Development of the pallial eye in Nodipecten nodosus (Mollusca: Bivalvia): insights into early visual performance in scallops

Scallop pallial eyes have been the most studied optical system in bivalve mollusks. Despite recent advances in our understanding of the function and evolution of scallop eyes, little attention has been focused on eye development and early visual performance. Here, the anatomy and development of pallial eyes were investigated in the scallop Nodipecten nodosus (Linnaeus, 1758) by means of integrative microscopy techniques (i.e., light, electron, and confocal microscopy). After metamorphosis, juvenile scallops bear small papillae that rapidly transform into minute ocular organs on the middle mantle fold. The distal epithelium gradually becomes pigmented, except for the cornea formed at the distal center of the eye. Internally, the optic vesicle comprises undifferentiated cells in the distal region, while mirror plates are secreted at the base of the eye, next to pigmented cells. Within the undifferentiated cell mass, the proximal retina is the first to be formed, followed by the distal retina and then by the lens. In this respect, the late development of the scallop lens from retina precursor cells may represent a unique condition among animal eyes. Adult eyes are characterized by large pigment distribution in the epithelium, tall columnar cornea, and lens above a slightly curved double retina. Whereas the pallial eyes from adult scallops are a complex visual system based on a mirror mechanism to form a focused image on the retina, early eye condition suggests a simple degree of directional photoreception, with no spatial vision.