the retina—in principle, exactly what a camera lens does
when focusing.
For sharp distant vision, the lens is pulled closer to
the retina with a special retraction muscle. For sharp
near-vision, the muscle relaxes and the lens slides out-
ward. This can be seen from the outside when part of
the lens closes the pupil and appears before the iris as a
hemisphere. The part of the spherical lens that projects
over the iris gives the fish a wide field of view to the left,
right, up, and down. Normally, the fish eye is adjusted to
near-vision; far-sightedness requires muscle power and
narrows the field of view.
Some small wrasses looking for food on the bottom
(e.g. genus Paracheilinus) are very adept specialists in
this regard, because their eyes have two lenses: a close-up
lens for focusing on prey and a wide-angle lens to watch
for predators concurrently.
DO FISHES SEE COLORS?
Bony fishes can distinguish four colors (this is known
as tetrachromatic vision), because in addition to the
red, green-, and blue-sensitive cone receptors they have
cones for UV radiation. They can see ultraviolet light and
perceive UV reflection patterns on the bodies of fellow
species. Human eyes lack UV-sensitive photoreceptors,
so we only see three colors (trichromatic vision).
LIGHT ADAPTATION
The eyes of terrestrial animals have irises that can in-
crease or decrease the size of the central aperture. In this
way, the eye regulates the amount of light that enters the
eyeball and reaches the retina. The eye of a fish has an
iris, the central opening of which forms the pupil. But as
mentioned above, we do not perceive this pupil as a black
hole, as we do when looking into our own eyes; we see a
transparent hemisphere, a part of the spherical lens, pro-
vided the eye of the fish is focused on near-vision.
Unlike land animals, however, whose irises contract
with intense light irradiation, the vast majority of fish
species cannot change their pupil diameter. In order to
protect the retina from excessive light, they have devel-
oped a completely different mechanism: Between the ret-
ina and the surrounding cornea there is a light-swallow-
ing layer of pigment, into which the stimulus-absorbing
parts of the very light-sensitive rod receptors (colorless
twilight vision) can migrate. If our aquarium fishes are
exposed to strong light radiation, the stimulating outer
structures of the rod receptors retreat into this pigment
layer, while the less light-sensitive cone receptors (sharp
color vision at high brightness) remain behind. If the
amount of light decreases, the stimulant parts of the rod
receptors move back out of the pigment layer.TAPETUM LUCIDUM
Brightness adjustment, however, is not the only trick
with which different light intensities are managed. Fishes
can also adjust to darkness. The quantity ratio between
the cone receptors for color vision and the rod receptors
for colorless perception at low brightness varies greatly
in the eyes of fishes, and not only differs from species to
species but is adapted to the fish’s preferred habitat. Spe-
cies that are primarily active in the dark have far more
rod receptors than fishes that are active during daylight.
It is even more interesting that many species of fish
in low-light habitats have a special adaptation that al-
lows them to make the best use of the available light. In
some respects it is reminiscent of the mirroring in bi-
valves: in the choroid, which lies under the retina, there
is a reflective layer. This is called the tapetum lucidum
(Latin for light carpet), and it captures and reflects light
that has reached the receptors of the retina so that it
can excite the receptors again. This phenomenon is also
found in some land animals—including cats, dogs, and
many birds.SINGLE CELL WITH LENS
Even single-cell organisms can have a lens eye, as
strange as that may sound. In 2015, Gregory Gavelis D. KNOPRhinomuraena quaesita eyes. You can clearly see the
domed cornea and the part of the spherical lens that
is pressed through the iris and closes the pupil.