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How do Bony Fish Eyes Work?
Ardan Huck July 6, 2002

Fish are near-sighted, can see colors, can adjust to the refraction of light (bending of light) in the water, and have a field of vision of 83 degrees to 97 degrees in each eye. They can see almost all the way around them because their eyes are on the sides of their bodies. Fish can see very well if an object is close. They can see to both sides at the same time.

In the back of the fish's eye where the image focuses, it is called the retina (just like our eyes). In the retina, two types of special pigments are located. One is called "rods" which are "light sensitive" (50 times more sensitive than cones) and are used in darkness. The other pigments are called "cones" and are color sensitive, and are used in light. Fish have 3 types of cones (mammals only have two). Fish have one type of cone for each of the colors red, green, and blue (reptiles and birds have a fourth type for UV light).

These connect to nerve cells which then transmit the images to the brain. This is the largest part of a fish's brain (the optic lobe). It takes a fish "15 minutes to five hours" to adjust from conditions of light to conditions of darkness and vice versa. Fish cannot regulate the amount of incoming light as they do not have an iris, as we do (we dilate and constrict). They use a "screening pigment" which covers the photoreceptors. This is why fish can be startled so easily by light and light changes. Fish cannot close their eyes either, because they don't have eyelids. Fish seem to have a "memory" system to start the adjustment process for light changes at different times of day.

Colors of light penetrate water differently. Red light does not penetrate water as deep as blue light does. Therefore the first color the fish see in the morning in sunlight and the last color they see for the day is blue.

If water is calm, fish use the surface of the water as a mirror to reflect an image from behind an obstacle.

Fish use eyesight mainly for feeding and it also plays a big part in "schooling habits."

The outer surface of the eye is the "cornea". It contains the fluid and components of the eye. It also keeps foreign objects out of the eye. It has a refractive index similar to water. Fish do not have tears as their eyes are constantly bathed in water.

The middle part of the eye is the lens. Light enters through the cornea and passes through the lens, which focuses the light on the retina. The lens is large and spherical and has a very short focal length (2.5 times the radius of the lens (Sinclair, 1985)). This is the highest refractive index of any vertebrate! This is why they are near-sighted. They can accommodate slightly by moving the lens slightly away or toward the retina, but they cannot change the spherical shape.

Fish use their sense of smell to aid in seeing. They also use the lateral line system to "feel" around them, sensing pressure changes. Fry have very poor eyesight. Other types of fish eyesight varies somewhat (deep sea fish, sharks, and bottom dwellers).


Sources:
  • "How Animals See" by Sandra Sinclair, 1985
  • "World Book Encyclopedia" by World Book, Inc., 1990
  • "Anatomy and Physiology" by Anthony and Thibodeau, 1983
  • "Handbook of Tropical Aquarium Fishes" by Dr. Herbert R.Axelrod and Dr. Leaonard P. Schultz, 1990
  • "The Complete FishKeeper" by Joseph S. Levine, 1991


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