Morphological differences between two closely related East Asian flatfish species of the genus Pleuronichthys, with particular emphasis on growth-related changes

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Morphological differences, including growth-related changes, were examined in two closely related East Asian flatfish species, Pleuronichthys lighti and Pleuronichthys cornutus. Body measurements of both species indicated frequent growth-related proportional changes, head length, pelvic fin length (both fins), eye diameter (both eyes), inter-orbital ridge length and the lengths of both jaws all showing relative decreases with growth. On the other hand, whereas caudal fin length became relatively shorter and snout length relatively greater with growth in P. lighti, those proportions remained unchanged with growth in P. cornutus. Conversely, although body depth became relatively greater and the longest dorsal and anal fin rays relatively shorter with growth in P. cornutus, those proportions remained unchanged with growth in P. lighti. Counts of scales below the lateral line and gill rakers both tended to increase with growth in P. cornutus, but remained stable in P. lighti. New key characters to distinguish between the two species included ocular side pectoral fin length and the area ratio between the upper and lower eyes. The ocular side pectoral fin of P. cornutus was generally longer than in P. lighti, with plots of the fin proportions in the two species showing modest separation. In addition, because both the horizontal and vertical diameters of the upper eye of P. cornutus were generally greater than those of P. lighti, graphical representation of the relationship between the upper and lower eye areas showed almost complete separation of the two species.