In Isahaya Bay, Kyushu, Japan, one of the edible ray species, red stingray, Hemitrygon akajei tends to acceleratedly accumulate total mercury (THg) that was released from Mt. Unzen beside the bay as it grows. In this study, we collected 20 individuals of the female ray with wide variety of body sizes (16 to 65 cm in disc size, 140 to 9,540 gww in body wet weight), and aimed to clarify its mechanism. The stomach content analysis revealed ontogenetic changes of feeding habits among the immature females, females in transitional maturity, and mature females in its long life of 10 to 15 years or more. Immature females favor to preying on epifaunal macro-benthic animals such as shrimps, which are classified as “Low THg content group” of the secondary consumers, while mature ones mainly feed on short-neck clam, Ruditapes philippinarum, and polychaete, Nectoneanthes ijimai, which are “High THg content group” of the primary consumers. Therefore, mature females with larger bodies tend to acceleratedly accumulate THg and reached a max. 1,370 ng g⁻¹ dw, although the stable isotope analysis of carbon and nitrogen of the muscles of the mature females indicates the descent of the trophic position to the immature ones in the food web system.

We examined two groups of quantitative structure–activity–activity relationship (QSAAR) models for predicting Japanese medaka (Oryzias latipes) early-life stage (ELS) toxicities of chemicals for the purpose of chronic hazard and environmental risk assessments. The models included not only typical molecular descriptors but also acute Daphnia magna toxicity data, ELS test conditions, and information about chemical categories (e.g., pesticides). We found that acute Daphnia magna toxicity was an important descriptor for predicting fish ELS toxicity, along with molecular descriptors. The group II models, which were based on 119 training data for three warm freshwater species (fathead minnow, Japanese medaka, and zebrafish) had higher predictivity than the group I models, which were based on a 172 training data for four freshwater species (fathead minnow, Japanese medaka, zebrafish, and rainbow trout). In addition, the group II models had higher predictivity than the QSAAR models we reported previously (SAR QSAR Environ. Res. 29 : 9, 725–742 and 30 : 11, 825–846). Models developed by means of the strategy used to develop the group II models would be usable for estimating chronic fish toxicity in screening assessments such as those required under the Japanese Chemical Substances Control Law.

The early life stages of fish are particularly useful for chronic toxicity assessment of chemicals; however, thus far, only a few test methods have been established for marine fishes. The present study aimed to establish a short-term and chronic toxicity assay system using embryo and sac-fry stages of an euryhaline fish mummichog, Fundulus heteroclitus. To achieve this goal, we examined the optimal exposure duration and endpoints for the toxicity assay in seawater (30 PSU). The survival, hatchability, and dry weight of larvae were evaluated as endpoints after exposure to two typical toxicants, copper (Cu) and 3,5-dichlorophenol (3,5-DCP). In the Cu experiment, the lowest observed effect concentration (LOEC) was 0.3 mg/L for larval growth (dry weight), but even the maximum concentration (9 mg/L) had no effect on survival and hatchability. The LOEC of 3,5-DCP on larval growth was also lower than that of the other endpoints (1 mg/L vs. 3 mg/L). These LOECs for growth did not differ between the two test durations (embryonic to larval stage vs. only larval stage) for both toxicants. These results indicate that larval growth is the most sensitive and suitable endpoint for short-term and chronic toxicity assessments in this fish species.

Microplastic contamination in the environment is an emerging concern worldwide. In this review, by putting a particular focus on the ecological risk assessment of microplastic particles, we introduced several results of hazard assessments that have focused on the hazardous concentration for 5% of the species (HC5) estimated using species sensitivity distribution. In these previous studies, point estimates (mean/median) for HC5 were derived on the order of 0.01–1 µg/L (based on particle number, 10⁵–10⁶ particles/m³). We then summarized and discussed the relevant issues on the hazard assessments for microplastic particles: (1) identification of effect mechanisms relevant for risk assessment, (2) consideration of the influences of microplastic properties on effect concentrations, (3) identification of a proper concentration unit (i.e., mass-based or particle-based concentration), (4) handling of effect concentrations obtained from experiments that failed to establish concentration-response relationships, (5) consideration of bioavailability of microplastic particles, (6) consideration of environmentally realistic exposure conditions, (7) consideration of naturally occurring particles, (8) consideration of the influences of chemical additives and preservatives on effect concentrations, and (9) application of uncertainty factors to effect concentrations.

In order to develop an optimal method for the investigation of relative potencies of dioxin-like compounds using Japanese medaka, the present study conducted five independent early-life stage toxicity tests using 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as the positive control. First, both short- and long-term effects of TCDD were examined to determine sensitive and highly reproducible endpoints. Then, the outcome and threshold of TCDD toxicities under different experimental conditions, e.g., exposure timing and duration, were compared to develop a cost-effective method. Finally, we decided to observe yolk sac/pericardial edema, hatching failure, and mortality of medaka embryos/larvae within a 28-d experimental period, after 6-h exposure to TCDD at 0-d post fertilization. We obtained the LC₅₀ values of TCDD at 28-d post fertilization based on its concentration in water (6.84 ng/L) or in egg (753 pg/g-egg), and the LC₅₀ values were comparable with those reported in the literature. Thus, in our future studies, relative toxic potencies of dioxin-like compounds will be tested using the method established in this study.

The whole effluent toxicity (WET) test with freshwater aquatic organisms has already been validated for freshwater effluent management. Because the effluent is directly discharged into the coastal areas in Japan, there is an urgent need to establish a WET protocol for marine organisms. We have developed a draft protocol for a short-term chronic toxicity bioassay system to assess the effect on embryonic to larval stages of marine and brackish fish species (draft version). This study performed an inter-and intra-laboratory test using three marine fish species, Mummichog (Fundulus heteroclitus), Java medaka (Oryzias javanicus), and Red seabream (Pagrus major). Moreover, the validity and reproducibility of the draft protocol were assessed. The survival and hatchability of three fish and larval growth of F. heteroclitus and O. javanicus were evaluated as endpoints after exposure to two typical toxicants, hexavalent chromium (K₂Cr₂O₇; Cr) and 3,5-dichlorophenol (3,5-DCP). First, a satisfactory validity for the survival and hatchability for the control was obtained. Thereafter, we calculated the coefficient of variation (CV) of EC₂₅, obtained from inter-laboratory trials to examine the reproducibility of the test for respective endpoints. As a result, CV for at least one of the endpoints was within the allowable range (35%) for individual fish species, indicating a robust validity of the WET protocol for marine organisms. Future studies are necessary to clarify the usefulness of the protocol by applying it to effluents discharged into the coastal areas.

The chemical 2-phenoxyethanol (2-PE) is used as a preservative in pharmaceuticals and cosmetics. In this study, toxicity testing was conducted to determine the chronic toxicity of 2-PE on reproduction of Daphnia magna. Its toxicity on the growth of D. magna was also evaluated as an optional endpoint. The no observed effect concentration (NOEC) for growth and reproduction was 46.9 mg/L. Our results indicate that the concentration of 2-PE, which induces adverse effects on the growth and reproduction of D. magna, was much higher than the concentrations measured in Japanese urban rivers.

Since the Agricultural Chemicals Regulation law in Japan has been revised in 2019, we should conduct the ecotoxicology tests for the pesticide registration under the rules in the OECD guideline tests. However, there were several differences between the Japanese and OECD guideline tests, such as the body size of test fish in the fish acute toxicity test. For instance, recommended body size in Cyprinus carpio before the revision of Japanese guideline was 2–6 cm (total length), whereas that in the OECD test guideline (OECD TG) 203 is 2–4 cm. To confirm the toxicity in the different body sized C. carpio (total length: 3–6 cm) to Pentachlorophenol sodium (PCP-Na), we conducted the fish acute toxicity tests according to the OECD TG 203. The median lethal concentrations (96 h-LC₅₀ values) (0.12–0.26 mg/L) in all the test groups were comparable to each other. However, the median lethal time (LT₅₀ value) (as a sub-parameter in this research) was correlated with the fish body size, suggesting that this parameter is easily affected by the small difference in body size. Considering the test reproducibility, it may be difficult to use the LT₅₀ value for the guideline test. In summary, the previous experimental data using 3–6 cm test fish may be applied to the revised guideline test in Japan.