Meiofauna and nematode community composition in a hydrothermal vent and deep-sea sediments in the Central Indian Ridge.

The hydrothermal ecosystem is very unusual, yet little research has so far been conducted on meiofauna in hydrothermal zones. To identify the communities of both meiofauna and nematodes around the Onnuri Vent Field (OVF), we collected sediment from a hydrothermal zone in the OVF and deep-sea (DS) sediments (non-vent) outside the OVF. Sampling was conducted at seven stations using multiple corers on the Research Vessel ISABU in June 2018 and June-July 2019. The average densities of meiofauna ± standard deviation ranged from 21.7 ± 5.2 to 122.3 ± 45.0 individuals/10 cm2. The structure of the meiofaunal community differed between the OVF and DS. The two most dominant groups of meiofauna in both environments were nematodes and harpacticoids. Statistical analyses showed significant differences in the structure of the nematode community between OVF and DS. We also found that the richness, evenness, and diversity of nematodes in the OVF were lower than those in the DS.

Effect of increased pCO2 in seawater on survival rate of different developmental stages of the harpacticoid copepod Tigriopus japonicus.

The rapid increase in carbon dioxide levels in seawater is causing ocean acidification and is expected to have significant effects on marine life. To explore the ability of the harpacticoid copepod Tigriopus japonicus to adapt to an increased concentration of dissolved carbon dioxide (CO2) in seawater, we compared the survival rates of adult and nauplius stages at 400, 1000, and 1550 ppm pCO2 over a 14-day period. The survival rate of T. japonicus dramatically decreased over time with increase in pCO2 concentration. At 1550 ppm, the survival rate showed a decrease of more than 20% at the end of the experimental period over that at 400 ppm. Furthermore, the survival rate decreased by a greater amount at all concentrations in nauplii than in adults, with a greater effect in wild-collected specimens than in culture-derived individuals. The results suggest that future ocean acidification may negatively influence the sustainability of T. japonicus and thus may eventually influence benthic ecosystems.

Effect of the Hebei Spirit oil spill on intertidal meiofaunal communities in Taean, Korea.

In December 2007, approximately 10,900tons of oil from a crude carrier spread rapidly onto the coast of South Korea. We studied the effects of oil on meiofauna by comparing two contaminated intertidal sites with an uncontaminated site. During 2008-2009, the density of meiofauna fluctuated among the contaminated sites but did not vary by season. Seasonal changes in density were observed at contaminated sites 3years after the oil accident. Meiofauna appeared to be more sensitive to oil pollution stress at the more heavily contaminated site than at the less contaminated site. CLUSTER analysis showed that meiofauna communities in the 3years immediately following the accident significantly differed from those sampled later. A non-metric multidimensional scaling analysis showed that nematode species composition in the first month after the accident significantly differed from those sampled later. Long-term monitoring is needed to assess the effects of oil on the meiofaunal community.