Cephalopod Eyes excerpt from
http://archives.evergreen.edu/webpages/curricular/20112012/m2o1112/web/cephalopods.html
Camera-type Eye
The cephalopods possess camera-type eyes similar to those of
vertebrates. These eyes contain an iris, a rectangular pupil, a nearly
circular lens, vitreous cavity and photoreceptors forming a retina.
The pupil shape for different cephalopods varies. The octopus has a
rectangular pupil, the cuttlefish a 'W' shaped pupil and the squid a
circular pupil. They use their pupils to control the amount of light that
hits the retina. The cephalopod eye resembles the vertebrate eye but
is less sophisticated. Of all the cephalopods, the octopus has the
most developed eye with a completely closed cornea. The octopus
focuses by moving its entire (stiff) lens back and forth within the eye.
Human use small muscles to change the shape of the lens and its
distance to the retina.
Considering that cephalopods are water-dwelling creatures, one of
their most interesting features is their ability to keep their eye
orientation the same in relation to gravity regardless of their body
position in the three dimensional water column. Essentially, their pupil
will stay horizontally aligned regardless of the angle of their body. In
order for this to work, they have a balance organ called a statocyst
that controls the optic muscles. A statocyst is a sense organ that is
half filled with both tiny calcium granules in a jelly-like fluid and lined
with hair cells. The statocyst gives the cephalopod input about
orientation and acceleration similar to the utricle and saccule in
human ears.
Evolutionary Significance
The likeness between the camera-type cephalopod eye and the
vertebrate eye has been used as a classic example of convergent
evolution for almost 140 years. Convergent evolution is the
independent evolution of similar structural or functional components
in two or more unrelated or distantly related lineages. The other
possible explanation is parallel evolution wherein two lineages
evolve from a common ancestor and in a common trajectory towards
a similar endpoint, resulting in the similar or shared trait in different
species. The convergence theory of the camera-type cephalopod eye
is now being challenged. A conserved gene network has been
discovered which includes the gene Pax6. Pax6 governs eye
development in both vertebrate and invertebrate lineages. This gene
conservation may indicate a common origin of eyes in vertebrates
and invertebrates. If this is the case then the similarity between the
camera-type cephalopod eye and the vertebrate eye would be a
result of constraint in development and parallel evolution. However, it
is both possible and probable that this likeness is the result of a
combination of both convergent and parallel evolution, as the eye is a
complex organ with many components which may evolve
independently of one another.