[1] They are the most common type of cone cells in fish, reptiles, birds, and monotremes such as the platypus and are present in most vertebrates, though they have been noted as absent in most placental mammals (including humans), elasmobranches and catfish.
[2] There are many gap junctions between the cells of fish double cones.
[1] Their function, if they have any unique function compared to single cones, is largely unknown; proposed uses include achromatic (non-colour vision) tasks such as detecting luminance, motion and polarization vision.
[3] Behavioural research on the reef dwelling triggerfish Rhinecanthus aculeatus has provided evidence that individual members of double cones can act as independent channels of colour information.
[2] In a book about vision in fishes,[3] James Bowmaker writes that double cones tend to be sensitive to longer wavelengths of light than single cones.