Are there any creatures that have only one type of cone cell plus rod cells?
For reasons that are entirely mysterious, all whales, dolphins, seals, sea lions etc. seem to fit this bill. They have lost the ability to make shortwave sensitive cones. The genes coding for the pigments have been sequenced, and the S-cone genes are non-functional, so you don’t have to worry about the cones being just rare… the machinery for making them is broken. And it is broken in different ways in the different groups of animals, so it’s not something that whales and seals obtained from the same ancestor. This is particularly odd in that ocean water is enriched in short wave radiation relative to the light that terrestrial animals are exposed to. But there you go. Also, raccoons and at least some of their relatives have only one type of cone. For other sorts of “intermediates,” you might look into fish. Individual fish species generally live at a restricted range of depths, and fish that live in deeper waters tend to have fewer types of cones. In the deepest lake (Lake Baikal) y
