Comparative histological analysis of the pallial eyes of Zygochlamys patagonica and Argopecten purpuratus, two scallop species from the southeastern Pacific

The visual system of the Pectinidae family of bivalve mollusks is exceptional because of the number and complexity of their eyes. The non-cephalic eyes of pectinids consist of a cornea, a lens, a double retina, a pigment epithelium, and a concave mirror. While prior studies have addressed the anatomy and putative functions of pectinid eyes, the cellular components and their adaptations to different habitats have been explored in only a few scallop species. The Northern scallop Argopecten purpuratus and the Patagonian scallop Zygochlamys patagonica are important hydrobiological resources which inhabit different latitudes in the southeastern Pacific (5°S to 33°S and 40° to 48°S, respectively). The present study characterized and compared the components of the pallial eyes of the two species with light, confocal and electron microscopy, to improve our understanding of the sensory adaptations to their different habitats. While most cellular elements, including the ciliary and rhabdomeric photoreceptors, are morphologically similar and ostensibly conserved, some structures such as the cornea and the lens display significant differences between A. purpuratus and Z. patagonica. Overall, the cellular complexity and structural sophistication of pectinid eyes highlight the critical role of vision in scallop ecology and behavior.