irritated. This ability is because crocodiles have a transparent, protective membrane over their eyes, almost like a second eyelid that comes over the eye selectively. A video by the Smithsonian Channel Aviation Nation refers to the protective membrane of crocodiles as “a pair of built-in goggles.”

Fish have their own analogous adaptations that allow them to see underwater with high resolution. First, the eyes of many species of fish “are protected by a thin mucous layer, known as the conjunctiva, which helps to protect their eyes from debris and bacteria in the water.” Unlike crocodiles, this is not a second eyelid, rather an extra protective layer on their eyes. In fact, most fish do not have eyelids and sleep with their eyes fully open. 

Another visual problem humans experience while underwater is light refracting as it hits the water. This optic interaction makes objects appear to be in a different position than they actually are because observed light lands further behind the retina of a human eye than it otherwise would. A similar mechanism occurs in far-sightedness.  Fish have evolved a spherical lens that optimally focuses refracted light on the retina, allowing them to see clearly up to a meter away. When it is necessary to see across further distances, fish can automatically retract their lens. Fish can also see in ultraviolet light, helping them identify prey and predators better, and giving them access to violet-like colors beyond human perception

How can humans improve their vision?

Each time we switch from bright light to darkness, 11-CR is transformed into the toxic ATR. The build-up of ATR throughout one’s life may be related to age-related blindness, called age-related macular degeneration (AMD). Researchers believe that rhodopsin plays a part in age-related macular degeneration because humans are constantly in bright conditions even during the night. Though artificial, this extended period of being in bright light causes more ATR to be produced than usual.

According to the Centers for Disease Control and Prevention (CDC) and the Vision Health Initiative (VHI), some ways we can prevent AMD is by quitting smoking and having a healthy lifestyle that lowers cholesterol. Additionally, beta-carotene found in carrots is used to synthesize vitamin A and then 11-cis-retinal. Thus, eating carrots can help restore rhodopsin to a complex with 11-cis-retinal, minimizing levels of toxic ATR in the eye.

Going Forward

The evolutionary adaptations that allow some organisms to see underwater with high resolution are fascinating, with many still not fully understood. For example, Dr. Castiglione is currently studying the genetic differences in the rhodopsin of animals with diurnal activity or in animals that surface from the deep sea, such as whales. Future research will elucidate a better understanding of this critical photoreceptor and help us understand how to better prevent loss of vision.

Resources

1.  Castiglione, Gianni M, et al. “Adaptation of antarctic icefish vision to extreme environments.” Molecular Biology and Evolution, vol. 40, no. 4, 10 Feb. 2023, https://doi.org/10.1093/molbev/msad030. 
2. Castiglione, Gianni M., Yan L.I. Chiu, et al. “Convergent evolution of dim light vision in owls and deep-diving whales.” Current Biology, vol. 33, no. 21, Nov. 2023, https://doi.org/10.1016/j.cub.2023.09.015. 
3. Demetre, D.C. “Do Fish Blink? Fisheye Facts.” Natureweb, 3 July 2023, natureweb.co/do-fish-blink/#:~:text=Fish%20eye%20protection%20and%20moisture,that%20may%20affect%20their%20vision. 
4. Vision Health Initiative. “Learn about Age-Related Macular Degeneration.” Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, 23 Nov. 2020, www.cdc.gov/visionhealth/resources/features/macular-degeneration.html#:~:text=Quitting%20smoking%2C%20or%20never%20starting,can%20cause%20permanent%20vision%20loss. 
5. “Rods & Cones.” Rochester Institute of Technology, www.cis.rit.edu/people/faculty/montag/vandplite/pages/chap_9/ch9p1.html#:~:text=Rods%20are%20responsible%20for%20vision,is%20populated%20exclusively%20by%20cones. Accessed 23 Feb. 2024. 
6. “Astounding Facts about Crocodile Eyes.” Facebook, Smithsonian Channel Aviation Nation, 24 Feb. 2020, www.facebook.com/watch/?v=1058992727767151. 
7. “Rhodopsin.” Wikipedia, Wikimedia Foundation, 1 Jan. 2024, en.wikipedia.org/wiki/Rhodopsin. 
8. Castiglione Lab @ Vanderbilt, Vanderbilt University, castiglionelab.com/. Accessed 23 Feb. 2024.