How fish see their underwater world has fascinated humans for a long time. As air-breathing terrestrial creatures, the way fish perceive their environment seems alien to us. Yet fish occupy the same shared Earth as us. Understanding how fish see can help bridge the gap between our very different worlds.
Do fish see like us? The short answer is no. Fish eyes work very differently from human eyes. Fish inhabit a vastly different visible environment than humans. And their brains process visual information unlike humans. But fish eyes and vision are remarkably adapted to see clearly underwater. Fish visual abilities can even exceed humans in some ways. Exploring fish vision provides an intriguing window into how evolution shapes perception of the natural world based on an animal’s niche.
Fish Eyes vs Human Eyes
Human and fish eyes come in very different shapes, sizes, and structures. The most obvious difference is that fish eyes are much more spherical than human eyes.
| Eye Feature | Fish Eyes | Human Eyes |
|---|---|---|
| Shape | Spherical lens resulting in a spherical retina | Ellipsoid lens resulting in a spheroidal retina |
| Accommodation | Focus using lens movement | Focus using lens thickening and thinning |
| Aqueous humor | Less viscous | More viscous |
| Fovea | Absent | Present |
| Cones | Single cone type | Multiple cone types |
The spherical shape of fish eyes allows light to focus on a spherical retina. This enables fish to have a nearly 360-degree field of view. Human eyes have a smaller field of view of around 180 degrees.
Fish change focus by moving their spherical lens forwards and backwards. Humans change focus by altering the thickness of their ellipsoid lens.
The aqueous humor that fills fish eyes has less viscosity than in humans. This assists with visual clarity underwater and rapid focus changes.
Many fish lack a fovea, the central high acuity region of the human retina. Instead, fish visual resolution is usually more uniform across the retina.
Fish normally have a single type of cone photoreceptor, compared to humans’ three cone types. As a result, many fish are thought to have color vision limited to discriminating colors from shades of grey.
Seeing Underwater
Water presents a vastly different visual environment than air. When light enters water, its speed changes, it gets absorbed and scattered. These effects influence what fish can see underwater.
Light travels through air extremely quickly at around 299,792,458 meters per second. In water, light only travels at around 225,000,000 meters per second. This means light’s speed reduces by around 25% entering water.
Absorption selectively removes colors from white light as it passes through water. The longest wavelengths like red and infrared get absorbed first at around 3 meters deep. Orange and yellow vanish by 10 meters. Green, blue and violet penetrate deepest to around 100 meters. Water color itself can further absorb and scatter light. As a result, fish color vision adapts to the residual colors available.
Scattering diffuses and reflects light underwater. This causes glare and blurring effects that fish eyes and brains compensate for. Scattering essentially sets the maximum visual range underwater far shorter than in air. The most clear tropical waters may allow up to 200 meter visibility. But in many waters, visibility quickly decreases to just tens of meters or less.
To cope with these effects, fish eyes possess several adaptations. Many fish have ultra-spherical corneas and lenses to focus scattered light. Their pupils dilate widely to capture more photons in dim waters. Shorter wavelength sensitive retinal photopigments matched to blue/green light dominate. Multi-layered retinas and 5+ retinal photopigment macula filters outlets to counter light and color distortion. Contrast detection allows resolution of blurred images.
Fish Vision Variations
Fish inhabit nearly every possible underwater habitat, from ultrabright coral reefs to the blackness of the deep sea. Over 33,000 species of fish exist with a staggering diversity of body forms, behaviors and lifestyles. Unsurprisingly, fish eyes and vision also vary enormously based on habitat and ecology.
Fish can be roughly grouped by visual ability. One simple classification divides fish into diurnal (daytime) and nocturnal (nighttime) vision specialists. Diurnal fish typically have high resolution colorful vision adapted to well-lit shallow waters. Many popular aquarium fish like cichlids, tetra and guppies fall into this group. Nocturnal fish often show eye enlargement, retinal sensitivity and light gathering adaptations for maximizing vision in dim waters. Deep sea fish and catfish demonstrate excellent night vision.
Other visual groups include fish with:
– Ultraviolet Vision – Seen in fish like rainbow trout and goldfish that detect UV wavelengths invisible to humans. Useful for zooplankton detection.
– Polarized Light Detection – Found in rainbow trout and herring allowing them to see polarization effects humans miss. Enhances contrast and prey detection through scattering.
– Tetrachromacy – Possessed by species like goldfish and guppies giving them 4+ cone types for expanded color vision. Allows color discriminations invisible to human eyes.
– Doubled-Up Retinas – Some diurnal fish have two retina layers, one tuned to day and one to night vision. Allows optimizing vision under changing light levels.
– Tubular Eyes – Deep sea dragonfish have eyes shaped like long tubes to maximize light capture at 500-1000 meter depths in permanent blackness.
Fish eyes can also move independently, swivel forward like binoculars and even split light to different retinal regions. Truly fish vision capacities are far wider than typically appreciated.
Fish Brain Visual Processing
Seeing goes far beyond simply having functioning eyes. Complex visual processing occurs in the fish brain that allows interpreting the visual scenes focused by their eyes.
Fish brains contain tectal and cortical regions dedicated to analyzing visual information much like the mammalian visual cortex. Signals from retinal photoreceptors get preprocessed by interneurons into edge detections, movement detectors and looming object detectors. This allows fish to rapidly identify objects, motion and threats.
Some fish have demonstrated impressive visual capabilities through learning. Archerfish can judge distance and perspective to hit aerial prey with water. Manta rays and groupers can recognize and remember human faces. African cichlids distinguish individual facial features of other fish. Even goldfish have shown they can recognize simple 2D shapes and patterns.
But fish brains differ from mammals in how they process vision. Fish brains lack the massive cerebral hemispheres of the mammalian cortex. Visual information appears processed in a more decentralized, modular manner. Split-brain studies also show limited interhemispheric integration in fish. As a result, fish vision seems more instinctual and less analytical than primate vision.
Conclusion
Exploring how fish see their liquid world reveals many parallels, but also differences, with human vision. While fish eyes work very differently from our own, they are supremely adapted to seeing through water, not air. Variations in fish eyes, vision and brains across species and habitats also showcase evolution in action. But fundamentally fish see the same shared world as we do, just from their own unique visual perspective. Understanding the viewpoint of fish can allow us to better connect with these fellow Earth inhabitants.
