What can box jellyfish tell us about early eye evolution

The eyes of visually prominent animals such as vertebrates, cephalopods, and arthropods generally subserve a multitude of visual tasks. Naturally, early stages in the evolution of these complex visual organs must have been simpler, and subserved a smaller number of visual tasks. Hence, eye evolution is driven by a consecutive accumulation of visual tasks. Each task adds to the requirements on eye structure, making it gradually more complex. For these reasons, reconstructions of eye evolution should ideally be based on an understanding of the sequential addition of visual tasks. In particular, it is interesting to ask what the first visual tasks might have been, and what requirements these would have placed on the structure and function of early eyes. With this objective, we have investigated vision in a group of simple and phylogenetically basal animals, the box jellyfish. Behavioural experiments indicate that these animals use vision primarily for positioning in the habitat, and for negotiating obstacles. To serve these tasks, the eyes are tuned for low spatial frequencies and are colour-blind. The findings indicate that low resolution is not just sufficient, but in fact desirable in early stages of eye evolution.