Debate over the smallest eye requires a definition of what makes an eye. The answer to this question, generally given by those considered authorities, is that an eye is an organ that receives and recognises light and has the ability to define spatial detail. Specifically, an eye can compare the amount of light coming from different directions. This definition means that an unusual collection of photoreceptors that may perceive light, but cannot tell direction or form of an image, such as those found in the tail of some of the sea snakes, would not be considered an eye.

By most definitions then, the planktonic dinoflagellate, Erythropsidium, must have among the smallest of eyes, since the creature is only 50–70 μm in diameter. Next to nothing is known about its genetics or visual mechanisms; nevertheless, we do know that this remarkable eukaryote speaks volumes about evolution’s creativity and emphasis on vision, although the term “vision” goes beyond an eye and must include some degree of interpretation of the image. It is doubtful that this organism interprets any image.

Dinoflagellates are protists, or single celled nucleated …