Molecular Identification of an 'Abnormal Tuna' Caught in the Taiwan Strait off Southwestern Taiwan

A female tuna of odd appearance, with the size of a Pacific bluefin tuna (Thunnus orientalis Temminck and Schlegel, 1844), anal fin count of bigeye tuna (Thunnus obesus Lowe, 1839), pectoral fins of yellowfin tuna (Thunnus albacares Bonnaterre, 1788), caught in waters west of the island of Xiaoliuqiu, Pingtung, Taiwan, received considerable media attention after its landing at Tungkang fishing port on 14 December 2020. This tuna has now been identified by molecular systematic means. Fishermen and fish merchants suggested it was a hybrid form, unique in living memory; however, it had matured ovaries and developing ova. To confirm whether this fish was a hybrid, we determined its parental species by analysing the cytochrome b gene (cyt b) and cytochrome c oxidase subunit 1 (COI) sequences of the mitochondrial DNA for maternal inheritance, and the internal transcribed spacer 1 (ITS1) gene sequence from the nuclear DNA to confirm both parents’ lineages. Genomic DNA was isolated from fast-skeletal muscle, and primers were designed based on the known sequences of conserved regions among tunas. According to cyt b and COI, the mother of the peculiar tuna was a Pacific bluefin tuna (T. orientalis), and the ITS1 sequence showed that both parents were of this species. We therefore conclude that despite the mixed morphological appearance, this abnormal tuna was a Pacific bluefin tuna, not a hybrid.