Although the eyes of the octopus,
and of other cephalopods, and the eyes
of vertebrates have evolved entirely
independently, each has a retina, a cornea,
an iris, a lens,
and a fluid-filled interior. These similarities of structure, despite different
origins, provide a classic example of biological convergence.
However, the cephalopod and vertebrate eyes are also very dissimilar in
some respects. For example, the photoreceptor cells in the eye of the octopus
point toward the incoming light whereas our own rod
cells and cone cells point backward
and absorb light reflecting from the back of the eye. Another difference
is that the octopus eye, like those of other invertebrates, develops as
an invagination, or in-pocketing, of the skin, and not, as in the case of
the vertebrate eye, as an extension of the brain.
The method of focusing too is quite distinct. Cephalopods have a stiff lens
of fixed focal length, which is normally focused on objects fairly nearby.
The focus is changed by moving the entire lens closer or farther from the
retina with the ciliary muscle. We,
on the other hand, use our ciliary muscles to change the shape of our eye
lens to bring objects at varying distances into focus.
A unique characteristic of the cephalopod eye is its ability to rotate and
maintain a constant orientation with respect to gravity. Using its statocyst,
a balance organ common to many invertebrates, an octopus can always keep
its slit-shaped pupils in a horizontal position. Consequently, the brain
can always safely interpret visual information on the basis that the eyes
are horizontally aligned, though the body may be at any angle.
Like insects, cephalopods are sensitive to
the polarization of light. The chromatophores
and iridescent cells on the skin of cephalopods can create a visual pattern
that coincides with polarized light. Octopuses and squid can recognize these
light patterns and since the chromatophore patterns change depending on
mating season, behavior, and stress, they can effectively communicate with
each other. Polarized vision also allows cephalopods to detect otherwise
transparent prey such as jellyfish and