Can Fish See?

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russ:
Yes, they can see.

Not withstanding other sensory mechanisms, all fish do not see things the same. Species from turbid waters, clearer waters, and darker(tannin stained) waters have to make refractive adjustments to maintain clear sight. Depending on the species, mechanisms for formulating colors are also different.

Ever wonder why a fish eye lens is called a fish eye lens? The eye structure of a fish is basically the same as land critters in the sense they both have a cornea, and iris, a lens, and a retina that contains rods or rods and cones. Due to the differences in density between air and water, features of the fish?s eye have been modified to function at a high efficiency under water. But, the main differences between terrestrial vertebrates and majority of fishes is the shape of the lens.

Because of the difference between the densities of air and water, the angle of refraction will be greater as the light passes from one to the other. When our eyes are exposed to light in air, the majority of refraction occurs at the air-cornea interface. Terrestrial vertebrates' lenses fine tune this refraction and focus the light on the retina. Under water, the density of water and our eye is about the same. Because our eye lenses are more flattened, and little refraction occurs as light passes, the margin for a clear focus is greatly reduced.

To compensate for the lack of refraction, most fish eyes is spherical. This shape, and the high density of the fish?s lens, results in fishes having the highest refractive index of any vertebrate. (I like to interject at this point that a shark?s eye is a little different from other fishes, and for the sake of space and reading and writing time, I won?t get into this now).

Fishes also have a different method of focusing. We focus light on the retina by changing the shape of the lens. Most fishes cannot do this and maintain the optimal refractive index. So, they accomplish this much the same way as a camera lens, by changing the position of the lens. In the forward or relaxed position, near objects are in focus to the fish. Focusing on distant objects is accomplished by moving the lens back toward the retina.

There is another difference between most fish and terrestrial eyes. Majority of fish eye lenses protrude through the pupil. Along with the shape of the retina, this allows for an extremely wide angle of vision. When fishes swim with a slight side-to-side motion, it allows them to view objects all around them and eliminate blind spots.

The organization of the retina is similar in fishes and other vertebrates. Nerve cells and fibers, and photosensitive rods and cones. The outer most area is pigmented. There is another area of most fishes that contain a mirror like substance. This increases the sensitivity of the eye in low light by reflecting light back over the rods. There is more of this substance in some fishes than others.

In a nut shell, rods handle vision in low/dim light situations and cones primarily for color vision in clearer or brighter lit situations.

The number of rods and cones, along with their distribution in the retina, will vary with different species of fish. Fishes that are active mostly at night or in dim light environments have a higher ratio of rods to cones than those fishes that live in more brightly lit environments.

Our eyes control the amount of light striking the rods and cones by dilating the pupil. Because the fish?s eye extends through the pupil, synchronized movements of the rods, cones, and pigment granules is employed.

In bright light, the pigment granules move toward the outer ends of the rods and cones. At the same time, the rods move back into the pigment and the cones move toward the light. This action also protects the rods from harmful effects of bright light by enclosing them in the pigment while allowing the cones exposure to the light. In low/dim light situations this action is basically reversed.

Here is a very important point to remember when turning your aquarium lights on in the morning or evening. The time required for this light/dark adaptation of a fish is considerably longer than the dilation of ours and other vertebrates' pupils. Actually, it can take anywhere from several minutes to an hour! So care should always be practiced in acclimating your fishes in light/dark situations.

Because of the light scattering and absorption properties of water, the light environment of fish is more varied than most land critters. Because of the large differences in quantity and quality of light in water, fishes have evolved the widest range of visual pigments than any other vertebrates. I won?t get into detail about the two basic groups of pigments other than to say that one group is primarily employed by marine fishes, and the other by freshwater fishes.

All of the visual pigments studied in fishes have two major components (opsin and chromophore). Within these groups, differences in opsins determine which wavelength of light are absorbed by the pigment. The pigment undergoes changes that result in the generation of an electrical impulse. This impulse is transmitted to the brain and the sensation of sight occurs.

Fishes living in deeper water (example, ocean) where almost all light except blue has been filtered out, have pigments that absorb these shorter wavelengths. Fishes living in more shallow waters or more turbid waters, have pigments shifted to the longer wavelengths in the yellow-green. Still, other fishes have pigments that allow them to maximize the contrast between prey and background light.

Color receptors in higher vertebrates are represented by cones. Because fish eyes also contain cones, it can, and has been concluded that fish do have color vision. In order to distinguish differences in color (wave lengths) from differences in brightness, an organism must have some type of receptors that will respond to different wavelengths.

There are three basic types of color receptors that account for color vision. This was correctly concluded in 1801 by a scientist (or doctor, I forget) Thomas Young. Humans have three basic cones with pigment sensitivity in the blue, green, and yellow. Fishes also have this to various degrees in addition to some variances into more of the red.

Cones sensitive to those colors (wavelengths) respond with the resultant sensation to that color (the whole impulse to the brain thing). Colors that happen to be combinations of these will cause the cones to register more or less in proportion to those primary receptor colors.

I realize that there is more to whether your fish recognizes you when you walk into a room than sight. But, since that was the original question, I decided to dwell on that part without getting into hearing and equilibrium, the fishes lateral line system, or imprinting factors. Hope I didn't dwell too long...lol

Personally, I think a fish or group of fishes responding in-kind can recognize individuals. Not in all instances though. There would need to be a combination of appropriate lighting and brightness. Also, this would depend on the species. In ideal conditions, I believe a great many fish can see us at the same distance across the room as we can see them. 

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