DNA barcoding based on a fragment of the cytochrome c oxidase subunit I (COI) gene in the mitochondrial genome is widely applied in species identification and biodiversity studies. The aim of this study was to establish a comprehensive barcoding reference database of fishes in the Taiwan Strait and evaluate the applicability of using the COI gene for the identification of fish at the species level. A total of 284 mitochondrial COI barcode sequences were obtained from 85 genera, 38 families and 12 orders of fishes. The mean length of the sequences was 655 base pairs. The average Kimura two parameter (K2P) distances within species, genera, families, orders and classes were 0.21%, 6.50%, 23.70% and 25.60%, respectively. The mean interspecific distance was 31-fold higher than the mean intraspecific distance. The K2P neighbor-joining trees based on the sequence generally clustered species in accordance with their taxonomic classifications. High efficiency of species identification was demonstrated in the present study by DNA barcoding, and we conclude that COI sequencing can be used to identify fish species.

During this research, the average surface temperature, salinity, dissolved oxygen, and pH were 24.65 ± 1.53 (°C), 34.21 ± 0.07 (PSU), 6.85 ± 0.18 (mg/L), and 8.36 ± 0.03, respectively. Based on these environmental parameters, stations were arranged into three groups. Group A represents stations located around Keelung Island with the relative highest average dissolved oxygen, lowest average temperature, and pH values. Instead, the lowest average dissolved oxygen and highest average temperature, salinity, and pH values were recorded at the offshore stations. Keelung Island area was charged by cold water masses, which were driven by the Northeast monsoon, and stations in group C were affected by the Kuroshio Current. Kueishan Island area was mainly affected by mixed water masses resulting from the Kuroshio intrusion and monsoon-derived cold water. In this study, a total of 108 copepod species were identified, with an average abundance of 774.24 ± 289.42 (inds. m⁻³). Most species belong to the orders Calanoida and Poecilostomatoida, with an average relative abundance (RA) of 62.96% and 30.56%, respectively. Calanoid copepodites were the most dominant group, with a RA of 28.06%. This was followed by Paracalanus aculeatus, with a RA of 18.44%. The RA of Clausocalanus furcatus and Canthocalanus pauper was 4.80% and 3.59%, respectively. The dominant species P. aculeatus, C. pauper, Paracalanus parvus, and Temora turbinata were positively correlated with dissolved oxygen and negatively correlated with temperature in the surface waters. pH showed a negative correlation with P. parvus and T. turbinata, while the temperature was negatively correlated with these two dominant species. Indicator species were selected by an indicator value higher than 50%. Temora turbinata, Calanopia elliptica, C. pauper, Euchaeta concinna, Temora discaudata, Acartia pacifica, Macrosetella gracilis, Corycaeus speciosus, and P. parvus were considered as monsoonal cold water indicator species in Group A. Indicator copepod species for the Kuroshio Current were Farranula concinna, Copilia mirabilis, Candacia aethiopica, Corycaeus agilis, Farranula gibbula and Acrocalanus monachus in the study area. Paracandacia truncata, Oncaea clevei, P. aculeatus, and Centropages furcatus were considered suitable indicators for mixed water masses.

Taxonomic information is important to fisheries management and conservation. In this study, we collected 95 specimens of larval fish from Hawaiian waters and a total of 92 mitochondrial COI barcode sequences were obtained, representing 20 species, 18 genera and 12 families. The determined average Kimura-2-parameter distances within species, genera, families, and orders were 0.72%, 25.99%, 26.30%, and 27.50%, respectively, and the mean interspecific distance was found to be 36-fold higher than the mean intraspecific distance. Phylogenetic tree analysis showed that individuals belonging to the same species were clustered together and could be clearly distinguished. The findings of this study can make a valuable contribution to our knowledge of the diversity and dispersal of larval fish in Hawaiian waters.

The abundance of adult males, females, and copepodites, and sex ratio of a Temora turbinata population and seawater hydrology were studied from 2018 to 2019 in waters off northeast Taiwan, northwest Pacific Ocean. The hydrological parameters showed significant differences between sampling months caused by interactions of Kuroshio, East China Sea water, and the China Coastal Current. The highest average abundance was recorded from the June 2018 cruise with 2903.92 ± 3499.47 (inds. m⁻³), followed by a cruise in June 2019 with an average abundance of 1990.64 ± 1401.55 (inds. m⁻³). The sex ratio ranged between 0.25 and 2.33; the records were significantly higher in samples of June 2018 than during other sampling cruises (one-way ANOVA). The spatiotemporal distribution of T. turbinata showed a clear pattern of seasonal changes among sampling stations and distribution zones. Abundance of females and copepodites correlated significantly positive (r = 0.755, p < 0.001), whereas sex ratio (r = 0.247, p = 0.119) did not correlate significantly. The present study revealed that the abundance of T. turbinate was highest in June and was positively correlated with seawater temperature; furthermore, this is the first time the in situ sex ratio of T. turbinata in western Pacific waters has been reported.

The Ovulidae (Gastropoda, Cypraeoidea) is a family of small to medium Mollusca in the order Littorinimorpha, and Sandalia is a very small genus containing only three extant species. In the present study, 132 specimens of Ovulidae were collected, belonging to seven genera and nine species, including 54 Sandalia bridgesi and three Sandalia triticea individuals. The cytochrome c oxidase I gene, 16S rRNA, and ITS1-5.8S-ITS2 sequences were obtained from all specimens and compared with sequences downloaded from GenBank to calculate genetic distances and construct phylogenetic trees. The sequences of S. bridgesi and S. triticea exhibited a high degree of similarity, and S. bridgesi does not form a separate clade, supporting the proposal that S. bridgesi should be synonymised with S. triticea.