Octopus Color Vision

One of the greatest questions I've ever asked myself was:

How does an octopus see color?

I quickly came to discover what the immediate answer is:

We're not exactly sure!

After a lot of research, manual raytracing, computer simulation, and praticed curiosity, this is how I understand it:

Octopuses lack color-sensitive cone cells in their retina pursuant to traditional vertebrate color vision, making their retina monochromatically sensitive. They're colorblind! Their color camouflage is a sign of complex and fast interpretation of the color of their environments, however, indicating another mechanism for seeing different wavelengths of light.

The leading thoery is that an octopus changes their pupil shape to occlude the area around the middle of their eye, using an evolutionary understanding of the chromatic aberrations and diffraction of the slit of their radial pupil to interpret color information!

How does an octopus change colors?

Many cephalopods have skin covered in chromatophores, special sacs of pigment that may inflate/deflate depending on nervous signal from the animal, depicting a combination of colors based on which sacs are inflated. They function a lot like RGB displays!

Papers related to cephalopod color vision & my summary of them:

Spectral discrimination in color blind animals via chromatic aberration and pupil shape [DOI]
Stubbs & Stubbs

wowowow a description of the optical phenomena potentially responsible behind diffraction-based color vision

Visual phototransduction components in cephalopod chromatophores suggest dermal photoreception [DOI]
Alexandra C. N. Kingston, Alan M. Kuzirian, Roger T. Hanlon, Thomas W. Cronin

The second leading theory is that cephalopods just have something in their skin (specfically in chromatophores) that can see color which is just bananas (I don't believe in this one)

Colour-blind camouflage [DOI]
Marshall, N., Messenger, J.

There's evidence that points towards cephalopods not being able to distinguish color at all, rather intuiting that an object's shape corresponds to a color they should turn into based on lots of evolution! This is compelling, but I tend to believe they understand what color they are, many of these other papers demonstrate that!

The Colours of Octopus: Using Spectral Data to Measure Octopus Camouflage [DOI]
Luis Nahmad-Rohen , ORCID,Yusuf H. Qureshi 1 andMisha Vorobyev

wowowowowow this paper measured the spectra given off by backgrounds in octopus habitats and the octopus when camouflaging to that background. Comparing those measurements shows a significant difference between the spectra of the background and octopus, so their camouflage is not perfect (they're made of different stuff than their backgrounds!). The interesting conclusion of this article is that octopus

Color matching on natural substrates in cuttlefish, Sepia officinalis [DOI]
Lydia M. Mäthger, Chuan-Chin Chiao, Alexandra Barbosa & Roger T. Hanlon

I haven't read this yet, but I will get to it!

Octopus skin ‘sight’ and the evolution of dispersed, dermal light sensing in Mollusca [DOI]
Ramirez, Merce Desmond

Need to read!