Left–right asymmetry of the visual system in the scallop Nodipecten nodosus (Bivalvia: Pectinidae)

Left–right asymmetries are consistent differences between the left and right sides and represent an intriguing feature of molluscan morphology. Interestingly, external asymmetries, such as inequivalve shells, are often coupled with lateralization in the nervous system, which often includes functional and structural specializations of the left or right sides. In the case of visual asymmetries, lateralized phenotypes frequently include left–right differences in eye position, structure and use. To investigate lateralization and visual asymmetries among bivalves, we examined the visual and nervous systems of the scallop Nodipecten nodosus. We characterized different eye morphologies in the species and determined eye-side frequencies. We also studied the anatomy of the nervous system, focusing on the parietovisceral ganglion (PVG). Our results reveal that the visual and nervous systems of the scallop N. nodosus comprise consistent left–right asymmetries: (1) a greater number of eyes on the left side, (2) increased size of the left PVG lateral lobe, (3) larger glomeruli in the left PVG lateral lobe and (4) radial pallial nerves associated with the left lateral lobe spaced more widely than the more compactly arranged pallial nerves associated with the right lateral lobe. In addition, eyes with a distinctive morphology, where the optic vesicle is rotated, are more frequent on the left side. Considering the habit of this species to rest on the right valve, we hypothesize that curved eyes might contribute to expanding the overall visual field. Even though bivalves are not typically considered in the literature on lateralization, accumulating evidence for scallops, as suggested by our results, indicates their potential to reveal novel patterns of visual asymmetry in benthic invertebrates.