Meeting Abstract
Animal eyes are morphologically diverse, variably complex, and essential for understanding the evolution of complex biological traits. While eyes evolved many times in bilaterian animals with elaborate nervous systems, image-forming and simpler eyes also exist in cnidarians, ancient non-bilaterians with simple nerve nets and neural condensations to process information. How often eyes of varying complexity, including image-forming eyes, arose in animals with such simple neural circuitry remains obscure. Here, we couple large-scale phylogenies of Cnidaria and their photosensitive proteins with a compilation of a vast literature on eyes and light sensing behavior to show cnidarian eyes originated at least eight times, with complex lensed-eyes having a history separate from the others. Our experiments show widespread light sensing behavior in eyeless cnidarians and comparative analyses support ancestors without eyes that already sensed light with dispersed photoreceptor cells. The history of expression of the photoreceptive protein opsin demonstrates distinct eye origins involved separate co-option of non-visual opsins into eyes. Overall, our results show eyes evolved repeatedly from ancestral photoreceptor cells in non-bilaterian animals with simple nervous systems, co-opting existing precursors similarly to what occurred in Bilateria. Our study underscores the potential for multiple distinct visual systems and underlying developmental pathways even in animals with simple nervous systems.
