Comparison of Morphological Differences Among Four Species of the Genus Thymallus

The genus Thymallus, belonging to the Salmoniformes, Salmonidae and Thymallinae, is distributed in three main areas in China, namely, the Irtysh River Basin, the Heilongjiang River Basin and the Yalu River Basin. Research has shown that the diversity of extant Thymallus in China is much higher than that in the vast European and North American regions, but the taxonomic results of some species are still controversial. Among them, those whose taxonomic status has been clarified and whose nomenclature is not controversial are T. arcticus, T. grubii and T. tugarinae, and the one that is still controversial is T. yaluensis. Named by Mori in 1928, T. yaluensis is mainly distributed in the Yalu River Basin and is geographically isolated from the T. arcticus, T. grubii and T. tugarinae. In 2007, a study on the comparison of countable traits of Thymallus found that the countable traits of the T. yaluensis were in the middle between the T. grubii and T. tugarinae, and the validity of the taxonomic status of T. yaluensis has been formally questioned ever since; in 2008, a study using mitochondrial DNA control region sequences to study the evolutionary relationships of fishes in the genus Thymallus pointed out that the T. yaluensis is synonymous with the T. grubii. In 2016, researchers used mitochondrial genome data to confirm once again that there was no difference between the T. yaluensis and the T. grubii. However, this conclusion does not seem to be accepted by the academic community, and in recent years, there are still some scholars who regard the T. yaluensis as an independent species. Morphological methods are the most commonly used methods in species identification, which can be categorized into traditional morphological method and framework measurement method. The traditional morphological method mainly analyzes the horizontal and vertical traits of the fish body, and it is mostly limited to the head and tail traits, which can't measure the whole surface of the fish body comprehensively. The framework measurement method, however, is based on a certain number of anatomical coordinate points and measures the entire fish body in multiple directions, which can provide a more comprehensive response to the morphological differences of the fish body. So far, the studies on the interspecific differences of Thymallus in China have mostly focused on the molecular biology level, while the morphological differences have been less studied, only in T. grubii and T. tugarinae. The rapid development of molecular marking technology provides an important auxiliary means for species classification, but the apparent morphological differences are still recognized as the most intuitive basis for species classification. In order to clarify the morphological differences of Thymallus in China, this study comprehensively used traditional morphological methods and framework measurement method, and took the T. grubii, T. arcticus, T. yaluensis and T. tugarinae as the research objects, Statistical analysis was conducted on their 10 segmentation characteristics and 32 standardized metric features. The results showed that: (1) there was no significant difference (P > 0.05) between all the subsection characteristics of T. yaluensis and T. grubii, and there were significant differences (P < 0.05) in more than five segmentation characteristics between T. grubii, T. arcticus and T. tugarinae. (2) The coefficient of variation analysis showed that except for caudal peduncle length/body length, the differences in morphological characters between T. yaluensis and T. grubii were at the level of differences between geographic populations within species. (3) The principal component analysis screened out eight principal components with eigenvalues greater than 1, and the cumulative contribution rate reached 81.170%, among which the first and second principal components with the highest contribution rate mainly explained the differences in trunk and head between Thymallus, respectively. The construction of scatter plots of the first and second principal components revealed that the individual crossover between T. yaluensis and T. grubii had a high degree and could not be distinguished individually, while T. tugarinae and T. grubii show a lower level of overlap and partial distinction. Only T. arcticus can be distinctly separated. (3) Discriminative analysis was used to screen out 14 standardized measurement features to establish the Fisher linear discriminant function, and the discriminative rate of individuals being correctly assigned to their respective groups was as high as 97.30%. (4) Cluster analysis showed that compared with T. grubii, T. yaluensis had the highest degree of similarity with it, followed by T. tugarinae, and T. arcticus had the lowest degree of similarity with T. grubii. The comprehensive analysis showed that: (1) T. yaluensis should not be designated as a valid species and its scientific name should be revised to T. grubii. (2) Morphological differences among T. grubii, T. arcticus and T. tugarinae are mainly reflected in their segmentation characteristics and the measurement characteristics of trunk and head. The results of this study will provide a reference for the study of species classification and evolutionary relationships of the genus Thymallus.