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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), FST 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.

The population genetic structure and historical demography of the spotted scat Scatophagus argus along the coast of the South China Sea were examined on the basis of a 1141-bp segment of the mitochondrial DNA cytochrome b gene to resolve difficulties in artificial breeding in Taiwan. In all, 213 samples were collected from 15 locations, and 75 haplotypes were obtained. The haplotypes were divided into two sympatric lineages. The genetic distance between these two lineages was 2.55%. The observed genetic structures are the result of complex interactions of historical events and oceanographic currents. During glaciations, sea level fluctuations played major roles in isolation, and changes in environmental factors affected diversification. The observed genetic patterns imply that these two lineages are not only genetically distinct, but also ecologically distinct populations. Such divergence may contribute to the development of reproductive isolation. These results suggest the existence of cryptic species within S. argus. We considered it possible that many populations from Taiwan Island were introduced from South-east Asia and that the populations in the Gulf of Tonkin originated from these introduced populations from Taiwan. This study offers important resources for the further study of conservation genetics, hybridisation and speciation.

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Hemigymnus melapterus belongs to the family Labridae, which inhabit in coastal and continental shelf waters. The entire mitochondrial genome of H. melapterus is 16,527 base pairs (bp) in length and contained 13 protein-coding genes, two rRNA genes, and 22 tRNA genes. The overall base composition is 27.56% A, 25.58% T, 30.02% C, and 16.85% G, showing AT-rich feature (53.14%). Phylogenetic analysis based on 13 protein-coding genes shows the H. melapterus has the closest evolutionary relationship with Stethojulis strigiventer. This work provides valuable genome variation information, which will be useful for phylogenetic analysis and population genetics research.

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 tRNAPro 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.

Summary The length–weight relationships were determined for three fish species: Histiopterus typus (Temminck & Schlegel, 1844), Branchiostegus argentatus (Cuvier, 1830), and Ilisha elongata (Anonymous [Bennett], 1830). Samples were collected from the South China Sea with commercial trawls in June 2016 using trawl nets. Prior to this study the length–weight relationships for these three species were unknown.