Fish perceive the world very differently than humans do. Their underwater habitat results in major differences in vision compared to land animals. Fish have unique adaptations that allow them to see clearly underwater. These include:
Fish Have Spherical Lenses
The lenses in fish eyes are spherical, unlike the flattened lenses in human eyes. Spherical lenses are optimal for focusing light underwater. Light refracts when it moves from water to air. The curved lens of a fish eye corrects for this refraction. This allows fish to see clearly in both mediums.
Fish lenses are also closer to spherical than those of land animals. Spherical lenses produce little or no optical distortion. This grants fish excellent dynamic vision essential for life in the aquatic environment.
More Rods Than Cones
Human eyes contain cones and rods that detect light and color. Cones provide high visual acuity and color vision. Rods operate in low light. Most fish have many more rods than cones in their retinas.
Extra rods help fish see in the dimly lit undersea world. They provide excellent night vision critical for nocturnal feeders. More rods also boost motion detection. This allows fish to spot prey and avoid predators in murky waters.
Fish that live deeper down with no sunlight depend entirely on rods. Others like shallow water reef fish have more cones. But even reef dwellers have way more rods than people. The tradeoff is lower color vision for increased dim light and motion detection.
Fish focus on objects at different distances using a process called accommodation. Fish change the shape of their eye lenses to focus light from near or far much like humans. But unlike humans, most fish adjust their focus by moving their lenses rather than reshaping them.
Fish lenses are spherical and permanently fixed in shape. Moving the rigid lens back and forth allows fish to rapidly change focus. This is called aphakic accommodation. It provides sharp vision at all depths and distances in the underwater scene.
Most bony fish have tubular shaped eyes that bulge out from the sides of their heads. This layout increases their field of view compared to human eyes. Tubular eyes allow fish to see nearly 360 degrees in every direction except right behind them.
Full peripheral vision gives fish the ability to spot predators and prey from all angles. Some fish even have eyes on each side of their head that can move independently. This grants simultaneous panoramic views above and below.
Many fish possess a structure called the tapetum lucidum behind their retinas. This reflective layer bounces light back through the retina giving photoreceptor cells a second chance to be stimulated.
The tapetum lucidum significantly improves vision in low light. It allows fish to see up to 10 times better than humans can under the same dim conditions. This grants critical extra light sensitivity for life underwater at night.
Some fish can see ultraviolet (UV) light thanks to special visual pigments in their eyes. UV waves are invisible to human eyes but penetrate deep into water. UV vision gives certain fish a secret communication channel.
Fish use UV signals for important functions like mating displays, territorial markings and recognizing prey. Being able to see UV provides a major advantage in the aquatic environment full of UV cues.
Polarized Light Detection
Light scatters and polarizes as it filters down through water. Many fish have polarized retinas able to detect the direction light vibrates in. This grants them a form of “stereo” vision without overlapping fields of view.
Polarization vision improves fish’s ability to perceive contrast, depth and distance. It helps them detect transparent prey and safely navigate through reflections and sun flicker.
Differences Between Saltwater and Freshwater Fish Vision
Fish living in oceans and lakes face different visual challenges. Their eyes adapt to their specific light environments.
Saltwater fish need to see longer distances in open water. Their eyes tend to be larger, more tubular and located high on their heads. This gives an unobstructed horizontal view ideal for open ocean life.
In contrast, freshwater fish inhabit smaller ponds and rivers with more plants and debris. They need to look upward to find gaps in vegetation and avoid obstacles. Their eyes are generally smaller and positioned more laterally to maximize overhead viewing.
Fish eyes possess special adaptations that allow them to see clearly underwater. Spherical lenses, extra rods, aphakic accommodation and tubular shape grant fish superior vision compared to land animals. UV sensitivity, polarization detection and translucent debris-filtering lenses provide additional advantages for life in the aquatic world.
Evolution shaped fish eyes into the perfect optical systems for their watery habitat. Their unique vision gives fish the visual edge they need to thrive in the marine environment.
Here are some references used as sources for this article:
- Collin, S. P., & Hart, N. S. (2015). The retinal basis of vision in fishes. In Sensory Biology of Aquatic Animals (pp. 35-77). Springer, New York, NY.
- Douglas, R. H., & Hawryshyn, C. W. (1990). Behavioural studies of fish vision: an analysis of visual capabilities. In The Visual System of Fish (pp. 373-418). Springer, New York, NY.
- Fritsches, K. A., & Marshall, N. J. (2002). Independent and conjugate eye movements during optokinesis in teleost fish. Journal of Experimental Biology, 205(8), 1241-1252.
- Jerlov, N. G. (1976). Marine optics. Elsevier.
- Lythgoe, J. N. (1979). The ecology of vision. Oxford: Clarendon Press.