We need light to see. There are times, though, when things are so gloomy it is difficult for light to shine through. So, to beat the darkness, either we create our own light or adapt to the lack of it. In Nature, the natural habitat of many vertebrates is water - sea water or fresh water. Unlike sea water, lakes or streams can be very turbid and, thus, less easy to move around in. Over a century ago, scientists had already observed that the eyes of freshwater fish have a visual pigment system that is not the same as those of marine fish or, for that matter, land animals. Freshwater fish, for instance, are able to navigate through hazy waters the way a bird would fly across a cloudless sky. This is because they can perceive wavelengths of light that humans, for instance, cannot. For us, everything would remain blurry. More surprising, perhaps, are certain vertebrates such as bullfrogs and salmon that have both visual pigment systems, and switch from one to the other depending on the environment. How can they do this? Thanks to an enzyme, known as cytochrome P450 27C1. More