Simple Eyes, Extraocular Photoreceptors and Opsins in the American Horseshoe Crab

The eyes and photoreceptors of the American horseshoe crab Limulus polyphemus have been studied since the 1930s, and this work has been critical for understanding basic mechanisms of vision. One of the attractions of Limulus as a preparation for studies of vision is that it has three different types of eyes-a pair of later compound, image-forming eyes and two types of simple eyes, a pair of median ocelli, and three pair of larval eyes. Each eye type is tractable for experimentation. Limulus also has extraocular photoreceptors in its segmental ganglia and tail. The current contribution focuses on photoreceptors in Limulus larval eyes and ocelli and its extraocular photoreceptors with the goal of summarizing what is currently known and not known about their physiology and function and the opsins they express. The Limulus genome encodes a surprisingly large number of opsins (18), and studies of their expression pattern have raised new questions about the role of opsin co-expression, the functions of peropsins expressed outside of eyes, and the physiological relevance of opsins with apparently very low expression levels. Studies of opsin expression in Limulus lead one to wonder whether photoreceptors yet to be discovered might be present throughout its central nervous system.