PLEURONECTIFORM

also called flatfish, any member of the order Pleuronectiformes, which includes about 600 species of bony, oval-shaped, flattened fishes such as the flounder, halibut, and turbot. The pleuronectiforms are unique among fishes in being asymmetrical. They are strongly compressed, with both eyes on one side, while other fishes and vertebrates in general are bilaterally symmetrical. The asymmetry is believed to have evolved from a generalized, symmetrical percoid (sea bass) body pattern in a fish that habitually rested on its side. Larval flatfishes have an eye on each side of the head, but during a period of rapid body change (metamorphosis) one eye migrates to the other side of the head, after which the larvae settle to the bottom. Osteological changes resulting from the eye migration are responsible for the asymmetry in the flatfish skull. Additional reading J.T. Cunningham, A Treatise on the Common Sole, Solea vulgaris, Considered Both as an Organism and as a Commodity (1890), a monograph on the morphology, development, life history, and economics of this species, including a taxonomic review; C.L. Hubbs, Phylogenetic Position of the Citharidae, a Family of Flatfish, Misc. Publs. Mus. Zool. Univ. Mich., no. 63, pp. 538 (1945), presents a definition of the family Citharidae and phylogenetic relationships of the flatfish families; H.M. Kyle, Flat-fishes (Heterosomata), Rep. Danish Exped. Medit. 2:1150 (1913), a major treatise on the development of some flatfishes in the families Bothidae, Soleidae, and Cynoglossidae (illustrations of larvae included when known); and The Asymmetry, Metamorphosis and Origin of Flatfishes, Phil. Trans. R. Soc., Series B, 211:75129 (1921), a major paper on the origins of pleuronectiform metamorphosis; J.R. Norman, A Systematic Monograph of the Flatfishes (Heterosomata), vol. 1, Psettodidae, Bothidae, Pleuronectidae (1934, reprinted 1966), a classic on the taxonomy of flatfishes, the only paper dealing with these families on a worldwide basis; B. Bennet Rae, The Lemon Sole (1978).