Do fish blink their eyes? Many of us have wondered this, especially those of us who have spent time observing our underwater friends. For most fish, the answer is no.
Fish have unique eye structures that help them see clearly and effectively in their aquatic habitats. While humans and other land animals blink to keep the surface of our eyes moist and free of debris, fish are immersed in water, which necessitates a different type of blinking.
Fish eye protection and moisture come primarily from their surrounding water, which keeps their eyes constantly wet. This eliminates their need for the same blinking mechanisms found in terrestrial animals. However, fish do have other methods of dealing with irritants and debris that may affect their vision.
Various adaptations and behaviours in fish help keep their eyes clean and functioning well, such as specialized eye movements or secretions. The wide array of eye characteristics among fish species can offer insights into their habitats and how they have adapted to maintain their vision and eye health.
For example, some deep-sea fish, like bioluminescent flashlight fishes, possess specialized light-producing organs beneath their eyes to illuminate their prey in the ocean’s dark depths. This adaptation allows these predators to enhance their vision in low-light conditions and successfully locate and capture prey.
Fish Eye Structure
The cornea is an essential part of fish eyes, as it serves as the outer protective layer that allows light to enter the eye. Fish corneas are usually thicker than those of other animals, as they need to withstand underwater pressure and protect the sensitive tissues within the eye.
The fish cornea is a transparent layer, enabling the light to pass through and reach the other components of the eye, such as the lens and retina. The refractive power of the corneal surface is significantly reduced in freshwater due to the small difference between the refractive indices of the corneal surface and the water. Because seawater has a refractive index similar to that of the cornea, the cornea contributes even less to the total refractive power of the eye in marine environments.
Eyeball Composition
The composition of the fish eyeball includes the lens, retina, and various other components that assist in processing visual information.
Unlike many terrestrial animals, fish do not have eyelids. Their eyes are protected by a thin mucous layer, known as the conjunctiva, which helps to protect their eyes from debris and bacteria in the water.
Fish lenses are generally larger and more rounded compared to those of land-dwelling animals, helping them to focus light more effectively underwater. These lenses are also quite flexible, and fish can adjust the shape of their lenses by contracting or relaxing the nearby muscles, allowing them to easily focus on various distances.
The retina, the light-sensitive tissue lining the back of the eyeball, is crucial for processing visual information in fish eyes. Like other vertebrates, fish retinas have both rod and cone cells that detect light and colour, respectively.
Some fish have a retroreflector behind the retina, which enhances vision in low light. Many fish species adapt to their specific environment, with some possessing more rod cells for better low-light vision or more cone cells for distinguishing fine details and colours.
Blinking Mechanisms in Fish
Certain fish – particularly sharks of the Carcharhiniformes order – have a nictitating membrane. This transparent eyelid slides across the eye surface, allowing these fish to protect and moisten their eyes without obstructing their vision.
The thin, opaque nature of the nictitating membrane enables sharks to have an unhindered view of their surroundings even while blinking. This is particularly important for aquatic creatures as it allows them to effectively navigate their underwater environment and swiftly detect possible predators or prey.
Other sharks, such as the Great White Shark, have no eyelids and instead rotate the eyeball backward when attacking active prey. This keeps the more sensitive parts of the eye safe.
Some species, like mudskippers, exhibit a blinking motion when they are exposed to air. This blinking action helps keep their eyes moist, which is essential for maintaining their vision when they are out of water.
Mudskippers blink by sucking down their eye into their eye socket, where they are covered by a stretchy membrane called a dermal cup. The evolution of this behaviour did not necessitate the evolution of many new parts, such as new muscles or special glands.
Instead, mudskippers used their existing set of eye muscles in a novel way. The mudskipper’s blink lasts about the same amount of time as a human blink.
By comparing the anatomy and behaviour of mudskippers to the fossil record of early tetrapods, researchers from Georgia Institute of Technology, Seton Hill University, and Pennsylvania State University concluded in a 2023 paper that blinking emerged in both groups as an adaptation to transition life on land. These findings contribute to the understanding of our own biology and raise a host of new questions about the variety of blinking behaviours observed in living species.
Do Fish Sleep?
Fish, like many other living organisms, require periods of rest. However, their sleep patterns and behaviours can be quite different from those of mammals or birds. Even when they are asleep, fish continue to swim in order for their gills to absorb oxygen for the body continuously. Fish sleep can be classified into a few distinct types: suspended animation, reduced activity, and rapid eye movement (REM) sleep.
Suspended animation is a state of rest in which a fish’s body functions slow down significantly, allowing it to conserve energy. This can be observed in some species that burrow into the mud beneath dried stream beds, such as the African lungfish.
Protopterus annectens, an African lungfish, can survive in suspended animation (aestivation) for three to five years without food or water. When water becomes available, they awaken. Animals in suspended animation are inactive for an extended period of time, consuming no food or water and producing no urine or waste.
Reduced activity sleep, on the other hand, is more common among fish. During this phase, fish experience a noticeable decrease in their movement and responsiveness to external stimuli. They may seek out shelter or hover in one place while still maintaining a level of alertness to potential threats or opportunities.
An example of this can be seen in zebrafish, which have been studied for their sleep regulation. Zebrafish have also been shown to experience REM sleep, which is characterized by rapid eye movement and resembles mammalian sleep in certain aspects.
Though the fish’s eyes do not move during REM sleep, their brain and muscle signatures are similar. Fish, of course, do not close their eyes when they sleep because they lack eyelids.
Do marine mammals like dolphins and whales blink?
Unlike fish, marine mammals such as dolphins and whales do have eyelids, as they are warm-blooded mammals. However, they do not blink as frequently as humans and other terrestrial mammals.
Instead, these marine mammals have well-adapted eyes to cope with the underwater environment, and they can produce a thick mucus to protect their eyes from the saltwater without needing to blink as often.
References:
- Aiello BR, Bhamla MS, Gau J, et al. The origin of blinking in both mudskippers and tetrapods is linked to life on land. Proc Natl Acad Sci U S A. 2023;120(18):e2220404120. doi:10.1073/pnas.2220404120
- Collin, S.P. and Collin, H.B. (2006), The corneal epithelial surface in the eyes of vertebrates: Environmental and evolutionary influences on structure and function. J. Morphol., 267: 273-291
- Helfman, Gene; Collette, Bruce B.; Facey, Douglas E.; Bowen, Brian W. (2009). The Diversity of Fishes: Biology, Evolution, and Ecology. John Wiley & Sons. ISBN 978-1-4443-1190-7
- Hiong KC, Ip YK, Wong WP, Chew SF (2015) Differential Gene Expression in the Liver of the African Lungfish, Protopterus annectens, after 6 Months of Aestivation in Air or 1 Day of Arousal from 6 Months of Aestivation. PLoS ONE 10(3): e0121224
- Land, M. F.; Nilsson, D. (2012). Animal Eyes. Oxford University Press. ISBN 9780199581146
- McCosker, J. E. (1977). Flashlight Fishes. Scientific American, 236(3), 106–115.
- Poscai, Aline & Rangel, Bianca & Casas, André & Wosnick, Natascha & Rodrigues, Alexandre & Rici, Rose & Kfoury, José. (2017). Microscopic aspects of the nictitating membrane in Carcharhinidae and Sphyrnidae sharks: a preliminary study. Zoomorphology. 136. 1-6. 10.1007/s00435-017-0351-1
- Zhdanova, I. (2011). Sleep and its regulation in zebrafish. , 22(1), 27-36.