The Japanese firefly squid (Watasenia scintillans), which inhabits the deep ocean, has three visual pigments located in different parts of the retina that are likely to allow colour discrimination as they each have distinct spectral sensitivities.

Colour vision in mammals

Although sex-linked colour “blindness” in human males is relatively common, for the great majority colour vision is an integral, and indeed delightful, part of our world. As is well-known the ability of different animals to perceive colour is widely variable, but nevertheless there are many striking instances of convergence including close parallels to our trichromatic vision (notably in New World monkeys and some marsupials).

Spectral discrimination in Japanese Firefly squid

Turning to consider vision in cephalopods, it is worth noting that despite the many remarkable convergences between the camera eyes of the cephalopods (including octopus) and the vertebrates, perhaps oddly the evidence is that generally the cephalopods do not possess colour vision. This does, however, require some qualification. First, in the Japanese Firefly Squid Watasenia scintillans, otherwise known as the Sparkling Enope Squid which inhabits the deep ocean, there are three visual pigments that are located in different parts of the retina and are likely to allow colour discrimination as they each have distinct spectral sensitivities. Nevertheless, there are oddities. In particular the spectral discrimination relies on three different types of retinol (the molecule related to vitamin A) in the chromophore, whereas more usually colour discrimination depends on specific amino acid substitutions in the visual proteins known as the opsins.

As noted, cephalopods other than Watasenia seem to be effectively monochromatic. Nevertheless, there is evidence that the opsins do show specific substitutions in some species that permit spectral shifts. It has also been remarked that the absence of some sort of colour vision is all the more remarkable given their superb abilities in camouflage where they blend into different backgrounds, not to mention the astonishing examples of sexual mimicry and predatory mimicry.