Re-evaluation of the taxonomic status of three nominal species, Cynoglossus abbreviatus, C. trigrammus and C. purpureomaculatus in Cynoglossus (Pleuronectodei: Cynoglossidae)

The morphological characteristics of some species of the genus Cynoglossus are highly similar, making it difficult to identify and distinguish between species. For example, in the case of tongue soles with three complete lateral lines on the eye side, C. abbreviatus (Gray, 1834), C. trigrammus (Günther, 1862) and C. purpureomaculatus (Regan). There are three opinions on the validity and interspecies relationship of the three species: only C. abbreviatus as a valid species; C. abbreviatus and C. trigrammus as a valid species, and C. purpureomaculatus is a synonym of of C. trigrammus; all three species are valid species. In order to resolve this classification confusion, the 28 morphological characteristics of the specimens from the original descriptions of three tongue soles, type specimens measured in this study, and the specimens from the coast of mainland China were compared and the COⅠ Barcoding were amplified. The results show that values of eight meristic, 16 proportional characters and two descriptive features among the three species were consistent, containing and largely overlapping; only other two descriptive features showed difference. One is the relative size of the snout length to the length of the upper margin of the upper eye from the base of the dorsal fin, which in C. abbreviatus and C. purpureomaculatus is the continuous values (slightly longer, equal or slightly shorter), while significantly longer in C. trigrammus.The other difference is the coloration of the ocular-side body. For C. purpureomaculatus, the standard length is usually less than 200 mm, and there are irregular dark-brown speckles spread over the entire body; the standard length of C. abbreviatus is usually more than 250 mm, and the speckles disappear gradually with increasing of body length . But the cloud-shaped dark-brown blotches sparsely over the area between posterior head to anterior 3/4 of the body in C. trigrammus. The COⅠ genetic distances between C. abbreviatus and C. purpureomaculatus was 0.000-0.002, suggesting the differences are intra-specific, while that between these former two species and C. trigrammus were 0.141-0.145, suggesting inter-specific differences. Based on the above morphological and molecular results, the conclusion of this study is C. abbreviatus and C. trigrammus were affirmed as a valid species, and C. purpureomaculatus is a junior synonym of C. abbreviatus. The results of this study provide a scientific basis for the correct classification of Pleuronectiformes.