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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 ...

Two types of multi-cellular eyes have been identified in the Bivalvia. Paired cephalic eyes occurring internally above the anterior end of the ctenidia are seen only in representatives of the Arcoidea, Limopsoidea, Mytiloidea, Anomioidea, Ostreoidea, and Limoidea. These eyes, comprising a pit of photo-sensory cells and a simple lens, are thought to represent the earliest method of photoreception. Many shallow-water marine, estuarine, and freshwater bivalves also possess simple photoreceptive cells in the mantle that enable them to respond to shadows. In some other marine, shallow-water taxa, however, a second type of more complex photoreceptors has evolved. These comprise ectopic pallial eyes that can be divided into three broad categories, in terms of their locations on the (i) outer mantle fold in representatives of the Arcoidea, Limopsoidea, Pterioidea, and Anomioidea, (ii) middle fold in the Pectinoidea and Limoidea, and (iii) inner fold in the Cardioidea, Tridacnoidea, and Laternulidae (Ano...