Reidentification of Decapterus macarellus and D. macrosoma (Carangidae) reveals inconsistencies with current morphological taxonomy in China

Decapterus macarellus and D. macrosoma are economically important pelagic fish species that are widely distributed in tropical and subtropical seas. The two species are often mistakenly identified due to their morphological similarities as described in the Chinese literature on fish identification. In this study, D. macarellus and D. macrosoma samples were collected in the Eastern Indian Ocean and the South China Sea and reidentified using morphological and DNA barcoding techniques. The characteristics that distinguish the two species primarily include the scute coverage of the straight portion of the lateral line (the most indicative characteristic for classification), the shape of the predorsal scaled area and its location relative to the middle axis of the eye, and the shapes of the posterior margin of the maxilla and the posterior margin of the operculum. The results revealed a large number of misidentified sequences among the homologous cytochrome oxidase (COI) sequences of the two species in the NCBI database and that the genus Decapterus may include cryptic species. In terms of genetic structure, the Sundaland has not blocked genetic exchange between D. macarellus populations in the South China Sea and the Eastern Indian Ocean, giving rise to a high level of genetic diversity. In this study, we made corrections to the Chinese classification standards for D. macarellus and D. macrosoma and the erroneous reference sequences in the NCBI database, thereby providing accurate reference points for the future exploration of cryptic species in the genus Decapterus.