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.

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.

Capitulum mitella is an important species in the intertidal ecosystem and in the economy in China. This study explored the genetic diversity and population structure of C. mitella using mitochondrial 16S ribosomal subunit (16S rRNA) gene sequences. The researchers sequenced a 527 bp fragment of 16S rRNA from 390 samples collected from six localities along Western Taiwan Strait and identified 45 distinct haplotypes with an average haplotype diversity (h) of 0.451 and an average nucleotide diversity (π) of 0.00124. The pairwise statistics and AMOVA indicated a lack of significant population structure. The results indicated a recent expansion of the population of C. mitella in the study area. This expansion, combined with the species’ high dispersal capabilities, limited physical barriers, and the influence of oceanic currents, may be the factors contributing to its current phylogeographic structure. This study provides particularly important information for managing and protecting the resources of C. mitella in Southern China.

A new species of jellyfish, Phyllorhiza yurena Chen, Hu & Xing sp. nov., is described from the East China Sea based on an integrative approach combining morphological and molecular data. Specimens were collected from coastal waters and cultured in the laboratory to observe their complete life cycle, including polyps, ephyrae, and medusae. Distinct morphological features such as the blue lappet stems in ephyrae, white warts on the exumbrella, and multiple short club-shaped appendages on the mouth arms distinguish the new species from its congeners. Phylogenetic analyses based on mitochondrial (COI, 16S) and nuclear (18S, 28S) gene markers confirmed its taxonomic distinctiveness, with COI and 16S markers providing the most resolution. The study highlights the importance of combining life history observations with molecular tools for accurate jellyfish taxonomy and provides a revised identification key for the genus Phyllorhiza. This discovery contributes to the understanding of jellyfish biodiversity in the East China Sea and offers a baseline for future ecological and biogeographic studies.