Cephalopod eyes are fascinating. Just like us vertebrates they have camera-type eyes, a hollow liquid-filled chamber with an opening, the iris, and a lens through which light enters and is projected onto the photosensitive surface, the retina. Despite their similarities, vertebrate and cephalopod camera-type eyes have different origins and evolved independently. There are some striking differences that highlight this:
Unlike us, the photoreceptor cells of cephalopods point outwards towards the source of the light rather than inwards. This not only means the we have “inverted” retinas, it also means that cephalopods don’t have a blind spot because the nerve fibers that transmit the visual impulses from the retina to the brain collect and exit the eye behind the retina rather than in front of it. The developmental origins of the eye tissues are also different. For instance, in vertebrates the complex layers of the retina develop from nerve tissue, while the lens develops from skin tissue. In cephalopods both tissues develop from progenitor skin cells.
Cephalopods have excellent vision, and use complex visual cues to communicate with each other, camouflage themselves, and send signals to their environment. To do this they use highly adaptible pigment-filled cells in their skin called chromatophores. The capricorn night octopus (Callistoctopus alpheus) in the photo looks blue, but if it would open all its chromatophores it would turn deep red with bright white polka dots.
Photo credit: David Liittschwager, National Geographic.