To understand the effects of variable water masses in the northeastern East China Sea (Korea South Sea), planktonic copepods were seasonally sampled. Out of a total of 106 copepod species, 85 were oceanic warm-water species, and the number of species varied in summer, autumn, spring, and winter. The study area was divided into two or three regions according to the degree of influence of the water masses. Canthocalanus pauper, Clausocalanus furcatus, Oithona plumifera, Oncaea venella, Oncaea venusta, and Paracalanus aculeatus showed a positive correlation with water temperature and salinity and were indicator species of warm currents. Calanus sinicus, known as an indicator species of the Yellow Sea Bottom Cold Water, showed a high abundance and occurrence ratio in the western sea of the study area from spring to autumn. Moreover, Acartia pacifica indicated the extension of coastal waters to offshore areas. Several oceanic warm-water species (A. danae, Centropages gracilis, Labidocera acuta, Rhincalanus nasutus, and Temoropia mayumbaensis) were considered indicator species of the Taiwan Warm Current. Our results suggest that the spatiotemporal distribution patterns of indicator species are partly explained by different water masses.

The recent accelerated ocean acidification and global warming caused by increased atmospheric carbon dioxide may have an impact on the physiology and ecology of marine animals. This study was conducted to determine the egg production rate (EPR) and hatching success (EHS) of Acartia ohtsukai in response to the combined effects of an increase in temperature and a lower pH. Acartiaohtsukai with fresh surface seawater were collected in the northwestern Yeoja Bay of Korea in September 2017. The temperature and pH conditions applied included two different pH levels (representing the present: 7.9 and the future: 7.6) and three temperature values (26 °C, 28 °C, and 30 °C). In the pH 7.9, EPR significantly increased with increased temperature, but in pH 7.6, it significantly decreased as the temperature increased. EHS was lower in pH 7.6 than in pH 7.9. These results suggest that changes in the marine environment due to global warming and ocean acidification may affect Acartia populations and cause overall fluctuations in copepods of the genus Acartia.

Acartia erythraea, a copepod species commonly found in the summer, was sampled weekly using a conical net, in Gamak Bay (southern Korea) from 19 July to 14 November 2013. Egg production rate (EPR) and hatching success rate (HS) were estimated weekly during the study period, for egg types with distinct surface morphologies, i.e. smooth or spiny. EPR ranged from 1.0 to 25.0 eggs female⁻¹ day⁻¹ with a mean of 12.0 eggs female⁻¹ day⁻¹. The weight-specific growth rate (WSGR) ranged from 0.01 to 0.19 day⁻¹ with a mean value of 0.09 day⁻¹. Of these, all the eggs were smooth until August, whereas spiny eggs were produced from September, and the spiny eggs contributed 88% of the total eggs in November. Both egg types hatched within 12 to 48 h, and the average HS was 80% or higher. The EPR and WSGR were significantly related to water temperature but weakly related to chlorophyll-a concentration. Contrary to previous studies, our findings suggest that it is impossible to determine whether an egg is subitaneous or diapause based on the egg surface structure. Spines on the surface of A. erythraea eggs in Gamak Bay could be an adaptation to seasonal variations and a rapidly changing coastal environment.

Research to understand the distribution and density of jellyfish is actively being conducted using training ships, but this is hindered by the high cost of manpower and the limitations of the irradiation area. Unmanned aerial vehicles (UAVs or drones), however, provide cost-effective means for assessing marine animal populations. Therefore, we tested the application of UAVs in estimating jellyfish density and probed the altitude-dependent suitability of these devices. We analyzed images obtained by a drone as well as by manual counting and used ImageJ to measure the density of Nemopilema nomurai off Sang-Chuja Island, Jeju, South Korea. Analysis of the image obtained at altitudes of 5–120 m allowed for the identification of 2–173 individuals, while 1.49–9.09 individuals were identified per 100 m². Jellyfish density data measured by manual count and by ImageJ did not show any difference below 90 m; however, a difference was presented at altitudes of 100 m (98%) and 120 m (95%). These results demonstrate the potential of drones for jellyfish monitoring and recommend an optimal altitude for observation.

This study was carried out to elucidate the causes of massive outbreaks of Aurelia coerulea in Geoje Bay, Korea, from November 2022 to October 2023. Adult medusae consistently spawn with planulae, and the populations of A. coerulea in Geoje Bay could be categorized into current-year and overwintering populations. The current-year population began with the emergence of ephyrae in February and grew until October, while the overwintering population comprised a mixture of surviving current-year population and additional individuals that joined during the warm season. The size of the planulae are significantly larger than the annual average during the cold season. These results appear to be the energy accumulation of planulae for polyp formation under low water temperatures. Planulae form polyps within a temperature range of 5–25 °C, suggesting the possibility of year-round polyp recruitment. In Geoje Bay, the highest appearance rate of A. coerulea was in April (8.71 ± 12.5 ind. m⁻³), with ephyrae experiencing higher growth rates up to the young medusa stage. However, from April, a decline in zooplankton biomass resulted in reduced growth rates in adults, indicating that jellyfish growth was primarily regulated by food availability. Additionally, submersed oyster shells in oyster farms served as the main habitat for jellyfish polyps. A. coerulea populations were also characterized by the continuous spawning of planulae throughout the year. In conclusion, this study suggests that stable polyp habitats, abundant food supply during the initial developmental period of the population, and suitable ranges of water temperature were significant factors inducing the massive outbreak of A. coerulea in Geoje Bay, Korea.

In this study, we investigate the distribution of calanoid copepod eggs in the sediments of Masan Bay (South Sea, Korea), in which hypoxic conditions occur every summer. In August (2011), hypoxia was observed at all stations, while normoxia was observed in April (2012). The pH and hydrogen sulphide concentration at the sediment-water interface in the inner bay during August were 7.7 and >20 mol L⁻¹, respectively, and there was no significant difference between the stations in April. The abundance of eggs in the sediment ranged from 0.69 to 1.49 × 10⁶ eggs m⁻² in August, and from 0.59 to 1.08 × 10⁶ eggs m⁻² in April. Notably, the proportion of abnormal eggs was high (a maximum of 77.1%) in August, and a uniform distribution of normal eggs (>80%) was observed in April. A generation of abnormal eggs hatched with deformed nauplii in Masan Bay during the summer, and a failure to hatch was likely due to the high H₂S concentration and low pH caused by hypoxia. In this paper, we discuss the effects of hypoxia and seabed environments on the hatching success of calanoid copepod eggs.