How fish see
Butterfly cichlid © Christina Will
Fish see in color, but in a rather broader spectrum than humans. Pisces tend to be short-sighted. In detail, the eyesight of fish varies greatly depending on the species.
Many fish seem to recognize who is feeding them. Every fish keeper knows that virtually all fish will come to the feeder as soon as the keeper approaches the aquarium. However, it is unclear whether they recognize this with their sense of sight or whether they react to currents, vibrations and sound with their lateral line organs, for example. So they could not recognize their breadwinner visually, but possibly by their characteristic cadence and pedaling strength. People often recognize at least acquaintances by the way they move or walk.
Appropriate experiments have shown that fish can distinguish between shapes and colours, including patterns. How well the fish can distinguish patterns depends on the species.
Zebrafish are relatively bad at it. They are typical shoal fish and the survival of the species depends less on the individual individuals and more on the shoal.
Peacock cichlids, Astronotus ocellatus, are not schooling fish. Keepers report again and again that these fish distinguish who is in front of the aquarium.
Astronotus react very differently up to a distance of four meters, whether the owner has empty hands or whether he is carrying a measuring cup with live food. If the hands are empty, the animals wait. If a cup is in hand, they swim back and forth excitedly.
If they only perceived the impact sound, the different reactions could not be explained.
Scalars are very curious. You will definitely see movements of up to 20 to 30 centimeters in front of the aquarium. This can be tested by sitting quietly in front of the aquarium and then moving your index finger back and forth. Similar to detecting the eye movement of small children.
Angelfish clearly track the finger up to about 30 centimeters away. Vibrations should be excluded. If the distance is greater, they are no longer interested. They probably still see there, but probably no longer sharply, or details the size of a finger.
There is evidence that scalars can see up to 10 meters away. At these distances and movements, however, it is not certain whether they really see body outlines, for example, or feel vibrations. For example, if you just move back and forth on a chair, i.e. without vibrations, then they don’t seem to react.
Angelfish are particularly interested when dust is being vacuumed. It’s probably related to vibration.
How fish see color
© Mirko Rosenau – Fotolia
Unfortunately, it is impossible to exactly understand the color perception of animals and thus of fish, because humans have a slightly different sense of sight.
Seeing colors is a result of processing in the brain. Certain sensory cells in the retina of an eye, the so-called cones, are used to see colors. In addition, there are so-called rods in the eye, with which black and white can be seen in weak light.
When light falls on an object, the object absorbs certain wavelengths of light and reflects the rest. The reflected light determines what a viewer perceives as color. Each color is composed of pure or mixed wavelengths.
In each cone there is a specific pigment that absorbs light of specific wavelengths. Color perception arises from the interaction of different types of cones, which have different pigments and therefore recognize different ranges of wavelengths. A certain wavelength, the absorption maximum, is recognized best. The brain determines which cone types recognize which wavelengths to what extent and uses the overlaps to put together the color impression.
There are four different color pigments in vertebrates that arose early in the evolution of vertebrates:
- Pigments for long-wave yellow light
- Medium wavelength pigments visible to humans
- Pigments for short-wave blue light visible to humans
- pigments for ultraviolet light
In the course of further evolution, mammals have lost two pigments and thus the associated cone types. Possibly because mammals were first nocturnal animals. Mammals originally retained the pigments for longwave and ultraviolet light.
In the primates, which also include humans, a new, third type of cone later emerged. Presumably the gene for a pigment has duplicated. A mutation then created a gene for a new pigment from one of the duplicate genes. The reason for this development was possibly that the diurnal primates were able to recognize fruit better.
The gene for the long-wavelength light pigment probably duplicated and mutated. The new pigment also recognizes long-wave light best, but the absorption maximum is no longer in the yellow range, but in the green range.
During this development, the absorption maxima of the other pigments also shifted.
370nm 424nm 445nm 508nm 530nm 565nm 560
During the development of mammals, oil inclusions, ie small oil droplets in the cones, were also lost. These droplets enable birds, for example, to distinguish more colors than would be possible without these inclusions.
Studies on birds, for example, indicate that birds see ultraviolet light as colors that combine with other wavelengths to form mixed colors. Because many fish also have the original four color pigments, it may be similar with fish. Fish perceive colors that we can perceive in a similar way to us.