2017-09-01 Coral Magazine

levels of brightness. However, fishes have developed oth-
er mechanisms (see below).
The functional principle of sight is always the same:
Light radiation reaches the photoreceptors, exciting the
downstream nerve cell, which transmits this information
as an electrical pulse. Enhanced eyes improve directional
vision, resolution, and focusing at certain distances.
Although the eye development of all animals
is triggered by the control gene PAX6, the actual
development process differs in different organ-
isms because it depends on the animal’s anato-
my and physiology. Even if the eyes of different
types are very similar and have similar functional
components, such as a retina or a lens, there may
be considerable differences. This is particularly obvi-
ous when comparing the eyes of vertebrates with those
of the octopus.

VERTEBRATE EYES
Vertebrate eyes—in both fishes and humans—are pro-
duced in part from the brain. From the neural tube of
the embryo, which later forms the central nervous sys-
tem with the brain, two structures called optical vesicles
are produced. The inversion of the surface results in the

formation of the lens, vitreous body, and cornea. This

form of development causes a problem that could be

described as “evolutionary baggage”: the light radiation

coming from the lens to the retina must first penetrate

a relatively thick layer of nerve cells before it strikes the

photoreceptors, which actually perceive light. This leads

to an enormous loss of contrast. For this reason, we per- ALL: D. KNOP

A

B

C D

E

A B

C

Lens eye and ciliary

sense organ of the

giant clam Tridacna

maxima: A) lens, B)

ciliary sense organ, C)

photoreceptor cells.

The lens eye of the vertebrate (this is a pig’s eye, a historic

Heidenhain specimen from the author’s collection): A) cornea,

B) iris, C) lens, D) retina, E) optical nerve.

This enlargement of a pig’s eyeball shows the opening of

the optic nerve and how many tissue layers are above the

photoreceptors (arrow).

s.

The eye of Wunderpus photogenicus

comes closer to the ideal eye than

the vertebrate eye.