Comparative Morphology of the Concave Mirror Eyes of Scallops (Pectinoidea)

The unique, double-retina, concave mirror eyes of scallops are abundant along the valve mantle margins. Scallops have the most acute vision among the bivalve molluscs, but little is known about how eyes vary between scallop species. We examined eye morphology by immunofluorescent labeling and confocal microscopy and calculated optical resolution and sensitivity for the swimming scallops Amusium balloti (Bernardi, 1861), Placopecten magellenicus (Gmelin, 1791), Argopecten irradians (Lamarck, 1819), Chlamys hastata (Sowerby, 1842), and Chlamys rubida (Hinds, 1845) and the sessile scallops Crassadoma gigantea (Gray, 1825) and Spondylus americanus (Hermann, 1781). We found that eye morphology varied considerably between scallop species. The eyes of A balloti and P. magellenicus had relatively large lenses and small gaps between the retinas and mirror, making them appear similar to those described previously for Pecten maximus (Linnaeus, 1758). In contrast, the other five species we examined had eyes with relatively small lenses and large gaps between the retinas and mirror. We also found evidence that swimming scallops may have better vision than non-swimmers. Swimming species had proximal retinas with inter-receptor angles between 1.0 ± 0.1 (A. balloti) and 2.7 ± 0.3° (C. rubida), while sessile species had proximal retinas with inter-receptor angles between 3.2 ± 0.2 (C. gigantea) and 4.5 ± 0.3° (S. americanus). Distal retina inter-receptor angles ranged from 1.7 ± 0.1 (A balloti) to 2.8 ± 0.1° (C. rubida) for swimming species and from 3.0 ± 0.1 (C. gigantea) to 3.6 ± 0.2° (S. americanus) for sessile species, but did not appear to correlate as strongly with swimming ability as proximal retina inter-receptor angles did. Finally, we found that optical sensitivity differed between species, measuring from 3 ± 1 (A. balloti) to 21 ± 10 µm2 · sr (C. hastata) for proximal retinas and from 2 ± 1 (C. gigantea) to 8 ± 5 µm2 · sr (C. hastata) for distal retinas. These differences, however, did not appear to correlate with ecological factors such as a scallop species' swimming ability, preferred substrate type, or range of habitat depth. In light of these and previous findings, we hypothesize that scallop distal retinas may perform tasks of similar importance to all species, such as predator detection, and that proximal retinas may perform tasks more important to swimming species, such as those associated with the visual detection of preferred habitats.