The genetic structure and demography of the yellowbelly threadfin bream, Nemipterus bathybius, in the northern South China Sea were examined using the mitochondrial DNA cytochrome b gene (1141 bp). High levels of haplotype and nucleotide diversities (0.98 and 5.26 × 10⁻³, respectively) showed that all populations exhibited a high level of genetic diversity. Analysis of molecular variance (AMOVA), F_ST statistics, and haplotype networks suggested the absence of significant genetic differentiation along the coast of the northern South China Sea. Although the results suggested that the lack of differentiation within the population structure of N. bathybius was shaped by ocean currents, our results also showed that the Qiongzhou Strait limited their migration between Beibu Gulf and the northern South China Sea. Neutrality tests and mismatch distributions indicated population expansion, but the Bayesian skyline plots and approximate Bayesian computation approaches suggested that the population sizes recently contracted. The diversification of multiple stocks, which were induced by two ocean current systems, contributed to these discordant results. Although these analyses of demographic history revealed no evidence for recent population bottlenecks, the population demography needs to be evaluated further.

Four Trichiurus species, T. japonicus, T. lepturus, T. nanhaiensis, and T. brevis, from the coasts of the China Seas, have been identified and their entire mitochondrial genomes (mitogenomes) have been sequenced by next-generation sequencing technology. A comparative analysis of five mitogenomes was conducted, including the mitogenome of T. gangeticus. The mitogenomes contained 16.568–16.840 bp and encoded 36 typical mitochondrial genes (13 protein-coding, 2 ribosomal RNA-coding, and 21 transfer RNA-coding genes) and two typical noncoding control regions. Although tRNA^Pro is absent from Trichiurus mitogenomes, when compared with the 22 tRNAs reported in other vertebrates, the gene arrangements in the mitogenomes of the studied species are consistent with those in most teleost mitogenomes. The full-length sequences and protein-coding genes (PCGs) in the mitogenomes of the five species had obvious AT biases and negative GC skew values. Our study indicate that the specimens in the Indian Ocean are neither T. lepturus nor T. nanhaiensis but they are T. gangeticus; the Trichiurus species composition in the Indian Ocean is totally different from that in Pacific and Atlantic oceans; there are at least two Trichiurus species in Indian Ocean; and the worldwide systematics and diversity of the genus Trichiurus need to be reviewed.

This study constitutes the first record of the Por’s goatfish, Upeneus pori Ben-Tuvia et Golani, 1989, from the South China Sea. This fish had formerly only been reported in the western Indian Ocean. Six specimens of U. pori collected from the South China Sea were identified through comparisons of morphology and molecular analysis. Upeneus pori and Upeneus tragula Richardson, 1846 are very similar. The difference is that the former has seven spines in the first dorsal fin, lacks any spots or blotches on its body, and exhibits white or creamy white barbels. The latter has eight spines in the first dorsal fin, exhibits black spots on its body, and displays orange-colored barbels, along with different caudal fin patterns. Pairwise genetic distance computation demonstrated that U. pori exhibits a very low genetic distance from sequences of the other three recorded U. pori species found in the type locality of Israel, Red Sea, as documented in GenBank. These findings provide compelling evidence of the Por’s goatfish’s presence in the South China Sea.