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Body symmetry in animals came very early as shown by the metazoan imprints from the Ediacaran period (600–543 million years ago). Even now, symmetry represents a sign of fitness.
Why, then, did the approximately 500 species of flatfish of the order Pleuronectiformes discard such a predominant characteristic primarily with the eye and orbit?
Flatfish, as represented by the sand dab, Citharychthys, on the cover, hatch from eggs as bilaterally symmetrical, transparent fry that live near the ocean surface more or less as plankton and have the same fusiform body shape as most other fish. In these animals, transparency, except for the pigment in the eye, is a camouflage technique and is common to many piscine species. High in the water column these young flatfish feed on smaller planktonic creatures until metamorphosis. Their metamorphosis is vastly different from other species of fish because they become flattened and compressed and simultaneously move to deeper water to become bottom dwelling. That alone would not be so unusual, but along with the bodily compression, one eye, with corresponding bony changes, migrates to the opposite side of the body to join …
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