The nuclear 18S-rRNA gene has been used as a metabarcoding marker in massively parallel sequencing (MPS)-based environmental surveys for plankton biodiversity research. However, different hypervariable regions have been used in different studies, and their utility has been debated among researchers. In this study, detailed investigations into 18S-rRNA were carried out; we investigated the effective number of sequences deposited in international nucleotide sequence databases (INSDs), the amplification bias, and the amplicon sequence variability among the three variable regions, V1-3, V4-5 and V7-9, using in silico polymerase chain reaction (PCR) amplification based on INSDs. We also examined the primer universality and the taxonomic identification power, using MPS-based environmental surveys in the Sea of Okhotsk, to determine which region is more useful for MPS-based monitoring. The primer universality was not significantly different among the three regions, but the number of sequences deposited in INSDs was markedly larger for the V4-5 region than for the other two regions. The sequence variability was significantly different, with the highest variability in the V1-3 region, followed by the V7-9 region, and the lowest variability in the V4-5 region. The results of the MPS-based environmental surveys showed significantly higher identification power in the V1-3 and V7-9 regions than in the V4-5 region, but no significant difference was detected between the V1-3 and V7-9 regions. We therefore conclude that the V1-3 region will be the most suitable for future MPS-based monitoring of natural eukaryote communities, as the number of sequences deposited in INSDs increases.

Monitoring zooplankton communities is important to understand dynamics in marine ecosystems. However, it is difficult to identify cryptic species and immature stages of zooplankton using morphological classification, which is time-consuming and requires high skill levels. Here, we conducted a metagenetic analysis of the 18S region in 101 zooplankton samples collected weekly throughout 2014 and 2015 at the Okhotsk Tower in Mombetsu, Hokkaido, Japan, and compared the results of this analysis with those provided by morphological analysis. The metagenetic analysis detected 561 molecular taxonomic units (MOTUs), whereas the morphological analysis detected 201 taxonomic groups. Zooplankton communities were dominated by copepods throughout the sampling period; however, non-copepod taxa, which comprised high proportions of both MOTUs (mean 51.1%) and sequence reads (mean 19.1%), were also important. Cryptic diversity detected by the metagenetic analysis was primarily driven by Copepoda and by the larvae of benthic taxa such as Bivalvia, Gastropoda, and Polychaeta. Community structure and diversity varied between periods of warm and cold water, indicating strong correlations with water temperature and thus seasonality. Furthermore, metagenetic analysis revealed detailed seasonal changes in dominant taxa, including larval stages of metazoans with high taxonomic resolutions; these included commercially important organisms such as Japanese scallops. The metagenetic analysis revealed that changes in both water mass and bentho-pelagic interactions sustain ecosystems rich in zooplankton diversity in this area. Metagenetic analysis provides novel insight into zooplankton diversity, and generates massive sequence data that may be used in future research; thus, it is considered an effective tool for monitoring zooplankton communities.

No abstract

No abstract

To evaluate the temporal changes in zooplankton size spectra, optical plankton counter (OPC) measurements were made of high-frequency time-series zooplankton samples collected at approximately 3.5-day intervals in Mombetsu Harbour, which is located in the southern Okhotsk Sea, from January to December 2011. Based on biomasses of 47 equivalent spherical diameter (ESD) size classes binned at 0.1 mm intervals across 0.35-5 mm, the Bray-Curtis similarity index separated the zooplankton community into six groups (A-F). The occurrence of each group was separated seasonally. Thus, groups A and B were observed during the ice-covered season and summer season, respectively. During March and June, groups C-F were observed. Their occurrence varied in the short term in relation to the exchange of water masses. Groups A and C, which were observed from January to April, showed flatter normalized biomass size spectra (NBSS) slopes (-0.85-1.1), which indicate low productivity. In contrast, the other groups showed steeper slopes (-1.31-1.52) from May to December, with high productivity. Throughout the year, the frequency of highly productive groups occurred at a high level (95.2%). Although the seasonal variability in zooplankton size and productivity in Mombetsu Harbour was mainly governed by water 3 mass exchanges, the productivity was continuously high throughout nearly all of the 31 one-year study period.

No abstract

Interannual variability in growth of walleye pollock, Theragra chalcogramma, was examined. Adult walleye pollock were collected from the central Bering Sea (Aleutian Basin) from 1978 to 1999. Average fork lengths were found to be approximately 47 cm during the 1970–80s, this increased to 56 cm in the late 1990s. Age was determined for 4805 individuals using the otolith break and burn method. Ages ranged from 5–23 years and the year classes of 1978 and 1989 were dominant in the 1980s and the 1990s, respectively. Fish had significantly larger length‐at‐age in the 1990s compared to the 1970–80s, and interannual variability in age–length relationship was clearly observed. Taking into consideration a recent decrease of the walleye pollock biomass in the central Bering Sea, density‐dependent growth was supported as one possibility of the growth variability. At the same time, we could not rule out the possibility that oceanographic variability affected the growth of walleye pollock in the area.

The whale shark Rhincodon typus is the world's largest fish and it occurs in tropical, subtropical and warm temperate waters. Here, the northernmost record of R. typus is reported, when it was found in the Sea of Okhotsk for the first time. This occurrence can be explained by the unusually high sea surface temperature during the summer of 2012.