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The structure of the pallial, ectopic eye of Patinopecten yessoensis is described and shown to be of the typical pectinid form, located on the middle mantle fold. The cornea is, however, a tall epithelium and, with the lens, forms a Cartesian oval, unlike the lens alone in other pectinids which functions to counter spherical aberration. The basal cell layer beneath the retina and argentea is poorly pigmented, although this may be countered by the fact that the optic tentacle epithelium itself is apically heavily pigmented.

It is generally assumed that pectinid pallial eyes, being able to perceive a moving image, are used to warn of vicinal predators, resulting in swimming. However, this does not seem to be the case; although crabs and starfish, in particular, are known predators of scallops, which respond by swimming to the latter, they do so (usually) on receipt of mechanical and chemical stimuli, not visual. Some scallop species live in seagrass beds and there is evidence that these may afford protection. Scallops are visually attracted to the waving fronds.

Scallops may also make relocation movements and, in more specialized taxa, e.g. species of Amusium, can swim for several metres. The visual sense of scallops is poorly understood and although pallial eyes may have developed early in the ancestry of the Pectinidae, as an antipredation sense organ, they may, in the descendants, now have a different, but perhaps related, function, although what this is, is unknown. Pectinid pallial eyes may improve the efficiency of photon capture in low light intensity subtidal habitats but for what purpose is unclear, since even if they are functioning as optical ‘burglar alarms’, what could they see in such a situation, especially as there is no brain to formulate an image?

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