cannot accurately assess mixed colors—for example, dark
yellow and light orange look absolutely the same—but it
saves time, a crucial advantage for a hunter with ultra-
fast prey-catching or killing tools. While other animals
optimized their nervous systems in order to improve
color detection, mantis shrimps evolved sophisticated
compound eyes and avoided the costly development of
the nervous system. These shrimps are able to see three-
dimensionally and estimate distances with each of their
two eyes. Humans use two eyes to focus on the same
object from different positions.
MIRROR EYE
Some animals of the class Bivalvia, especially swimming
species like the genera Pecten (scallops) and Spondylus
(thorny oysters), have mirror eyes. Their visual organs
are spherical and have a large opening with a primitive
lens. The main way these eyes function is to perceive
brightness, and they use a very refined strategy. At the
heart of this type of eye is a mirror surface on the back
of the eye, the argentea, which reflects the incident light.
The retina is free in the eye, not at the back of the eye,
and it is double-sided. It has two retinas, an outer (dis-
tal) and an inner (proximal) retina, so incident light
has four contacts with the photoreceptors: first with the
outer retina, then with the inner retina, and after being
reflected by the mirror layer, once again with both in
reverse order.
The mirror layer of the argentea consists of guanine
crystals, a natural substance that can reflect light. When
reflecting surfaces occur in the animal kingdom—for ex-
ample, the bright blue stripe on the Neon Tetra—guanine
crystals are often involved. But these crystals are so tiny
that numerous layers must be superimposed, and some
radiation components are only reflected by an underly-
ing guanine layer, resulting in light scattering. Therefore,
the light mirrored by the argentea is not sharp.
This type of eye can register even a very small amount
of light. Its light sensitivity is about five times as high as
that in the eyes of fishes. But the tradeoff is that images
are not sharp because the light radiation can only be fo-
cused on the distal retina. On the inner proximal retina,
the image is inevitably blurred, in addition to the scat-
tering by the mirror layer. These eyes serve their purpose,
however, signaling when the bivalve must snap itself
closed to avoid being eaten.LENS EYES
Lens eyes, with their dioptric apparatus (vitreous body,
cornea, lens), occur in humans and have developed inde-
pendently in several groups (annelids, cephalopods, ver-
tebrates). The lens eye greatly improves light sensitivity
and imaging. In many cases an iris diaphragm regulates
the light incidence and helps the eye adapt to varyingSchematic representation of the mirror eye of Spondylus varius:
A) lens, B) outer retina, C) inner retina, D) mirror.Two of the many mirror eyes
of Spondylus varius.
ABCD