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 (Anomalodesmata). Eyes do not occur in deep-sea bivalve taxa. Where ectopic pallial eyes occur, they measure amounts of light and integrate intensities from different directions, thereby supplying information to the individual possessing them about the distribution of light in its immediate environment. This does not mean, however, despite broad, phylogenetically related advances in pallial eye complexity, that any bivalve can perceive an image. A revised picture of the independent evolution of ectopic pallial eyes in the Bivalvia is provided. In bivalves, pallial fold duplication has resulted in improvements to the peripheral visual senses, albeit at different times in different phylogenies and on different components of the mantle margin. This has been achieved, it is herein argued, through: (i) selective gene-induced ectopism; (ii) pigment cup evagination in Category 1 eyes; (iii) invagination in Categories 2 and 3; and (iv) natural selection. The invaginated distal retina in representatives of the Pectinidae and Laternulidae provides the potential for image formation and the detection of movement. In the absence of optic lobes capable of synthesizing such information, however, these complex eyes must await matching cerebral sophistication.
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