Mitsuharu Yagi

The deep sea is increasingly recognized as a major sink for marine plastic debris, yet the biological availability of deposited macroplastics remains poorly understood. We investigated macroplastic ingestion by the deep-sea isopod Bathynomus doederleini in the continental slope and Goto submarine canyon of the East China Sea. Among 280 individuals collected during seven research cruises, macroplastics were detected in 13 individuals (4.6%), yielding 70 items identified by ATR-FTIR. One individual contained 37 items, and another showed putative intestinal obstruction associated with accumulated macroplastics. Ingestion probability increased significantly with body size. Macroplastics from the submarine canyon were smaller than those from the continental slope, and polymer composition differed between habitats. These findings provide the first evidence of macroplastic ingestion by B. doederleini and demonstrate that macroplastics deposited on the deep seafloor can enter the feeding ecology of deep-sea scavengers, highlighting a biological pathway for plastic entry into deep-sea food webs.

Quantitative data on seasonal variation in heterobranch sea slug assemblages remain scarce in warm-temperate coastal regions, despite their ecological importance as benthic consumers and potential indicators of environmental change. We conducted standardized, multi-season SCUBA surveys of sea slug assemblages at two rocky reef sites (Tatsunokuchi and Nomozaki-Akase) along the northwestern coast of Kyushu, Japan, from February 2024 to November 2025. During the study period, we recorded 82 species and 891 individuals. Observed species richness and total abundance varied among survey months and were generally higher in winter-spring than in summer-autumn. The relative contributions of individual species also varied among survey months, while observed richness, abundance, and diversity-index values were generally lower during warmer months. Water temperature showed a clear seasonal cycle and was negatively correlated with both species richness and total abundance. Because temperature covaried with season and other unmeasured environmental and biological factors, these relationships are interpreted as descriptive associations rather than evidence of a causal temperature effect. This 22-month study provides an initial quantitative baseline for sea slug assemblages in warm-temperate coastal Japan and a reference for future surveys using comparable methods.

Microplastics (MPs) adsorb hazardous substances and are ingested by a wide range of organisms; therefore, indicators for managing their environmental concentrations are needed. Ideally, threshold values should be based on health impacts. However, the diversity of MPs and the complexity of their environmental behavior make it difficult to establish unified environmental concentration standards. In this study, we propose a threshold for the presence of MPs on sandy beaches based on “visual cleanliness,” derived from the amount of MPs that people find psychologically unacceptable. Three types of MPs were used: white polypropylene (PP), blue PP, and white polystyrene (PS; expanded polystyrene). The survey was conducted in Japan, and the number of valid responses was 245. For defining a narrow-range cleanliness threshold, volume concentration was more appropriate than mass concentration. White particles were expected to be less noticeable because they tended to blend with white shell fragments, which are ubiquitous on beaches. In contrast, blue particles were expected to be less acceptable owing to their rarity. However, we found no difference in unacceptability between white PP and blue PP. The threshold, defined as the volume concentration at which half of the respondents find MPs psychologically unacceptable, ranged from 1 to 2 cm³-MPs/m²-sand. Gender, age, travel time to the beach, and frequency of beach visits did not influence unacceptability. Strong concern about marine plastic pollution and experience in cleaning public spaces were associated with a tendency toward low tolerance for MP contamination on beaches.

Sexual selection arises from individual differences in reproductive success, which can drive the maintenance of genetic polymorphisms in genes subject to balancing selection by the pleiotropic effects that trade-off between survival and reproduction. However, the extent to which sexual selection maintains genetic polymorphisms in wild populations remains unclear. Here, we explored on genomic signatures of balancing selection and selective sweep in the northern medaka, Oryzias sakaizumii in Japan by performing whole-genome resequencing of wild individuals. In addition, we re-evaluated the population genetic structure and admixture of Oryzias latipes and O. sakaizumii across the Japanese archipelago and detected genomic regions affected by introgression. Regions with signatures of selection from multiple statistics were located on eleven chromosomes. In particular, a region spanning 4.25 to 6.80 Mb on chromosome 18 showed high genetic diversity that could not be explained by sex differentiation or introgression from O. latipes in Eastern Japan. This pattern suggests that balancing selection maintains genetic polymorphisms in O. sakaizumii . Specifically, because a previously reported quantitative trait locus associated with female mating behavior overlaps with this region, we infer that sexual selection contributes to the maintenance of genetic polymorphism at this locus.

Bathynomus doederleini Ortmann, 1894, a deep-sea isopod, is a benthic scavenger in the northwestern Pacific. Little is known about its habitat and intraspecific variation in body size in relation to depth. We examined habitat depth, body-size distribution, and range limits of B. doederleini along the western coast of Kyushu, Japan using baited traps deployed at of 151–821 m deep. A total of 1,152 individuals were collected, with the highest catch per unit effort (CPUE) between 400 and 500 m. CPUE declined sharply below 700 m, likely due to thermal constraints and interspecific competition. Body size varied significantly with depth. Minimum body size increased with depth, whereas maximum body size remained constant. Smaller individuals were more abundant in shallower, warmer waters, suggesting ontogenetic habitat segregation, possibly driven by metabolic and competitive factors. No brooding individuals were captured, supporting previous findings that reproductive females tend to be underrepresented in baited traps. Results show that B. doederleini exhibits depth-related niche partitioning. Our study area likely represents the northernmost distribution limit of the species in the East China Sea. The study highlights the importance of trap type and environmental gradients in understanding the ecology of deep-sea species.

Small microplastics (SMPs) are increasingly recognized as a numerically dominant, but poorly constrained fraction of marine plastic pollution. Here, we present the first size-integrated assessment of SMPs (1 – 300 µm) throughout the water column off the coast of western Kyushu, Japan, combining micro-Fourier transform infrared spectroscopy (µ-FTIR) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDX) analyses of seawater samples using a conductivity-temperature-depth (CTD) sensor. SMPs were detected at all stations and depths (0.1 – 109 m), demonstrating vertical pervasiveness. Polymer confirmation showed that only 21% of optically identified particles were synthetic, highlighting substantial optical overestimation. In contrast, SEM revealed particle concentrations orders of magnitude higher in the <3 µm fraction, indicating that most SMPs occur below the reliable detection range of µ-FTIR-based surveys. Size distributions were strongly skewed toward small particles, with nearly half of µ-FTIR-identified SMPs in the 10 – 20 µm range and almost half of SEM-detected particles <3 µm. Vertical profiles were heterogeneous and weakly depth-dependent, suggesting that ultrafine SMPs increasingly behave like suspended particulates rather than buoyant debris. Integration of µ-FTIR, SEM, and neuston net data yielded a continuous size spectrum spanning nearly four orders of magnitude and following a steep power-law decline with increasing particle size [[EQUATION]], where y is particle abundance in particles m-3 and x is particle size in µm. These findings demonstrate that power-law models enable future monitoring and modelling efforts in microplastic quantification, fate, and distribution in marine ecosystems.

Microplastics (MPs) adsorb hazardous substances and are ingested by a wide range of organisms; therefore, indicators for managing their environmental concentrations are needed. Ideally, threshold values should be based on health impacts. However, the diversity of MPs and the complexity of their environmental behavior make it difficult to establish unified environmental concentration standards. In this study, we propose a threshold for the presence of MPs on sandy beaches based on “visual cleanliness,” derived from the amount of MPs that people find psychologically unacceptable. Three types of MPs were used: white polypropylene (PP), blue PP, and white polystyrene (PS; expanded polystyrene). For defining a narrow-range cleanliness threshold, volume concentration was more appropriate than mass concentration. White particles were expected to be less noticeable because they tended to blend with white shell fragments, which are ubiquitous on beaches. In contrast, blue particles were expected to be less acceptable owing to their rarity. However, we found no difference in unacceptability between white PP and blue PP. The threshold, defined as the volume concentration at which half of the respondents find MPs psychologically unacceptable, ranged from 1 to 2 cm3-MPs/m2-sand. Gender, age, travel time to the beach, and frequency of beach visits did not influence unacceptability. Strong concern about marine plastic pollution and experience in cleaning public spaces were associated with a tendency toward low tolerance for MP contamination on beaches.

Understanding long-term changes in marine biodiversity is essential for evaluating effects of climate change on coastal ecosystems. In this study, we compared heterobranch sea slug (hereafter referred to simply as ‘sea slugs’) assemblages in northwestern Kyushu, Japan, across three survey periods: (1) historical records from 1960–1980, for which the exact survey effort and duration were not documented; (2) surveys conducted from April 2001 to September 2003; and (3) recent surveys conducted from June 2023 to January 2024. A total of 47 sea slug species were recorded from our underwater surveys conducted in 2023–2024. Species diversity indices, including the Shannon–Wiener diversity index ( H ) and Simpson’s diversity index ( D ), showed higher values during this survey than those of 2001–2003. Comparative analysis of species composition revealed significant shifts, with 15 species exhibiting statistically significant differences in relative abundance from the past to the present. Notably, several species common in the past, such as Aplysia kurodai, were rarely observed in the recent survey, while many tropical–subtropical species appeared for the first time. The proportion of tropical–subtropical species increased markedly, whereas subarctic species were no longer detected. Jaccard’s coefficient indicated that the current community differs markedly from those in earlier periods. These findings suggest a major community reorganization, potentially driven by rising seawater temperatures and other environmental changes. This study highlights the importance of long-term monitoring using multiple indicators to detect and interpret climate-driven biodiversity shifts in coastal marine ecosystems.

Quantitative information on the seasonal dynamics of heterobranch sea slug assemblages remains limited in warm-temperate coastal regions, despite their ecological importance as benthic consumers and indicators of environmental change. Here, we conducted a standardized, multi-seasonal SCUBA-based survey of sea slug assemblages at two rocky reef sites (Tatsunokuchi and Nomozaki–Akase) along the northwestern coast of Kyushu, Japan, from February 2024 to November 2025. Across the study period, a total of 81 species comprising 892 individuals were recorded. Species richness and total abundance exhibited pronounced seasonal variation at both sites, with higher values in winter–spring and marked declines during summer. Assemblage composition shifted seasonally from relatively even communities in winter–spring to dominance by a few taxa in summer, a pattern reflected by concurrent changes in diversity indices. Water temperature displayed clear seasonal cycles and was negatively correlated with both species richness and total abundance, indicating a close association between thermal conditions and seasonal changes in sea slug assemblages. While causal mechanisms were not explicitly tested, these consistent patterns highlight the importance of temporal environmental variability in structuring heterobranch communities in this region. This study provides one of the few quantitative, multi-seasonal baselines of heterobranch sea slug assemblages in warm-temperate coastal Japan, offering a reference framework for future ecological monitoring and assessments of environmental change.