Scallops visually respond to the size and speed of virtual particles.

The unique eyes of scallops are abundant along the right and left valve mantle margins. These eyes form images by reflection off a concave spherical mirror, and give scallops an angular resolution of around 2 degrees , far better than the 13-40 degrees angular resolution provided by the eyes of other bivalves. It has been argued that bivalve mantle eyes primarily act as predator detectors, but behavioral studies have suggested that vision may serve additional purposes in scallops. By placing specimens of the bay scallop Argopecten irradians (Lamarck 1819) in a tank with 5-10 cm s(-1) flow, showing them simulated images of moving particles, and recording their behavior, we tested whether visual cues may influence feeding behavior in these animals. We found that scallops opened their anterior mantle gapes significantly more often when they were shown 1.5x1.5 mm virtual particles (with angular sizes of 3.4 degrees ) than when they were shown 0.6x0.6 mm particles (1.4 degrees ; P<0.001) or no particles at all (P<0.05). We also found that scallops opened their anterior mantle gapes significantly more often when they were shown virtual particles moving at 2.5 cm s(-1) (P<0.01) or 5 cm s(-1) (P<0.05) than when they were shown particles moving at 10 cm s(-1). Because scallops must open their anterior mantle gapes to feed, our findings suggest that these animals may visually detect the size and speed of moving particles and use this information to help identify favorable feeding conditions.