Dan-E. Nilsson, Lars Gislén, Melissa M. Coates, Charlotta Skogh and Anders Garm Advanced Optics in a Jellyfish Eye to appear in Nature Abstract: Cubozoans, or box jellyfish, differ from all other cnidarians by an active fish-like behaviour and an elaborate sensory apparatus. Each of the four sides of the animal carries a conspicuous sensory club (the rhopalium), which has evolved into a bizarre cluster of different eyes. Two of the eyes on each rhopalium have long been known to resemble eyes of higher animals, but the function and performance of these eyes have remained obscure. We here show that box-jellyfish lenses contain a finely tuned refractive index gradient producing near aberration-free imaging. This implies that even simple animals have been able to evolve the sophisticated visual optics previously known only from a few advanced bilaterian phyla. A further surprise is that the position of the retina does not coincide with the sharp image, leading to very wide and complex receptive fields in individual photoreceptors. We argue that this may be useful in eyes serving a single visual task. The findings suggest that tailoring of complex receptive fields may have been one of the original driving forces in the evolution of animal lenses. LU TP 05-05