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.

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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.

Ophichthidae is the most ecologically diverse family of the order Anguilliformes; however, high morphological similarity and a long larval stage make precise identification difficult. The aim of the present study was to examine the potential for barcoding of ophichthid fishes. A total of 81 mitochondrial cytochrome C oxidase subunit I (COI) barcode sequences were obtained from 23 species and 12 genera of ophichthid fishes collected in the Taiwan Strait. The average Kimura two-parameter distances within species, genera, and families were 0.48%, 18.25%, and 18.79%, respectively; thus, the mean interspecific distance was 31-fold higher than the mean intraspecific distance, which demonstrates good efficiency in DNA barcoding. The sequence divergence among species also varied substantially from 4.60% to 25.90%. This evaluation demonstrates the high efficiency of DNA barcoding for species identification to facilitate research on the diversity and conservation of ophichthid fishes.