Introduction to the Symposium “Molluscan Models: Advancing Our Understanding of the Eye”*

Since the time of Darwin, the eye has been a subject of evolutionary and comparative biologists alike who were intrigued by the structural complexity and morphological diversity of eyes in nature. Much of what we know about the eye—development, structure, physiology, and function—has been determined from only a handful of model organisms, specifically the mouse and the fly. One major phylum in particular, the Mollusca, has been underutilized in investigating the evolution and development of the eye. This is surprising as molluscs display a myriad of eye types, such as simple pit eyes without any apparatus to focus images, compound eyes that superficially resemble the eyes of flies, camera-type eyes that are similar to vertebrate eyes, and eyes with mirrors, just to name a few. As a result, molluscan eyes comprise more morphological diversity than seen even in the largest animal phylum, the Arthropoda. With all of this incredible diversity, how do we as researchers determine which mollusc species should be developed as models to study the eye? Serb provides background for eye research using traditional model organisms and how using molluscan species would be advantageous to understanding the eye. She describes the research potential of molluscan species as model organisms and identifies criteria that might be used to develop a molluscan model and the questions molluscan models might address. One application of molluscan models is to study the cellular biology of human eye disease. As many degenerative eye diseases, such as macular degeneration, have been linked to the mis-organization of the cytoskeleton within retinal cells, understanding the control of cytoskeleton organization and its influence on photoreceptor cell changes may lead to prevention and possible cures for some eye diseases. Gray, Kelly, and Robles utilize Octopus bimaculoides Pickford and McConnaughey, 1949 as a model organism to study the molecular controls of cytoskeleton organization in the retina. Their work identifies a cell signaling pathway (Rho GTPase) that mediates cytoskeleton rearrangements. Errors in this pathway may prove to be one of the factors that disrupts cytoskeleton formation, leading to retinal degeneration. After developing one or several molluscan models of the eye, how does one set about understanding this great diversity of eyes and place it in an evolutionary context? One way is to use a comparative approach to identify conserved and variable components of eye morphology, such as lens composition, photoreceptor number and organization, and overall eye shape. These morphological features can provide evidence for functional differences and visual capabilities among species. Several authors in the symposium take this approach. Speiser and Johnsen examine eye morphology in four species of scallop and a closely related spondylid (Spondylus americanus [Hermann, 1781]). They show that scallop eye structure varies among species, and these structural differences affect optical resolution and sensitivity. Further, they provide evidence that actively swimming species (e.g., Amusium balloti [Bernardi 1861]) have better optical resolution than non-swimming species. Speiser and Johnsen provide several new and exciting hypotheses on how the scallop eye performs and how visual requirements may differ between mobile and immobile species. Morton takes a broader perspective and reviews the diversity of noncephalic eye types in the Class Bivalvia. He hypothesizes a possible evolutionary path to create the double retina system in Pectinidae and Laternulidae through the duplication of sensory structures on the pallial folds. Zieger and MeyerRochow review the variation of cephalic gastropod eyes, concentrating on pulmonate species, which are the beststudied eyes in gastropods. They discuss eye anatomy, differences in retinal design, and the visual capabilities of different optical components. Finally, they describe the ultrastructure of “additional” or “accessory” eyes associated with cephalic eyes in several lineages. These data provide