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The water flea Daphnia magna is a small freshwater planktonic animal in the Cladocera. In this study, we assembled the genome of the D. magna NIES strain, which is widely used for gene targeting but has no reported genome. We used the long-read sequenced data of the Oxford nanopore sequencing tool for assembly. Using 3,231 genetic markers, the draft genome of the D. magna NIES strain was built into ten linkage groups (LGs) with 483 unanchored contigs, comprising a genome size of 173.47 Mb. The N50 value of the genome was 12.54 Mb and the benchmarking universal single-copy ortholog value was 98.8%. Repeat elements in the D. magna NIES genome were 40.8%, which was larger than other Daphnia spp. In the D. magna NIES genome, 15,684 genes were functionally annotated. To assess the genome of the D. magna NIES strain for CRISPR/Cas9 gene targeting, we selected glutathione S-transferase omega 2 (GST-O2), which is an important gene for the biotransformation of arsenic in aquatic organisms, and targeted it with an efficient make-up (25.0%) of mutant lines. In addition, we measured reactive oxygen species and antioxidant enzymatic activity between wild type and a mutant of the GST-O2 targeted D. magna NIES strain in response to arsenic. In this study, we present the genome of the D. magna NIES strain using GST-O2 as an example of gene targeting, which will contribute to the construction of deletion mutants by CRISPR/Cas9 technology.

The copepod Tigriopus japonicus has been widely used as an experimental species in the field of ecotoxicology. We have sequenced and assembled the whole genome of T. japonicus with comparative analysis of gene families that represent detoxification phases in two additional public genomes of Tigriopus spp., namely, T. californicus and T. kingsejongensis. The total length of the T. japonicus assembled genome was 196.6 Mb with an N50 value of 10.65 Mb and consisted of 339 scaffolds and 25,143 annotated genes. The detoxification gene families encoding cytochrome P450s (CYP450s), glutathione S-transferases (GSTs), and ATP-binding cassette (ABC) proteins in Tigriopus spp. have shown species-dependent diversity in several gene sets, suggesting that these genes have undergone a species-specific expansion to increase their fitness to different marine habitats and environmental pressures. Our study will provide a better understanding of the detoxification system in Tigriopus spp. and will contribute to various areas of research, including ecotoxicology.

Autophagy is a 'self-eating' system that regulates the degradation of cellular components and is involved in various biological processes including survival and development. However, despite its crucial role in organisms, the regulatory mechanism of autophagy remains largely unclear, particularly in invertebrates. In this study, conserved autophagy in the rotifer Brachionus koreanus in response to cadmium (Cd) exposure was verified by measuring acidic vesicle organelles using acridine orange (AO) and neutral red (NR) staining, and by detecting LC3 I/II on Western blot and immunofluorescence. We also demonstrated activation of p38 mitogen-activated protein kinase (MAPK) in response to Cd-induced oxidative stress, leading to the induction of autophagy in B. koreanus. This was further verified by analysis of MAPK protein levels and immunofluorescence of LC3 I/II after treatment with reactive oxygen species (ROS) scavengers and inhibitors specific to MAPKs. We propose a p38 MAPK-mediated regulatory mechanism of autophagy in B. koreanus in response to Cd-induced oxidative stress. This study will contribute to a better understanding of autophagic processes in invertebrates and its modulation by environmental stressors.

To evaluate the adverse effects of MeHg on the rotifer Brachionus koreanus and the copepod Paracyclopina nana , we assessed the effects of MeHg toxicity on life parameters (e.g. growth retardation and fecundity), antioxidant systems, and mitogen-activated protein kinase (MAPK) signaling pathways at various concentrations (1 ng/L, 10 ng/L, 100 ng/L, 500 ng/L, and 1000 ng/L). MeHg exposure resulted in the growth retardation with the increased ROS levels but decreased glutathione (GSH) levels in a dose-dependent manner in both B. koreanus and P. nana . Antioxidant enzymatic activities (e.g. glutathione S -transferase [GST], glutathione reductase [GR], and glutathione peroxidase [GPx]) in B. koreanus showed more positive responses compared the control but in P. nana , those antioxidant enzymatic activities showed subtle changes due to different no observed effect concentration (NOEC) values among the two species. Expression of antioxidant genes (e.g. superoxide dismutase [ SOD ], GSTs, glutathione peroxidase [ GPx ], and catalase [ CAT ]) also demonstrated similar effects as shown in antioxidant enzymatic activities. In B. koreanus , the level of p-ERK was decreased in the presence of 1000 ng/L MeHg, while the levels of p-ERK and p-p38 in P. nana were reduced in the presence of 10 ng/L MeHg. However, p-JNK levels were not altered by MeHg in B. koreanus and P. nana , compared to the corresponding controls. In summary, life parameters (e.g. reduced fecundity and survival rate) were closely associated with effects on the antioxidant system in response to MeHg. These observations provide a better understanding on the adverse effects of MeHg on in vivo life parameters and molecular defense mechanisms in B. koreanus and P. nana .

To examine the toxic effects of multi-walled carbon nanotubes (MWCNTs) in the marine environment, we first exposed the monogonont rotifer (Brachionus koreanus) to MWCNTs in the presence of copper. The acute toxicity of copper decreased significantly with a decrease in copper bioavailability resulting from MWCNT exposure. Furthermore, we examined the effects of MWCNT exposure on reproductive capacity, population growth rate, growth patterns, antioxidant systems, and mitogen-activated protein kinase (MAPK) activation. Reproductive capacity, population growth rate, and body growth rate were significantly suppressed in B. koreanus in response to 1.3-4mg/L MWCNT exposure. Furthermore, MWCNTs induced the generation of reactive oxygen species (ROS) and decreased the antioxidant enzymatic activities of catalase (CAT) and glutathione reductase (GR). However, the enzymatic activity of glutathione S-transferase (GST) was up-regulated after a 24 h-exposure to 100mg/L MWCNTs. Exposure to 100mg/L MCWNTs induced extracellular signal-regulated kinase (ERK) activation in B. koreanus, suggesting that p-ERK may mediate the adverse effects of MWCNTs in B. koreanus via the MAPK signaling pathway. Our results provide insight into the mechanistic basis of the ecotoxicological effects of MWCNTs in the marine environment.