Can Fish See Water? Fish Vision and Other Curiosities

Fish eyes sport a remarkably similar structure to ours. The core elements are the same (sans eyelids), yet we struggle to see clearly underwater. Transparent as it may seem from the outside, once we submerge it presses our eyeballs, making our entire vision smudgy and somewhat unpleasant.

Is it the same for them? Can fish see water and does it blind them the way it blinds us?

Fish don’t see water and it doesn’t affect their vision negatively. Fish have evolved in it and their eyesight is perfectly well adapted to their natural habitat. So much so that some species can see through murky waters, and others can detect objects at incredible depths where light is scarce. 

There are species that are blind, even.

But those that see have the following general characteristics:

• Water is transparent and has a lower refractive index than the fish’s eyes, so they don’t see it. Air has even lower refraction which is why no living species see it, either.
• Fish eyes have a cornea, iris, and pupil, just like ours, but their eyes are more spherical.
• The spherical form of fish’s eyes refracts light to the retina differently from ours. It is perfectly adapted to the surrounding water.
• Fish can see colors. In fact, their color perception is greater than ours, as they see UV light on top of the basic red, green, and blue colors.
• Fish have almost complete 360° view of their surroundings, thanks to the spherical eyes.
• Most likely, fish don’t see as well as we straight ahead as their eyes are located on their sides, for most species. What’s more, fish cannot focus their eyes the same way we do. Their focus is very narrow, right at the center of their eyes. It works similarly to a photo camera lens.
• Regardless of these peculiarities, fish eyesight is among the best in the animal kingdom and they can see remarkably well underwater. Certainly much better than humans, even if the human eye is equipped with goggles.

Again, these are general characteristics. Fish species are incredibly diverse and all have evolved adaptations specific to their natural habitats. But what do they actually see?

How Do Fish See Their Surroundings?

Fish eyes have adapted to scan and constantly monitor their surrounding for food or predators. And since fish have no eyelids and sleep with their eyes open, many species are semi-aware of the waters around them even when actively resting.

The spherical lens of the fish eyes is excellent at absorbing visual input from all around the animal. Most species have practically 360° view at all times, making them very aware of their surroundings.

They can’t focus well on the entire visual field, as their focus is narrow and straight. Technically, this makes them shortsighted, but that’s alright as fish eyes don’t need to focus on distant details like the number of an incoming bus or the symbols on a road sign.

What Do Fish See?

Instead, their impressive field of vision is great at detecting motion and contrast. Both are important for orientation and for hunting and avoiding being hunted. This wide visual angle comes in handy when your environment is three-dimensional and danger can come from all sides: up, down, back, and front. The lateral line helps in dealing with threats or detecting prey big time, too.

Now, it is likely that some fish have small blind spots right in front of their noses and behind their tales. The sideway position of their eyes almost certainly doesn’t cover the entire viewing spectrum, but it is close enough that it doesn’t matter in practical terms. For the most part, fish are always on the move, constantly changing viewing angles.

Overall, fish eyes are considerably better than those of terrestrial animals. They might lack strong focus, but the amount of information they absorb at any given time is unsurmountable.

Take a picture with a wide-angle photo camera using a fish-eye lens. (The name is aptly chosen, as the shape of such lenses is modeled by the eyes of our aquatic pets.) Try to absorb all the visual information at once to get some remote idea of how much data a fish eye actually perceives.

And fish have two of those.

What Colors Can a Fish See?

Fish’s eyesight beats human’s not only in sheer data intake but also in color perception. On top of the red, green, and blue base colors, fish can see ultraviolet light. That trait is very useful in greater depths because water strips light of color quickly.

However, not all species are so gifted. Many have only two color cones, while others sport the four cones required to see UV light and the three base colors. Fish that live close to the surface and in tropical waters typically see more colors.

Deep-water and nocturnal fish, on the other hand, can see very well in low-light conditions, but they have practically no need to see colors, as their natural habitats are largely color-free.

Overall, very few species have been tested specifically, so take the above statements with a grain of salt. We might be in for new discoveries that change our ideas about what colors fish can see.

How Far Can Fish See?

In clear water, most fish can see reasonably far. They might be unable to focus very well on distant objects, but their blurred perception still can alert them of sudden movements or changes in brightness.

But this is in absolute terms. In reality, many fish, especially freshwater species, live in murky waters.

Can Fish See in Dirty Water?

The ability to see UV light comes in rather handy when light is low, be it because of depth or pollution of the water. Light cannot penetrate muddy waters much. As depth increases, red light is the first to disappear in clear water, but murky waters can be shallow and full of particles.

What happens then?

Then green and blue light, which are dominant in oceans and seas, as our eyes can testify, often are filtered out. Fish have adapted to such conditions through the enhanced production of the enzyme called Cyp27c1, which helps with red and ultraviolet light perception.

Do you know what’s crazy about it? Or, let’s be frank, crazier?

Fish like salmon, which live most of their life in the ocean, starts producing this enzyme when traveling upstream in rivers and streams. The latter, typically, are considerably murkier than the ocean.

And that’s not all. Elephantnose fish is among the species that coat their eyes in crystals to see through muddy waters. Their eyes are coated in cup-shaped photonic crystals! The underlying structure of their eyes allows them to see in extremely low-light conditions, detecting large objects. That’s a helpful feature when avoiding predators.

Fish eyes are insane!

They may not have eyelids, but they have some of the most fascinating night vision mechanisms in the animal kingdom.

Do Fish See Outside of Water?

Aquarium fish can see through the glass when they are close enough to it. Assuming the glass and the water column are clean, fish can see humans and even recognize faces. Heck, they can watch TV even! Fish sight beyond the aquarium walls follows similar principles to those outlined below.

In the wild, fish can see outside the water under certain conditions. These conditions have to do with physics and how the water’s surface refracts light, rather than the fish’s eyesight, per se. Something called Snell’s Window dictates the size and angle of the field of vision through the surface.

It’s a pretty interesting phenomenon and affects underwater vision as a whole, not only fish’s eyes. Snell’s Law describes in detail how water refracts light and creates this window, and if you want practical examples and explanations, this excellent article is the way to go.

Basically, Snell’s Window is a 96° “patch” through which light passes through the surface and reaches the eye. However, if you are deeper than a few feet or meters, the surface will reflect all the light, rendering the outside world invisible. Conversely, at a certain depth, the light is refracted in a stunning fashion — the entire 180° horizon is condensed into these 96° visible underwater.

It doesn’t matter if the eye in question is a spherical fish eye mounted on the side of the head or a human eye with goggles.

If you can’t be bothered with the theoretical details, take a look toward the sky next time you dive into the sea.

So, yes, fish can see through the surface. How much they can see on a given day depends on their depth, light intensity, air clarity, water clarity, waves, etc.

Can Fish See Air Bubbles?

Now, that’s an interesting question. Most fish can see air bubbles, just like we can. Air bubbles are tiny capsules of air pushing against the surrounding water. They create a distinct visual separation that is relatively easy to spot.

Do Fish Have Visual Memories?

Fish use their eyes to orient themselves. In most cases, that’s not their primary system for navigation and spatial conquest, but studies show that vision plays a part. Strong evidence suggests that fish form visual memories that help them avoid danger and find hiding places.

In aquariums, it is quite obvious that fish respond to visual cues with predictable behavior. Just get close to their feeding spot around feeding time. Chances are, most fish in the tank will cluster close to you, begging for food. Certain species can even recognize faces or, at least, the silhouette of their primary feeder.

Can Fish See While Sleeping?

Since fish have no eyelids, they can’t close their eyes. As a result, while sleeping, they can’t experience rapid-eye-movement sleep (the deepest level of sleep for mammals) and, certain species at least, are semi-aware of their surroundings.

They actively rest, their bodily functions slow down, and parts of their brains switch off to recuperate, but their awareness is never fully gone. Fish use their lateral lines — basically, their entire bodies — to sense motion and changes in their surroundings. Coupled with their ever-opened eyes, fish are quick to react to any danger.

Can Fish Live Without Eyes?

Species that have eyes would live much longer if their eyesight is intact. In the wild, a blinded fish will die quickly. In a fish tank, its likelihood of survival improves manifold.

However, there are fish species that are blind. And I am not talking about deep-water fish where light doesn’t penetrate. They actually have huge eyes that are extremely sensitive to the tiniest change in light (or level of darkness?).

Much closer to the surface and the aquarium hobby is the Mexican tetra or blind cave fish (Astyanax mexicanus). These little weirdos have scales where their eyes are supposed to be. Since their natural habitat is totally dark, they’ve foregone the useless organs and have boosted the perceptive power of their lateral lines.

Which Underwater Creatures Have the Best Vision

Fish certainly have amazing eyesight, but determining which underwater species have the best eyes is a tall order. What do you compare? How far they see or how much visual information they absorb at any given moment? Maybe how many colors?

Sharks certainly boast some impressive lenses that see very far, but what about the mantis shrimp? Its eyes are super complex and have 16 (!) eye rods. Yes, these are the rods that determine how many colors you can perceive. We have three, some fish have four.

And that’s not all. Each mantis shrimp has three pupils. Fish and mammals, humans included, have one. That provides the shrimp depth perception on each eye and extremely accurate triangulation. Comes in handy when you go out hunting.

Mantis shrimp are crazy!

But then again, each eye of a grown colossal squid can be up to 27 centimeters in diameter! That’s the size of an average frying pan.

As far as fish are concerned, archer fish has some amazing eyesight that allows them to detect prey above water and calculate its exact location. If you recall what we discussed about Snell’s Window, that’s not straightforward at all. Still, the archers can knock bugs off their perch by shooting powerful sprays of water. What’s more, the fish calculate where the hit bug will fall, so that it can eat it before someone else snatches it.

And what about the four-eyed fish? At first glance, it has only two eyes, just like anyone else. But its eyes are split — each has two pupils and two corneas. The bottom parts of their eyes have different photoreceptors from the top, adjusted to see underwater. Respectively, the top parts scan above water for potential prey.

Wicked!

How Do Fish See?

In conclusion, it is impossible to answer precisely how fish see without becoming an encyclopedia on fish vision. The variety among the species is far too great.

What’s clear is that:

• Fish don’t see water
• Fish don’t see air (but can see air bubbles)
• Some see colors
• Some see UV light
• Most are shortsighted
• Virtually all species have large field of vision
• Most fish absorb visual information even while sleeping
• Fish living in murky waters have adapted to see well in their habitat
• Fish living in low light have evolved to perceive even the slightest change in light intensity
• Mantis shrimp are insane

These are general characteristics, indeed. When you drill down to individual species, things become truly interesting!