Development of a promising fish model (Oryzias melastigma) for assessing multiple responses to stresses in the marine environment

Stranded heavy fuel oil exposure causes deformities, cardiac dysfunction, and oxidative stress in marine medaka Oryzias melastigma using an oiled-gravel-column system

Heavy fuel oil (HFO) stranded on the coastline poses a potential threat to the health of marine fish after an oil spill. In this study, an oiled-gravel-column (OGC) system was established to investigate the toxic effects of stranded HFO on marine medaka Oryzias melastigma. HFO 380# (sulfur content 2.9%) was chosen as one type of high sulfur fuel oil for acute toxicity tests. The marine medaka larvae were exposed to the OGC system effluents with oil loading rates of 0 (control), 400, 800, 1600, and 3200 µg HFO/g gravel for 144 h, respectively. Results showed that a prevalence of blue sac disease signs presented teratogenic effects, including decreased circulation, ventricular stretch, cardiac hemorrhage, and pericardial edema. Moreover, the treatments (800, 1600, and 3200 µg oil/g gravel) induced severe cardiotoxicity, characterized by significant bradycardia and reduced stroke volume with an overt decrease in cardiac output. Additionally, the antioxidant enzyme activities, including catalase (CAT), peroxidase (POD), and glutathione S-transferase (GST) were significantly upregulated at 800-3200 µg oil/g gravel except for a marked inhibition of CAT activity at 3200 µg oil/g gravel. Furthermore, significantly elevated protein carbonyl (PCO) levels were detected, suggesting that the organisms suffered severe protein oxidative damage subjected to the exposure. Overall, stranded HFO 380# exposure activated the antioxidant defense system (up-regulated POD and GST activities) of marine medaka and disrupted CAT activity, which could result in an oxidative stress state (elevated PCO levels) and might further contribute to cardiac dysfunction, deformities, and mortality.

Pectiniferosides A-J: Diversified Glycosides of Polyhydroxy Steroids Isolated from the Sea Star Patiria (=Asterina) pectinifera

To optimize the utilization of the sea star Patiria (=Asterina) pectinifera, which has demonstrated potential pharmaceutical properties in Chinese folk medicine, ten glycosides of polyhydroxy steroids, pectiniferosides A-J (1-10), were isolated and characterized. These compounds possess 3β, 6α, 8, 15α (or β), 16β-pentahydroxycholestane aglycones with sulfated and (or) methylated monosaccharides. The chemical structures of 1-10 were determined using NMR spectroscopy and HR-ESI-MS. The discovery of saponins with multiple substitution patterns in sea stars exemplified the molecular diversity of secondary metabolites in marine echinoderms. These compounds exhibited no embryotoxicity or teratogenicity at a concentration of 100 μM in a bioassay with marine medaka (Oryzias melastigma) embryos, implying that these compounds may not be ecologically toxic to marine fish embryos. In addition, none of the compounds exhibited significant cytotoxicity against five human cancer cell lines at 40 μM or anti-inflammatory activities at 50 μM, suggesting their potential for further structural optimization to enhance bioactivity. The research on the constituents of P. pectinifera provides a potential foundation for drug development and marine ecotoxicology.

Integrating Transcriptome and Metabolome Analyses Revealed Salinity Induces Arsenobetaine Biosynthesis in Marine Medaka (Oryzias melastigma)

Marine fish exhibit elevated levels of arsenobetaine (AsB), while the impact and underlying mechanism of salinity on AsB biosynthesis remain inadequately explored. In this study, marine medaka (Oryzias melastigma), typically inhabiting 30‰ high salinity, were gradually acclimated to low salinities of 20, 10, and 0‰. Following acclimation, the fish were exposed to arsenate (As(V)) in their diet for 30 days. Results showed a significant accumulation of total arsenic (As) and AsB concentrations in the muscle and head tissues of the exposed fish, with these accumulations exhibiting a positive correlation with water salinity. Transcriptome analyses revealed that exposure to As(V) at low salinity may disrupt membrane components and induce cytoskeletal injuries, while at high salinity, it triggered oxidoreductase activity and transmembrane transport. Metabolome analyses indicated that low salinity induced osmotic stress, resulting in an increased requirement for amino acids to upload intracellular osmotic equilibrium in O. melastigma. Furthermore, the key organic osmolytes and amino acids, including taurine, l-methionine, guanidinoethyl sulfonate, and N-acetyl-l-aspartic acid, exhibited a negative correlation with the AsB concentration. These findings indicated that salinity can regulate osmotic balance by influencing amino acid synthesis under low salinity and stimulating AsB synthesis under high salinity conditions in O. melastigma. This study provides insights into the impact of high salinity on AsB biosynthesis, the underlying regulatory mechanisms, and implications for managing As(V) risk.

Sex-specific divergent responses of marine medaka (Oryzias melastigma) towards long-term benzo[a]pyrene exposure revealed stronger resilience and recoverability in female fish

Benzo[a]pyrene (BaP), a representative five-membered polycyclic aromatic hydrocarbon, has been extensively studied as a pollutant for decades. Despite this, sex-specific responses to BaP exposure remain poorly understood. This study employed a life-cycle exposure approach to investigate the effects of prolonged BaP exposure on marine medaka (Oryzias melastigma), highlighting sex-specific responses. After a 90-day exposure period, significant variations in biometric measurements and oxidative stress markers were observed between male and female fish. BaP exposure resulted in weak detoxification defense in males, while females exhibited an opposite response. Transcriptomic analysis revealed 13 significantly enriched pathways in males and 11 in females, with varying numbers of differentially expressed genes between the sexes, highlighting distinct biological responses. Host resistance assay showed higher mortality rates among BaP-exposed males, and suppressed immune gene expressions and lysozyme activity, while females demonstrated enhanced immune genes and lysozyme activity post-challenge, indicating a more resilient defense response. Furthermore, after a one-month depuration period following BaP exposure, male medaka demonstrated slower recoverability compared to females. These findings underscore sex-specific effects of BaP exposure on fish, with females displaying stronger resilience. Understanding these distinctions are crucial for accurately assessing the impact of environmental pollutants on the aquatic population and ecosystem maintenance.

Long-term carbaryl exposure leads to behavioral abnormalities and reproductive toxicity in male marine medaka through apoptosis-mediated HPA and HPG axes dysregulation

Carbaryl is a widely used carbamate pesticide that has been detected in the marine environment, but its effects on marine fish are still unknown. This study was aimed to investigate the effects of long-term exposure of carbaryl on male marine medaka. For this purpose, we set up five exposure concentration groups of 0, 0.1, 1, 10, and 100 µg/L for 180 days. On the one hand, we observed increased aggression and decreased ability to avoid predators in males after exposure, which was affected by the levels of HPA-axis hormones, especially decreased cortisol level. On the other hand, after exposure, HPG axis hormone levels and gene transcription levels were disturbed. Males exhibited a decreased gonadosomatic index and a notable reduction in mature sperm proportion and the F1 generation displayed a significant increase in malformation rate. Additionally, the number of apoptotic cells and the transcription level of apoptosis-related genes in the brains of male marine medaka substantially increased after exposure. Apoptosis of brain cells may be responsible for the disturbance of HPA and HPG axes, consequently leading to behavioral and reproductive abnormalities. These findings provide novel insights into evaluating the toxic effects of carbaryl on male marine medaka and emphasizing the criticality of exploring the potential environmental risks posed by carbaryl in the marine environment, thus providing toxicity value basis for further strengthening marine environmental monitoring and the protection of biological resources.

Chronic exposure to environmental concentrations of benzo[a]pyrene causes multifaceted toxic effects of developmental compromise, redox imbalance, and modulated transcriptional profiles in the early life stages of marine medaka (Oryzias melastigma)

Polycyclic aromatic hydrocarbons (PAHs) accumulate and integrate into aquatic environments, raising concerns about the well-being and safety of aquatic ecosystems. Benzo[a]pyrene (BaP), a persistent PAH commonly detected in the environment, has been extensively studied. However, the broader multifaceted toxicity potential of BaP on the early life stages of marine fish during chronic exposure to environmentally relevant concentrations needs further exploration. To fill these knowledge gaps, this study assessed the in vivo biotoxicity of BaP (1, 4, and 8 μg/L) in marine medaka (Oryzias melastigma) during early development over a 30-day exposure period. The investigation included morphological, biochemical, and molecular-level analyses to capture the broader potential of BaP toxicity. Morphological analyses showed that exposure to BaP resulted in skeletal curvatures, heart anomalies, growth retardation, elevated mortality, delayed and reduced hatching rates. Biochemical analyses revealed that BaP exposure not only created oxidative stress but also disrupted the activities of antioxidant enzymes. This disturbance in redox balance was further explored by molecular level investigation. The transcriptional profiles revealed impaired oxidative phosphorylation (OXPHOS) and tricarboxylic acid (TCA) cycle pathways, which potentially inhibited the oxidative respiratory chain in fish following exposure to BaP, and reduced the production of adenosine triphosphate (ATP) and succinate dehydrogenase (SDH). Furthermore, this investigation indicated a potential connection to apoptosis, as demonstrated by fluorescence microscopy and histological analyses, and supported by an increase in the expression levels of related genes via real-time quantitative PCR. This study enhances our understanding of the molecular-level impacts of BaP's multifaceted toxicity in the early life stages of marine medaka, and the associated risks.

Neurobehavioral toxicity induced by combined exposure of micro/nanoplastics and triphenyltin in marine medaka (Oryzias melastigma)

Microplastics/nanoplastics (MNPs) inevitably coexist with other pollutants in the natural environment, making it crucial to study the interactions between MNPs and other pollutants as well as their combined toxic effects. In this study, we investigated neurotoxicity in marine medaka (Oryzias melastigma) exposed to polystyrene micro/nanoplastics (PS-MNPs), triphenyltin (TPT), and PS-MNPs + TPT from physiological, behavioral, biochemical, and genetic perspectives. The results showed that marine medaka exposed to 200 ng/L TPT or 200 μg/L PS-NPs alone exhibited some degree of neurodevelopmental deficit, albeit with no significant behavioral abnormalities observed. However, in the PS-MP single exposure group, the average acceleration of short-term behavioral indices was significantly increased by 78.81%, indicating a highly stress-responsive locomotor pattern exhibited by marine medaka. After exposure to PS-MNPs + TPT, the swimming ability of marine medaka significantly decreased. In addition, PS-MNPs + TPT exposure disrupted normal neural excitability as well as activated detoxification processes in marine medaka larvae. Notably, changes in neural-related genes suggested that combined exposure to PS-MNPs and TPT significantly increased the neurotoxic effects observed with exposure to PS-MNPs or TPT alone. Furthermore, compared to the PS-MPs + TPT group, PS-NPs + TPT significantly inhibited swimming behavior and thus exacerbated the neurotoxicity. Interestingly, the neurotoxicity of PS-MPs was more pronounced than that of PS-NPs in the exposure group alone. However, the addition of TPT significantly enhanced the neurotoxicity of PS-NPs compared to PS-MPs + TPT. Overall, the study underscores the combined neurotoxic effects of MNPs and TPT, providing in-depth insights into the ecotoxicological implications of MNPs coexisting with pollutants and furnishing comprehensive data.

Polyhydroxy steroids isolated from starfish (Asterina pectinifera) and their embryotoxicity

Many marine organisms possess an essential capacity to produce secondary metabolites that exhibit toxic characteristics. A new polyhydroxy steroid, 24-methyl-5α-cholestane-24(28)-ene-3β, 4β, 6α, 7α, 8, 15β, 16β, 26-octol-6-O-sodium sulphate (1) was isolated from starfish (Asterina pectinifera), along with five polar steroid compounds (2-6) that were previously identified. NMR (1H and 13C NMR, 1H-1H COSY, HSQC, HMBC, and NOESY) and HR-ESI-MS were employed for structure elucidations. The embryotoxicity and teratogenicity of the isolated compounds were assessed using embryos of marine medaka (Oryzias melastigma). Compound 5 exhibited moderate embryotoxicity (96h-LC50: 65 μM).

A study on the subchronic toxicity of triclocarban to the early-life development of oryzias melastigma and focused on the analysis of osmoregulatory regulation mechanisms

Triclocarban (TCC), a novel antimicrobial agent found in personal care products, has been extensively detected in marine environments. However, research on the toxic effects of TCC on marine organisms remains inadequate. This study delved into the subchronic toxic effects of TCC on the early life stages of marine medaka (Oryzias melastigma, O. melastigma), revealing that TCC could reduce embryo heart rate and hatching rate while diminishing the survival rate of larvae. Biomarker assays indicated that TCC could inflict damage on the embryos' antioxidant and nervous systems. Transcriptomic analysis suggested that TCC could impact cell growth, reproduction, and various life processes, activating cancer signaling pathways, increasing the likelihood of cancer, and exerting toxic effects on the immune and osmoregulatory systems. To validate and enhance our understanding of TCC's unique toxic impact on the osmoregulatory system of O. melastigma, we conducted homology modeling and molecular docking analyses on the protein involved in osmoregulation. The study intuitively revealed the potential binding affinity of TCC to sodium/potassium-transporting ATPase subunit alph (ATP1A1), indicating its ability to disrupt osmotic balance in marine fish by affecting this target protein. In summary, the results of this study will further enhance our comprehension of the potential toxic effects and mechanisms of TCC on the early stages of marine fish, with a specific focus on its unique toxic effects in osmoregulation.

Trophic transfer of micro- and nanoplastics and toxicity induced by long-term exposure of nanoplastics along the rotifer (Brachionus plicatilis)-marine medaka (Oryzias melastigma) food chain

Microplastics (MPs) and nanoplastics (NPs) are emerging pollutants in the ocean, but their transfer and toxicity along the food chains are unclear. In this study, a marine rotifer (Brachionus plicatilis)-marine medaka (Oryzias melastigma) food chain was constructed to evaluate the transfer of polystyrene MPs and NPs (70 nm, 500 nm, and 2 μm, 2000 μg/L) and toxicity of 70 nm PS-NPs (0, 20, 200, and 2000 μg/L) on marine medaka after long-term food chain exposure. The results showed that the amount of 70 nm NPs accumulated in marine medaka was 1.24 μg/mg, which was significantly higher than that of 500 nm NPs (0.87 μg/mg) and 2 μm MP (0.69 μg/mg). Long-term food chain exposure to NPs caused microflora dysbiosis, resulting in activation of toll-like receptor 4 (TLR4) pathway, which induced liver inflammation. Moreover, NPs food chain exposure increased liver and muscle tissue triglyceride and lactate content, but decreased the protein, sugar, and glycogen content. NPs food chain exposure impaired reproductive function and inhibited offspring early development, which might pose a threat to the sustainability of marine medaka population. Overall, the study revealed the transfer of MPs and NPs and the effects of NPs on marine medaka along the food chain.

Identification of Additives in Disposable Face Masks and Evaluation of Their Toxicity Using Marine Medaka (Oryzias melastigma)

The COVID-19 pandemic has resulted in huge amounts of face masks worldwide. However, there is a lack of awareness on the additives and their potential risk to aquatic ecosystems of face masks. To address this issue, the additives and their toxicity in 13 face masks (e.g., polypropylene, polyethylene, and polylactic acid) were determined using nontarget analysis and bioassays. A total of 826 organic additives including intermediates (14.8%), surfactants (9.3%), plasticizers (8.2%), and antioxidants (6.1%) were tentatively identified, with 213 compounds being assigned confidence levels of 1 and 2. Interestingly, polylactic acid masks contained more additives than most polypropylene or polyethylene masks. Among these additives, the concentration of tris(2-ethylhexyl) phosphate in masks was 9.4-978.2 ng/g with a 100% detection frequency. Furthermore, 13 metals such as zinc (up to 202.0 μg/g), copper (32.5 μg/g), and chromium (up to 5.7 μg/g) were detected in the face masks. The methanol extracts of the masks showed the developmental toxicity, swimming behavior, and/or endocrine disruption in embryos/larvae of Oryzias melastigma. The findings demonstrate that face masks contain various toxic additives to marine medaka, which deserves close attention to pollution by face masks.

Effects of microplastics and phenanthrene on gut microbiome and metabolome alterations in the marine medaka Oryzias melastigma

Plastic pollution of the oceans is increasing, and toxic interactions between microplastics (MPs) and organic pollutants have become a major environmental concern. However, the combined effects of organic pollutants and MPs on microbiomes and metabolomes have not been studied extensively. In the present study, to evaluate whether MPs and phenanthrene (Phe) act synergistically in the guts of marine medaka (Oryzias melastigma), we performed toxicity assessments, 16 S rRNA gene sequencing, and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analyses. Our investigations revealed increased toxicity induced by Phe, as well as disturbances in gut microbiota (known as dysbiosis) when MPs were present. Furthermore, combined exposure to Phe and MPs resulted in greater alterations to microbiota composition and metabolite profiles. Notably, MP exposure was distinctly associated with the abundance of Shewanella and Spongiibacteraceae, while Phe exposure was associated with the abundance of Marimicrobium. Among key microbiota, Marimicrobium and Roseibacillus were significantly correlated with metabolites responsible for coenzyme A and glycerophospholipid metabolism in medaka. These results suggest that interactions between Phe and MPs may have significant effects on the gut microbiota and metabolism of aquatic organisms and underscore the importance of acknowledging the interplay between MPs and contaminants in aquatic environments.

Exposure to manganese (II) chloride induces developmental toxicity, oxidative stress and inflammatory response in Marine medaka (Oryzias melastigma) embryos

Manganese (Mn) is an essential metal for organisms, but high levels can induce serious toxicity. To date, the toxic mechanism of Mn to marine fish is still poorly understood. In the present study, Oryzias melastigma embryos were exposed to different concentrations of MnCl₂ (0-152.00 mg/L) to investigate its effect on early development. The results showed that exposure to MnCl₂ caused developmental toxicity to embryos, including increased heart rate, delayed hatching time, decreased hatching rate and increased malformation rate. MnCl₂ exposure could induce oxidative stress in O. melastigma embryos, as indicated by increased the contents of malondialdehyde (MDA) and the activities of the antioxidant enzymes (superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase (CAT)). The heart might be an important target organ for MnCl₂ because of cardiac malformations and disruption in the expression of cardiac development-related genes (ATPase, epo, fg8g, cox1, cox2, bmp4 and gata4). In addition, the expression levels of stress- (omTERT and p53) and inflammation-related genes (TNFα and il1β) were significantly up-regulated, suggesting that MnCl₂ can trigger stress and inflammatory response in O. melastigma embryos. In conclusion, this study demonstrated that MnCl₂ exposure can induce developmental toxicity, oxidative stress and inflammatory response in O. melastigma embryos, providing insights into the toxic mechanism of Mn to the early development of marine fish.

Short-term exposure to enrofloxacin causes hepatic metabolism disorder associated with intestinal flora dysbiosis in adult marine medaka (Oryzias melastigma)

Enrofloxacin (ENR) is a widely used fluoroquinolone antibiotic that is frequently detected in the environment. Our study assessed the impact of short-term ENR exposure on the intestinal and liver health of marine medaka (Oryzias melastigma) using gut metagenomic shotgun sequencing and liver metabolomics. We found that ENR exposure resulted in imbalances of Vibrio and Flavobacteria and enrichments of multiple antibiotic resistance genes. Additionally, we found a potential link between the host's response to ENR exposure and the intestinal microbiota disorder. Liver metabolites, including phosphatidylcholine, lysophosphatidylcholine, taurocholic acid, and cholic acid, in addition to several metabolic pathways in the liver that are closely linked to the imbalance of intestinal flora were severely maladjusted. These findings suggest that ENR exposure has the potential to negatively affect the gut-liver axis as the primary toxicological mechanism. Our findings provide evidence regarding the negative physiological impacts of antibiotics on marine fish.

Enantiospecific Uptake and Depuration Kinetics of Chiral Metoprolol and Venlafaxine in Marine Medaka (Oryzias melastigma): Tissue Distribution and Metabolite Formation

The increasing use of chiral pharmaceuticals has led to their widespread presence in the environment. However, their toxicokinetics have rarely been reported. Therefore, the tissue-specific uptake and depuration kinetics of two pairs of pharmaceutical enantiomers, S-(-)-metoprolol versus R-(+)-metoprolol and S-(+)-venlafaxine versus R-(-)-venlafaxine, were studied in marine medaka (Oryzias melastigma) during a 28-day exposure and 14-day clearance period. The toxicokinetics of the studied pharmaceuticals, including uptake and depuration rate constants, depuration half-life (t_1/2), and bioconcentration factor (BCF), were reported for the first time. The whole-fish results demonstrated a higher S- than R-venlafaxine bioaccumulation potential, whereas no significant difference was observed between S- and R-metoprolol. O-desmethyl-metoprolol (ODM) and α-hydroxy-metoprolol (AHM) were the main metoprolol metabolites identified by suspect screening, and the ratios of ODM to AHM were 3.08 and 1.35 for S- and R-metoprolol, respectively. N,O-Didesmethyl-venlafaxine (NODDV) and N-desmethyl-venlafaxine (NDV) were the main venlafaxine metabolites, and the ratios of NODDV to NDV were 1.55 and 0.73 for S- and R-venlafaxine, respectively. The highest tissue-specific BCFs of the four enantiomers were all found in the eyes, meriting in-depth investigation.

Embryo developmental toxicity in marine medaka (Oryzias melastigma) due to parental and embryonic 17α-ethinylestradiol exposure

The synthetic estrogen 17α-ethinylestradiol (EE2) is a common component of hormone therapy and oral contraceptives and has been widely used for nearly 60 years. Numerous studies have shown that exposure to EE2 can affect embryonic development in a number of fish species. The effects of parental and embryonic EE2 exposure on embryo developmental toxicity and the underlying molecular mechanisms, however, have rarely been examined. In this study, embryos collected from parental EE2-exposed adult fish were examined to assess EE2-induecd toxicity during embryo development. The rate of embryo development including heart rate, hatching rate, and larval locomotion were measured to assess embryo developmental toxicity. The embryonic transcriptome was used to delineate the related developmental toxicity pathways. Our results suggest that parental and embryonic EE2 exposure resulted in growth retardation including a reduction in embryo heart rate, a delay in the appearance eye pigmentation, decreased hatching rate and impaired larval locomotion. In addition, gene ontology (GO) enrichment analysis, Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, and Ingenuity Pathway Analysis (IPA) of transcriptome revealed that these impairments are controlled by estrogen receptor and related to eye structure, neuronal and synaptic structure, and behaviour. The key factors identified, including PRKAA2, APOB, EPHB2, OXTR, NR2E3, and POU4F2, could serve as biomarkers for assessing EE2-induced embryo developmental toxicity. For the first time, our results show that eye pigmentation is a potentially sensitive marker of EE2-induced embryo developmental toxicity.

Bisphenol A alters sexual dimorphism and gene expression in marine medaka Oryzias melastigma

Bisphenol A (BPA) is an endocrine disruptor that is present in freshwater and marine environments. However, conclusive evidence for the toxicity of chronic BPA exposure to marine fishes remains lacking. Therefore, we investigated the influence of BPA on male marine medaka (Oryzias melastigma). BPA exposure induced formation of testis-ova at 2610 µg/L, and male-type anal fins became more female type in a concentration-dependent manner. Some males with female-type anal fins had normal testes, indicating that anal fin shape is more sensitive to BPA. Gonadal soma-derived factor (gsdf) expression decreased after BPA exposure in the 746 and 2610 µg/L exposure groups, although the changes were not statistically significant. Additionally, liver vitellogenin (vtg) expression increased in a dose-dependent manner and was significantly higher in all exposure groups. vtg and gsdf are likely to be useful biomarkers for the impact of estrogenic endocrine disrupters in O. melastigma.

Color preferences and gastrointestinal-tract retention times of microplastics by freshwater and marine fishes

We examined ingestion and retention rates of microplastics (MPs) by two freshwater (Japanese medaka and zebrafish) and two marine fish species (Indian medaka and clown anemonefish) to determine their color preferences and gastrointestinal-tract retention times. In our ingestion experiments, clown anemonefish ingested the most MP particles, followed by zebrafish, and then Japanese and Indian medaka. Next, we investigated color preferences among five MP colors. Red, yellow, and green MP were ingested at higher rates than gray and blue MPs for all tested fish species. To test whether these differences truly reflect a recognition of and preference for certain colors based on color vision, we investigated the preferences of clown anemonefish for MP colors under light and dark conditions. Under dark conditions, ingestion of MP particles was reduced, and color preferences were not observed. Finally, we assessed gastrointestinal-tract retention times for all four fish species. Some individuals retained MP particles in their gastrointestinal tracts for over 24 h after ingestion. Our results show that fish rely on color vision to recognize and express preferences for certain MP colors. In addition, MP excretion times varied widely among individuals. Our results provide new insights into accidental MP ingestion by fishes.

Widespread alterations upon exposure to the estrogenic endocrine disruptor ethinyl estradiol in the liver proteome of the marine male fish Cyprinodon variegatus

Quantitative proteomic changes in the liver of adult males of Sheepshead minnow (Cyprinodon variegatus) upon exposure to ethinyl estradiol (EE2) were assessed to provide an advanced understanding of the metabolic pathways affected by estrogenic endocrine disruption in marine fish, and to identify potential novel molecular biomarkers for the environmental exposure to estrogens. From a total of 3188 identified protein groups (hereafter proteins), 463 showed a statistically significant difference in their abundance between EE2 treatment and solvent control samples. The most affected biological processes upon EE2 exposure were related to ribosomal biogenesis, protein synthesis and transport of nascent proteins to endoplasmic reticulum, and nuclear mRNA catabolism. Within the group of upregulated proteins, a subset of 14 proteins, involved in egg production (Vitellogenin, Zona Pellucida), peptidase activity (Cathepsine E, peptidase S1, Serine/threonine-protein kinase PRP4 homolog, Isoaspartyl peptidase and Whey acidic protein), and nucleic acid binding (Poly [ADP-ribose] polymerase 14) were significantly upregulated with fold-change values higher than 3. In contrast, Collagen alpha-2, involved in the process of response to steroid hormones, among others, was significantly downregulated (fold change = 0.2). This pattern of alterations in the liver proteome of adult males of C. variegatus can be used to identify promising novel biomarkers for the characterization of exposure of marine fish to estrogens. The Whey acidic protein-like showed the highest upregulation in EE2-exposed individuals (21-fold over controls), suggesting the utility of abundance levels of this protein in male liver as a novel biomarker of xenoestrogen exposure.

Embryotoxicity of Polystyrene Microspheres of Different Sizes to the Marine Medaka Oryzias melastigma (McClelland, 1839)

Polystyrene microplastics (PS-MPs) are potentially harmful to marine organisms, especially during the early developmental stages, although the underlying mechanism remains unclear. The present study evaluated the growth and morphological characteristics of marine medaka Oryzias melastigma (McClelland, 1839) embryos exposed to PS-MP. PS-MPs of three different sizes (0.05, 0.5, and 6.0 μm with a concentration of 106 particles/L) were subjected to waterborne exposure for 19 d. The hatching time and rate of embryos exposed to 0.5 and 6.0 μm PS-MPs were significantly lower than those of the control, while no significant difference was observed in the 0.05 μm treatment. No significant differences were observed in the mortality rate of the embryos, embryo diameter, and relevant gene expression levels, including il6, il8, il-1β, jak, stat-3, nf-κb, hif-1α, epo, cyp1a1, ahr, sod, cat, and gpx, but with the exception of vtg. Fluorescent PS-MPs were found on the embryo surfaces when the embryos were exposed to 0.5 and 6.0 μm PS-MPs, but no signals were detected inside embryos using confocal microscopy. Therefore, the results indicate that PS-MPs having a diameter of 6.0 μm can only attach to the surface or villus of embryos and not enter the embryos through the membrane pores, whereas PS-MPs with diameters of 0.05 and 0.5 μm cannot enter the embryos.

Assessing the Effect of Modified Clay on the Toxicity of Karenia mikimotoi Using Marine Medaka (Oryzias melastigma) as a Model Organism

Blooms of the toxic dinoflagellate Karenia mikimotoi could threaten the survival of marine life, and modified clay (MC) is considered a promising method for the control of harmful algal blooms. Here, using marine medaka as the model organism, the toxicity of K. mikimotoi before and after MC disposal was investigated. The results showed that only a certain density of intact K. mikimotoi cells could cause obvious damage to fish gills and lead to rapid death. A systematic analysis of morphology, physiology, and molecular biology parameters revealed that the fish gills exhibited structural damage, oxidative damage, osmotic regulation impairment, immune response activation, and signal transduction enhancement. MC can flocculate K. mikimotoi rapidly in water and reduce its toxicity by reducing the density of intact algae cells and hemolytic toxicity. The results indicate that MC is an effective and safe method for controlling K. mikimotoi blooms.

Proteome Analysis of Whole-Body Responses in Medaka Experimentally Exposed to Fish-Killing Dinoflagellate Karenia mikimotoi

Karenia mikimotoi is a well-known harmful algal bloom species. Blooms of this dinoflagellate have become a serious threat to marine life, including fish, shellfish, and zooplanktons and are usually associated with massive fish death. Despite the discovery of several toxins such as gymnocins and gymnodimines in K. mikimotoi, the mechanisms underlying the ichthyotoxicity of this species remain unclear, and molecular studies on this topic have never been reported. The present study investigates the fish-killing mechanisms of K. mikimotoi through comparative proteomic analysis. Marine medaka, a model fish organism, was exposed to K. mikimotoi for a three-part time period (LT25, LT50 and LT90). Proteins extracted from the whole fish were separated by using two-dimensional gel electrophoresis, and differentially expressed proteins were identified with reference to an untreated control. The change in fish proteomes over the time-course of exposure were analyzed. A total of 35 differential protein spots covering 19 different proteins were identified, of which most began to show significant change in expression levels at the earliest stage of intoxication. Among the 19 identified proteins, some are closely related to the oxidative stress responses, energy metabolism, and muscle contraction. We propose that oxidative stress-mediated muscle damage might explain the symptoms developed during the ichthyotoxicity test, such as gasping for breath, loss of balance, and body twitching. Our findings lay the foundations for more in-depth studies of the mechanisms of K. mikimotoi's ichthyotoxicity.

Generation of a novel transgenic marine medaka (Oryzias melastigma) for highly sensitive detection of heavy metals in the environment

As typic priority pollutants in the marine environment, heavy metals can be accumulated in the human body leading to serious environmental and health problems. The metal regulatory elements (MREs) have been identified to be the main functional parts for the response to heavy metals. To develop a convenient biological monitoring tool for the detection of heavy metals in the oceans, we generated a transgenic marine medaka line Tg(OmMT: eGFP) with a truncated metallothionein promoter, which was only 193 bp and drove the expression of eGFP. After Tg(OmMT:eGFP) embryos were treated with four different heavy metals and different concentrations, the results showed that the expression level of eGFP was consistent with that of the endogenous mt. The transgenic embryos are very sensitive to Hg²⁺, and the fluorescence could be induced in the 0.0002 μM concentration, which is far lower than the primary water standard. The expression level of eGFP and mt showed a dose-dependent manner to heavy metals concentration. Taken together, the newly established marine medaka is a sensitive, efficient, and convenient tool for monitoring heavy metal pollution in the environment, especially seawater.

Developmental toxicity in marine medaka (Oryzias melastigma) embryos and larvae exposed to nickel

As an important trace metal, nickel (Ni) has been reported extensively in the studies on freshwater animals. However, the toxic effects of Ni on marine organisms are not clearly understood. Therefore, in order to investigate the toxic effects of Ni on the early development of marine fish, the marine medaka (Oryzias melastigma) embryos and larvae were immersed in 0.13-65.80 mg/L Ni solution. The results showed that Ni exposure changed the egg size and heart rate of the embryos, lowered the hatchability, increased the deformity rate, and shortened the total body length of newly hatched larvae. Besides, it was found that before organogenesis and post-hatching periods were the sensitive periods of embryos to Ni. The 25 d LC₅₀ value of embryos was 49.28 mg/L, and the 5 d LC₅₀ of larvae was 55.92 mg/L, indicating that the embryos were more sensitive to Ni than the larvae. Furthermore, the expressions of the metallothionein (MT) gene, the skeletal development-related gene (Cyp26b1) and the cardiac development-related genes (ATPase, smyd1, cox2 and bmp4) were determined, and the results showed that the expressions of ATPase and smyd1 were up-regulated, while MT, Cyp26b1 and cox2 were significantly down-regulated at 9 days post-fertilization (dpf). Overall, Ni exposure caused a significant toxic effect on the early development of the O. melastigma embryos and larvae. Our findings could provide an important supplement to the toxicity data of tropical Ni and provide a reference for the exploration of the molecular mechanisms of Ni toxicity.

Toxicity effects of hydrophilic algal lysates from Coolia tropicalis on marine medaka larvae (Oryzias melastigma)

Coolia tropicalis is a species of benthic and epiphytic toxic algae, which can produce phycotoxins that intoxicate marine fauna. In this study, the potential toxic effects of C. tropicalis on fish were investigated using larval marine medaka (Oryzias melastigma) as a model to evaluate fish behavior, physiological performance, and stress-induced molecular responses to exposure to two sublethal concentrations (LC₁₀ and LC₂₀) of hydrophilic algal lysates. Exposure to C. tropicalis lysates inhibited swimming activity, activated spontaneous undirected locomotion, altered nerve length ration, and induced early development abnormalities, such as shorter eye diameter, body as well as axon length. Consistent with these abnormalities, changes in the expression of genes associated with apoptosis (CASPASE-3 and BCL-2), the inflammatory response (IL-1β and COX-2), oxidative stress (SOD), and energy metabolism (ACHE and VHA), were also observed. This study advances our understanding of the mechanisms of C. tropicalis toxicity in marine fish in the early life stages and contributes to future ecological risk assessments of toxic benthic dinoflagellates.

Brazilian silverside, Atherinella brasiliensis (Quoy & Gaimard,1825) embryos as a test-species for marine fish ecotoxicological tests

The fish embryo test (FET) is an alternative to the classic freshwater toxicity test used to assess environmental hazards and risks to fish. This test has been standardized and adopted by the Organization for Economic and Cooperation and Development (OECD). As salinity may affect the substances’ toxicity, we describe the development of an alternative euryhaline test species for embryonic ecotoxicological tests: the Brazilian silverside Atherinella brasiliensis (Quoy & Gaimard, 1825). This species is broadly distributed along the coast of South America and is able to inhabit a broad range of environmental and saline conditions. Ours is the first study on the maintenance of a native South American species for natural reproduction and the generation of embryos for tests. The embryos used are transparent and possess fluorescent cells which have only been seen in a few species and which may be used as markers, making it an alternative assessment tool for the lethal and sublethal substances in marine and estuarine environments. We provide a detailed description and analysis of embryonic development under different salinities and temperatures. The embryos and larvae developed in similar ways at different salinities, however as temperatures increased, mortality also increased. We considered the effects of the reference toxicants Zn²⁺ and SDS using a protocol similar to the FET that was standardized for zebrafish. Brazilian silverside embryos are as sensitive as freshwater, or euryhaline fish, to the surfactant but are more resistant to metals prior to hatching. We were able to show the advantages of the Brazilian silverside as a model for a marine fish embryo test (FETm) with high levels of reproducibility and little contaminated waste.

Different effects of nano- and microplastics on oxidative status and gut microbiota in the marine medaka Oryzias melastigma

Plastic is regarded as a major environmental concern. In particular, nanoplastics and microplastics (NMPs) are attracting global attention due to their potential impact on aquatic organisms. Here, we examined the effects of NMPs (50 nm polystyrene microbead nanoplastics [NPs] and 45 µm microplastics [MPs]) on oxidative status and gut microbiota in the marine medaka Oryzias melastigma. The NP-exposed group exhibited stronger oxidative stress with higher activation levels of antioxidants compared to the MP-exposed group. However, the MP-exposed group demonstrated induction of intestinal damage (e.g., increased mucus ratio) with further alterations of gut microbiota, compared to the NP-exposed group. In particular, MPs caused more significant alterations of microbiota composition at both phylum and genus levels. Thus, in this study we show distinct toxicity pathways of NPs and MPs, an oxidative stress-mediated pathway (e.g., antioxidants) induced by NP exposure and dysbiosis of gut microbiota in association with immune dysfunction induced by MP exposure. Our results are helpful for expanding our knowledge about the impacts of NMPs as potentially harmful substances in the aquatic environment.

Long-read sequencing and de novo genome assembly of marine medaka (Oryzias melastigma)

BACKGROUND

Marine medaka (Oryzias melastigma) is considered as an important ecotoxicological indicator to study the biochemical, physiological and molecular responses of marine organisms towards increasing amount of pollutants in marine and estuarine waters.

RESULTS

In this study, we reported a high-quality and accurate de novo genome assembly of marine medaka through the integration of single-molecule sequencing, Illumina paired-end sequencing, and 10X Genomics linked-reads. The 844.17 Mb assembly is estimated to cover more than 98% of the genome and is more continuous with fewer gaps and errors than the previous genome assembly. Comparison of O. melastigma with closely related species showed significant expansion of gene families associated with DNA repair and ATP-binding cassette (ABC) transporter pathways. We identified 274 genes that appear to be under significant positive selection and are involved in DNA repair, cellular transportation processes, conservation and stability of the genome. The positive selection of genes and the considerable expansion in gene numbers, especially related to stimulus responses provide strong supports for adaptations of O. melastigma under varying environmental stresses.

CONCLUSIONS

The highly contiguous marine medaka genome and comparative genomic analyses will increase our understanding of the underlying mechanisms related to its extraordinary adaptation capability, leading towards acceleration in the ongoing and future investigations in marine ecotoxicology.

Removal of antibiotic resistant microbes by Fe(II)-activated persulfate oxidation

Sewage in WWTPs is one of main way to spread antibiotic resistant microbes (ARMs), and beach bay water is in direct contact with human skin. It is necessary to pay attention to remove the ARMs in WWTP sewage and bay water. Our results showed that ARMs and total microbes (TMs) can be effectively removed by S₂O₈^2-/Fe²⁺ in the effluent stage of WWTPs and bay water. Quenching experiments using tert-butyl alcohol, dimethyl sulfoxide and Al₂O₃ as scavengers confirmed that the primary reactive oxidants responsible for microbes removal during the Fe(II)-activated persulfate oxidation process might be SO₄^•- and Fe(IV), rather than •OH. The bacterial community shifted and the alpha diversity significantly reduced after treatment. In WWTP group, relative abundance of Firmicutes increased to 8.56%, and potential pathogens such as genus Vibrio decreased to 0.03% in bay water after treatment. The ecological toxicity to the environment of S₂O₈^2-/Fe²⁺ further illustrated that the mortality of indicator species Oryzias latipes did not increase after treatment, and the dosage of 60/30 μM can be potentially ideal dosage of S₂O₈^2-/Fe²⁺. This study revealed Fe(II)-activated persulfate oxidation as an eco-friendly and economical method could reduce TMs and ARMs in WWTP sewage and bay water.

Marine medaka heat shock protein 90ab1 is a receptor for red-spotted grouper nervous necrosis virus and promotes virus internalization through clathrin-mediated endocytosis

Nervous necrosis virus (NNV) can infect many species of fish and causes serious acute or persistent infection. However, its pathogenic mechanism is still far from clear. Specific cellular surface receptors are crucial determinants of the species tropism of a virus and its pathogenesis. Here, the heat shock protein 90ab1 of marine model fish species marine medaka (MmHSP90ab1) was identified as a novel receptor of red-spotted grouper NNV (RGNNV). MmHSP90ab1 interacted directly with RGNNV capsid protein (CP). Specifically, MmHSP90ab1 bound to the linker region (LR) of CP through its NM domain. Inhibition of MmHSP90ab1 by HSP90-specific inhibitors or MmHSP90ab1 siRNA caused significant inhibition of viral binding and entry, whereas its overexpression led to the opposite effect. The binding of RGNNV to cultured marine medaka hMMES1 cells was inhibited by blocking cell surface-localized MmHSP90ab1 with anti-HSP90β antibodies or pretreating virus with recombinant MmHSP90ab1 or MmHSP90ab1-NM protein, indicating MmHSP90ab1 was an attachment receptor for RGNNV. Furthermore, we found that MmHSP90ab1 formed a complex with CP and marine medaka heat shock cognate 70, a known NNV receptor. Exogenous expression of MmHSP90ab1 independently facilitated the internalization of RGNNV into RGNNV impenetrable cells (HEK293T), which was blocked by chlorpromazine, an inhibitor of clathrin-dependent endocytosis. Further study revealed that MmHSP90ab1 interacted with the marine medaka clathrin heavy chain. Collectively, these data suggest that MmHSP90ab1 is a functional part of the RGNNV receptor complex and involved in the internalization of RGNNV via the clathrin endocytosis pathway.

Hypoxia causes sex-specific hepatic toxicity at the transcriptome level in marine medaka (Oryzias melastigma)

Hypoxia, a low environmental oxygen level, is a common problem in the ocean globally. Hypoxia has been known to cause disruption to the endocrine system of marine organisms in both laboratory and field studies. Our previous studies have demonstrated the sex-specific response to hypoxia in the neural and reproductive systems of marine fish. In the current report, we aim to study the sex-specific hepatic response of fish at the transcriptome level to hypoxic stress. By using a comparative transcriptome analysis, followed by a systematic bioinformatics analysis including Database for Annotation, Visualization and Integrated Discovery (DAVID) and Ingenuity Pathway Analysis (IPA), we found that hypoxia altered expression of genes related to cell proliferation and apoptosis of hepatocytes, which are associated with human pathologies, such as liver inflammation hepatic steatosis and steatohepatitis. Furthermore, we observed sex-specific responses in the livers of fish through different cell signaling pathways. In female fish, hypoxia causes dysregulation of expression of genes related to impairment in endoplasmic reticulum structure and liver metabolism. In male fish, genes associated with redox homeostasis and fatty acid metabolism were altered by hypoxic stress. The findings of this study support the notion that hypoxia could cause sex-specific changes (hepatic toxicity and changes) in marine fish.

Production of high-quality two-dimensional gel electrophoresis profile for marine medaka samples by using Trizol-based protein extraction approaches

Background

Marine medaka is among the most popular models of fish species for ecotoxicology and environmental research and proteomic studies are useful tools for understanding the molecular responses of medaka upon exposure to different environmental stressors. The preparation of high-quality protein samples is the key to producing high-quality two-dimensional gel electrophoresis (2-DE) results for proteomic analysis. In recent years, Trizol-based protein extraction has been gaining popularity because of its promising performance in producing high-quality 2-DE as well as the convenience of the method.

Methods

Three Trizol-based approaches (Trizol method, Aliquot Trizol method and Trizol method with a commercial clean-up kit) were used to extract proteins from a marine medaka sample and 2-DE profiles were produced. Quality of the 2-DE profiles and effectiveness of the extraction methods were evaluated. For comparison, two common protein extraction methods (lysis buffer method and trichloroacetic acid (TCA)/acetone precipitation extraction) were also applied in parallel to Trizol-based approaches.

Results

Any of the three Trizol-based approaches produced a high-quality 2-DE profile of marine medaka compared with both lysis buffer method and TCA/acetone precipitation extraction. In addition, Trizol method with a commercial clean-up kit produced the best 2-DE profile in terms of background clarity, number of spots and resolution of proteins.

Conclusions

Trizol-based approaches offered better choices than traditional protein extraction methods for 2-DE analysis of marine medaka. The modified version of Trizol method with a commercial clean-up kit was shown to produce the best 2-DE profile.

The acute toxic effects of hexavalent chromium on the liver of marine medaka (Oryzias melastigma)

Chromium is toxic to marine animals and can cause damage to many of their organs, including the liver. To test the toxicity of chromium on marine organisms, we exposed the liver of the marine medaka (Oryzias melastigma) with hexavalent chromium [Cr(VI)]. Our results show that Cr enrichment in the liver demonstrates a positive correlation to the exposure concentration. With the increase of Cr(VI) concentration, pathological changes including nuclear migration, cell vacuolization, blurred intercellular gap, nuclear condensation, become noticeable. To further study changes in gene expression in the liver after Cr(VI) exposure, we used RNA-seq to compare expression profiles before and after Cr(VI) exposure. After acute Cr(VI) exposure (2.61 mg/l) for 96 h, 5862 transcripts significantly changed. It is the first time that the PPAR pathway was found to respond sensitively to Cr(VI) exposure in fish. Finally, combined with other published study, we found that there may be some difference between Cr(VI) toxicity in seawater fish and freshwater fish, due to degree of oxidative stress, distribution patterns and detailed Cr(VI) toxicological mechanisms. Not only does our study explore the mechanisms of Cr(VI) toxicity on the livers of marine medaka, it also points out different Cr(VI) toxicity levels and potential mechanisms between seawater fish and freshwater fish.

Construction of High-Resolution RAD-Seq Based Linkage Map, Anchoring Reference Genome, and QTL Mapping of the Sex Chromosome in the Marine Medaka Oryzias melastigma

Medaka (Oryzias sp.) is an important fish species in ecotoxicology and considered as a model species due to its biological features including small body size and short generation time. Since Japanese medaka Oryzias latipes is a freshwater species with access to an excellent genome resource, the marine medaka Oryzias melastigma is also applicable for the marine ecotoxicology. In genome era, a high-density genetic linkage map is a very useful resource in genomic research, providing a means for comparative genomic analysis and verification of de novo genome assembly. In this study, we developed a high-density genetic linkage map for O. melastigma using restriction-site associated DNA sequencing (RAD-seq). The genetic map consisted of 24 linkage groups with 2,481 single nucleotide polymorphism (SNP) markers. The total map length was 1,784 cM with an average marker space of 0.72 cM. The genetic map was integrated with the reference-assisted chromosome assembly (RACA) of O. melastigma, which anchored 90.7% of the assembled sequence onto the linkage map. The values of complete Benchmarking Universal Single-Copy Orthologs were similar to RACA assembly but N50 (23.74 Mb; total genome length 779.4 Mb; gap 5.29%) increased to 29.99 Mb (total genome length 778.7 Mb; gap 5.2%). Using MapQTL analysis with SNP markers, we identified a major quantitative trait locus for sex traits on the Om10. The integration of the genetic map with the reference genome of marine medaka will serve as a good resource for studies in molecular toxicology, genomics, CRISPR/Cas9, and epigenetics.

The untapped potential of medaka and its wild relatives

The medaka is a fish that has served as a model organism for over a century, yet there is still much to learn about its life in the wild. Here we summarize the current knowledge, highlight recent progress and outline remaining gaps in our understanding of the natural history of medaka. It has also become clear over time that rather than being a single species, medaka comprises an entire species complex, so disentangling the species boundaries is an important goal for future research. Moreover, medaka and other ricefishes exhibit striking functional diversity, little of which has been investigated to date. As such, there are opportunities to use the resources developed for medaka to study other ricefishes, and to learn more about medaka itself in an evolutionary context.

The genome of the marine medaka Oryzias melastigma

Marine medaka (Oryzias melastigma) is considered to be a useful fish model for marine and estuarine ecotoxicology studies and has good potential for field-based population genomics because of its geographical distribution in Asian estuarine and coastal areas. In this study, we present the first whole-genome draft of O. melastigma. The genome assembly consists of 8,602 scaffolds (N50 = 23.737 Mb) and a total genome length of 779.4 Mb. A total of 23,528 genes were predicted, and 12,670 gene families shared with three teleost species (Japanese medaka, mangrove killifish and zebrafish) were identified. Genome analyses revealed that the O. melastigma genome is highly heterozygous and contains a large number of repeat sequences. This assembly represents a useful genomic resource for fish scientists.

Immune competence assessment in marine medaka (Orzyias melastigma)-a holistic approach for immunotoxicology

Many anthropogenic pollutants in coastal marine environments can induce immune impairments in wild fish and reduce their survival fitness. There is a pressing need to establish sensitive and high throughput in vivo tools to systematically evaluate the immunosuppressive effects of contaminants in marine teleosts. This study reviewed a battery of in vivo immune function detection technologies established for different biological hierarchies at molecular (immune function pathways and genes by next generation sequencing (NGS)), cellular (leukocytes profiles by flow cytometry), tissues/organ system (whole adult histo-array), and organism (host resistance assays (HRAs)) levels, to assess the immune competence of marine medaka Oryzias melastigma. This approach enables a holistic assessment of fish immune competence under different chemical exposure or environmental scenarios. The data obtained will also be useful to unravel the underlying immunotoxic mechanisms. Intriguingly, NGS analysis of hepatic immune gene expression profiles (male > female) are in support of the bacterial HRA findings, in which infection-induced mortality was consistently higher in females than in males. As such, reproductive stages and gender-specific responses must be taken into consideration when assessing the risk of immunotoxicants in the aquatic environment. The distinct phenotypic sexual dimorphism and short generation time (3 months) of marine medaka offer additional advantages for sex-related immunotoxicological investigation.

Impact of ocean acidification on the early development and escape behavior of marine medaka (Oryzias melastigma)

Ocean acidification is predicted to affect a wide diversity of marine organisms. However, no studies have reported the effects of ocean acidification on Indian Ocean fish. We have used the Indian Ocean medaka (Oryzias melastigma) as a model species for a marine fish that lives in coastal waters. We investigated the impact of ocean acidification on the embryonic development and the stereotyped escape behavior (mediated by the Mauthner cell) in newly hatched larvae. Newly fertilized eggs of medaka were reared in seawater at three different partial pressures of carbon dioxide (pCO₂): control at 450 μatm, moderate at 1160 μatm, and high at 1783 μatm. Hatch rates, embryonic duration, and larval malformation rates were compared and were not significantly different between the treatments and the control. In the high pCO₂ group, however, the yolks of larvae were significantly smaller than in the control group, and the newly hatched larvae were significantly longer than the larvae in the control. In the moderate pCO₂ group, the eye distance decreased significantly. No significantly negative growth effects were observed in the larvae when exposed to pCO₂ levels that are predicted as a result of ocean acidification in the next 100-200 years. Larvae reared under control conditions readily produced C-start escape behavior to mechanosensory stimuli; however, in the moderate and high pCO₂ experimental groups, the probabilities of C-start were significantly lower than those of the control group. Therefore, the sensory integration needed for the C-start escape behavior appears to be vulnerable to ocean acidification. Altered behavior in marine larval fish, particularly behaviors involved in escape from predation, could have potentially negative implications to fish populations, and, further, to the marine ecosystems at the levels of CO₂ projected for the future.

The development of cellular immune defence in marine medaka Oryzias melastigma

Environmentally induced alterations of the immune system during sensitive developmental stages may manifest as abnormalities in immune organ configuration and/or immune cell differentiation. These not only render the early life stages more vulnerable to pathogens, but may also affect the adult immune competence. Knowledge of these sensitive periods in fish would provide an important prognostic/diagnostic tool for aquatic risk assessment of immunotoxicants. The marine medaka Oryzias melastigma is an emerging seawater fish model for immunotoxicology. Here, the presence and onset of four potentially sensitive periods during the development of innate and adaptive cellular immune defence were revealed in O. melastigma: 1.) initiation of phagocyte differentiation, 2.) migration and expansion of lymphoid progenitor cells, 3.) colonization of immune organs through lymphocyte progenitors and 4.) establishment of immune competence in the thymus. By using an established bacterial resistance assay for O. melastigma, larval immune competence (from newly hatched 1dph to 14dph) was found concomitantly increased with advanced thymus development and the presence of mature T-lymphocytes. A comparison between the marine O. melastigma and the freshwater counterpart Oryzias latipes disclosed a disparity in the T-lymphocyte maturation pattern, resulting in differences in the length of T-lymphocyte maturation. The results shed light on a potential difference between seawater and freshwater medaka in their sensitivity to environmental immunotoxicants. Further, medaka immune system development was compared and contrasted to economically important fish. The present study has provided a strong scientific basis for advanced investigation of critical windows for immune system development in fish.

Developmental toxicity and molecular responses of marine medaka (Oryzias melastigma) embryos to ciguatoxin P-CTX-1 exposure

Ciguatoxins are produced by toxic benthic dinoflagellates and cause ciguatera fish poisoning worldwide, but the toxic effects on developing marine fish have not been well investigated. The Pacific ciguatoxin (P-CTX-1), is a potent sodium channel agonist, which is one of the most toxic members among all CTXs. This study evaluated the toxic effects of microinjecting purified Pacific ciguatoxin-1 (P-CTX-1) on embryonic development of marine medaka Oryzias melastigma. A lower 96h-LD₅₀ value was estimated for eleuthero-embryos (1.32ngg^-1) than that for embryos (1.71ngg^-1), indicating that P-CTX-1 is more lethal to newly hatched medaka larvae. P-CTX-1 induced detrimental effects during embryonic development, including hatching failure, abnormalities in physical development (caudal fin malformation and spinal deformities), internal damage (green coloration of the gall bladder and hemorrhaging), immune dysfunction, and altered muscle physiology (bradycardia and hyperkinetic twitching). The results of a transcriptional expression analysis of genes related to the stress/immune responses, cardiac and bone development, and apoptosis supported the observed developmental abnormalities. This study advanced the understanding of P-CTX-1 mediated toxic mechanisms in the development of early life stages of a fish, and thus contributed to the toxicity assessment of CTXs in marine ecosystems.

Metabolomics approach reveals metabolic disorders and potential biomarkers associated with the developmental toxicity of tetrabromobisphenol A and tetrachlorobisphenol A

Tetrabromobisphenol A and tetrachlorobisphenol A are halogenated bisphenol A (H-BPA), and has raised concerns about their adverse effects on the development of fetuses and infants, however, the molecular mechanisms are unclear, and related metabolomics studies are limited. Accordingly, a metabolomics study based on gas chromatography-mass spectrometry was employed to elucidate the molecular developmental toxicology of H-BPA using the marine medaka (Oryzias melastigmas) embryo model. Here, we revealed decreased synthesis of nucleosides, amino acids and lipids, and disruptions in the TCA (tricarboxylic acid) cycle, glycolysis and lipid metabolism, thus inhibiting the developmental processes of embryos exposed to H-BPA. Unexpectedly, we observed enhanced neural activity accompanied by lactate accumulation and accelerated heart rates due to an increase in dopamine pathway and a decrease in inhibitory neurotransmitters following H-BPA exposure. Notably, disorders of the neural system, and disruptions in glycolysis, the TCA cycle, nucleoside metabolism, lipid metabolism, glutamate and aspartate metabolism induced by H-BPA exposure were heritable. Furthermore, lactate and dopa were identified as potential biomarkers of the developmental toxicity of H-BPA and related genetic effects. This study has demonstrated that the metabolomics approach is a useful tool for obtaining comprehensive and novel insights into the molecular developmental toxicity of environmental pollutants.

Discovery and functional characterization of novel miRNAs in the marine medaka Oryzias melastigma

The marine medaka Oryzias melastigma has often been used as a marine fish model to investigate the biological responses to environmental stresses and pollutants in marine environments. miRNAs are post-transcriptional regulators of many biological processes in a variety of organisms, and have been shown to be affected by environmental stresses, but the novel miRNA profile of marine medaka has not been reported. Using both genome and small RNA sequencings coupled with different bioinformatics analyses, we have discovered 58, 82, 234, and 201 unannotated miRNAs in the brain, liver, ovary and testis tissues of marine medaka, respectively. Furthermore, these novel miRNAs were found to target genes with tissue-specific roles such as neuron development and synaptic transmission in the brain, glucose and fat metabolism in the liver and steroidogenesis in the gonads. We here report, for the first time, novel miRNA profile of marine medaka, which will provide a foundation for future biomarkers and transgenerational studies for the assessment of environmental stresses and pollutions in the marine environments. In a boarder context, our data will provide novel insight into our knowledge of miRNome and miR research.

Endocrine disrupting effects of domestic wastewater on reproduction, sexual behavior, and gene expression in the brackish medaka Oryzias melastigma

The objective of this study was to investigate the endocrine disrupting effects of domestic wastewater on fish using the brackish medaka Oryzias melastigma as the animal model. Estuarine water samples were collected from Sihchong Creek and Baoli Creek estuaries, Taiwan, in March of 2012 to assess the whole effluent toxicity (WET) of domestic wastewater produced by the local residents and tourists. Chemical analysis detected various pharmaceuticals and personal care products (PPCPs) in the field water samples. Some of these PPCPs are endocrine disrupting chemicals. In the laboratory-based bioassay, breeding pairs were exposed to the water samples (Sihchong, Baoli, and control) for 21 days. Cumulative number of eggs spawned was significantly higher in the Sihchong group. While fish swimming activity was not affected, sexual behavior of the male fish was significantly induced in both Sihchong and Baoli groups. Male and female gonad histology was not affected. Expression level of biomarker genes CYP1A1, HSP70, and VTG was significantly induced in the Sihchong group. This study indicates that the mixture of contaminants contained in the estuarine water may cause endocrine disrupting effects in fish.

Omics of the marine medaka (Oryzias melastigma) and its relevance to marine environmental research

In recent years, the marine medaka (Oryzias melastigma), also known as the Indian medaka or brackish medaka, has been recognized as a model fish species for ecotoxicology and environmental research in the Asian region. O. melastigma has several promising features for research, which include a short generation period (3-4 months), daily spawning, small size (3-4 cm), transparent embryos, sexual dimorphism, and ease of mass culture in the laboratory. There have been extensive transcriptome and genome studies on the marine medaka in the past decade. Such omics data can be useful in understanding the signal transduction pathways of small teleosts in response to environmental stressors. An omics-integrated approach in the study of the marine medaka is important for strengthening its role as a small fish model for marine environmental studies. In this review, we present current omics information about the marine medaka and discuss its potential applications in the study of various molecular pathways that can be targets of marine environmental stressors, such as chemical pollutants. We believe that this review will encourage the use of this small fish as a model species in marine environmental research.

Tissue-specific transcriptome assemblies of the marine medaka Oryzias melastigma and comparative analysis with the freshwater medaka Oryzias latipes

Background

The marine medaka Oryzias melastigma has been demonstrated as a novel model for marine ecotoxicological studies. However, the lack of genome and transcriptome reference has largely restricted the use of O. melastigma in the assessment of in vivo molecular responses to environmental stresses and the analysis of biological toxicity in the marine environment. Although O. melastigma is believed to be phylogenetically closely related to Oryzias latipes, the divergence between these two species is still largely unknown. Using Illumina high-throughput RNA sequencing followed by de novo assembly and comprehensive gene annotation, we provided transcriptomic resources for the brain, liver, ovary and testis of O. melastigma. We also investigated the possible extent of divergence between O. melastigma and O. latipes at the transcriptome level.

Results

More than 14,000 transcripts across brain, liver, ovary and testis in marine medaka were annotated, of which 5880 transcripts were orthologous between O. melastigma and O. latipes. Tissue-enriched genes were identified in O. melastigma, and Gene Ontology analysis demonstrated the functional specificity of the annotated genes in respective tissue. Lastly, the identification of marine medaka-enriched transcripts suggested the necessity of generating transcriptome dataset of O. melastigma.

Conclusions

Orthologous transcripts between O. melastigma and O. latipes, tissue-enriched genes and O. melastigma-enriched transcripts were identified. Genome-wide expression studies of marine medaka require an assembled transcriptome, and this sequencing effort has generated a valuable resource of coding DNA for a non-model species. This transcriptome resource will aid future studies assessing in vivo molecular responses to environmental stresses and those analyzing biological toxicity in the marine environment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1325-7) contains supplementary material, which is available to authorized users.

Transcriptome analysis of medaka following epinecidin-1 and TH1-5 treatment of NNV infection

Nervous necrosis virus (NNV) infects a wide range of larval and juvenile fish species, thereby causing enormous economic losses in the aquaculture industry. Possible solutions to this problem include the use of antimicrobial peptides (AMPs), which directly inhibit bacterial growth, and also modulate host signaling mechanisms. The AMPs epinecidin (Epi)-1 and Tilapia hepcidin (TH) 1-5 have been demonstrated to be effective against Nervous necrosis virus infection in medaka (Oryzias latipes). However, the underlying molecular mechanisms are yet to be explored. Here, microarray analyses were performed to examine how NNV infection and/or epinecidin-1 or TH1-5 treatment affects gene expression in medaka; such analyses enabled the prediction of host signaling pathways affected by virus infection and/or regulated by epinecidin-1 and TH1-5. Transcriptome analysis revealed altered expression of genes involved in B cell activation, T cell activation, adipocytokine signaling, and mast cell activation. We subsequently used real-time PCR to analyze expression of key genes involved in these signaling mechanisms. Medaka infected with NNV exhibited up-regulation of PVALB, CEBPA, IFIM, IFN, IL-6ST, NF-kB2, SOC3, SP1, and TGFB1, and such increases were prevented by pre-treatment with epinecidin-1 or TH1-5. Immunohistochemistry using the anti-NNV antibody to stain brain and eye sections revealed that epinecidin-1 treatment during or after infection clears viral load, while TH1-5 treatment only reduces viral numbers if applied during infection. These observations demonstrate that epinecidin-1 and TH1-5 modulate NNV-induced host signaling mechanisms, thereby preventing viral multiplication in host organisms.