According to this post, Cephalopods have about $20,000 mm^{-2}$ photoreceptors in there eyes. However, Cephalopods (in my case, the Giant Squid) aren't born with cones (cells that dictate which colours you see). So are they born with about 20,000 rods (cells that help detect movement), or is there another type of cell involved?
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$\begingroup$ How do you get from 20,000 $mm^2$ to 20,000 cells? Note also that in the title you said 20,000 $m^2$. $\endgroup$– Remi.bCommented Oct 12, 2017 at 0:50
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$\begingroup$ You are missing a "/". The number in that post is 20,000 PER mm2. Overall though I think you need to do some more research on this on your own... rods are not only used for detecting movement, for example. Even that post you linked has a lot of information that you seem to be lacking. $\endgroup$– Bryan Krause ♦Commented Oct 12, 2017 at 3:09
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First squid lack both cones and rods, they evolved their eyes independently, the cells they use most closely resemble rods but they are by no means identical.
In vertebrates there are two types of cells rods and cones both of which can be used to see movement (movement is just detecting changes in the visual field). rods are better at it becasue they are smaller so they can be packed tighter, thus they can detect smaller changes, but that is by no means their primary function. Rods detect luminosity, that is the intensity of light, but react to light from all parts of the visual spectrum.
There is a lot of debate about how much color squid and octopi can see. behaviorally they appear to recognize color, but they don't have differentiated cells or opsins to detect them.
I appears they have stumbled on a completely different way to achieve color vision, they appear to use weird shaped pupils and chromatic aberration. I suggest reading the linked paper as you have to dive pretty deep into the physics of optics to understand. Basically different colors of light are bend by slightly different amounts by a lens, and they are using the multiple images of their U shaped pupils to gain color information.
