Freshwater Fish Research Articles

Diplozoids are ectoparasites that mainly infect the gills of freshwater fish. While the life cycles of Eudiplozoon nipponicum (Goto, 1891) Khotenovsky, 1985 and some Paradiplozoon Achmerov, 1974 species are documented, Sindiplozoon Khotenovsky, 1981, development remains unclear. During a survey of fish parasites, diplozoids were collected from the predatory carp, Chanodichthys erythropterus Basilewsky, 1855, in the Lancang River and cultured Kanglang fish, Anabarilius graham Regan, 1908, in Kunming. Morphological and molecular methods confirmed all specimens as Sindiplozoon coreius Cao, 2022, and five developmental stages with their typical features were observed through morphological observations: oval egg with filament; ciliated oncomiracidium with hooks and one pair of clamps; diporpa with additional clamps; X-shaped juvenile with developing clamps; and adult with complete clamps and mature reproductive system. Morphometric analysis showed the central hook grew significantly during the transition from oncomiracidium to diporpa ( p < 0.001). The buccal sucker, pharynx, and body length increased notably from juvenile to adult ( p < 0.001). Clamps developed steadily throughout the life cycle, reaching maximum maturity at the adult stage. This is the first detailed description of S. coreius development, confirming species identity and expanding its known host range and distribution.  Graphical Abstract

Environmental and biological drivers of 137Cs accumulation in freshwater fish across forested and downstream sites in Fukushima.

Flexible silver-metal-organic framework probe for highly sensitive and visual detection of tetracycline hydrochloride in freshwater fish.

Study on the residual dynamics of diazepam in freshwater fish of Zhejiang, China and its implications for human health.

Caddisfly larvae (Trichoptera: Limnephilidae) facilitate the uptake of microplastics by a freshwater fish (Ameiurus nebulosus).

The influence of boating disturbance on the parental care behaviors of smallmouth bass (Micropterus dolomieu).

Oxidative stress and caspase 3/Gsdme-dependent pyroptosis contributes to high fat diet induced-intestinal inflammation and lipotoxicity via Srebp1 cleavage at D444 site by caspase 3.

Assessing the human health risks from organophosphate esters: exposure assessment via wild freshwater fish consumption.

A mechanistic investigation of the effects of lactic acidosis on myocardial contractility in the Neotropical fast-swimming freshwater fish Brycon amazonicus.

Analysis of metabolic characteristics of highly resistant grass carp bred on the basis of constitutive expression level of innate immune molecules.

Sex-specific markers are important basic tools for the sex-controlled breeding of farmed fish. Here, we aimed to develop a rapid yet accurate, cost-effective method for determining the genetic sex of the Chinese hooksnout carp (Opsariichthys bidens), a freshwater fish. Using whole-genome resequencing technology, along with bulked segregant analysis (BSA) and chromosome quotient (CQ) methods, sex-specific regions were screened, and corresponding primers were designed to validate the screening results. A total of 45 sex-specific regions were successfully screened through BSA sequencing and CQ analysis, and 50 pairs of primers were designed for use in the screening verification. The Mar28 primer pair showed stable sex specificity in multiple populations of O. bidens, accurately distinguishing male from female individuals. This primer pair amplified two bands (509 and 814 bp) in males, but only one band (509 bp) in females. The genetic sex identification method established here provides a theoretical basis for studying the mechanism of sex determination in O. bidens, has implications for the monosex culture and molecular breeding of O. bidens, and has significant scientific and practical value.

PFAS-induced immunotoxicity in freshwater fish of inland China: mechanisms and ecological risks.

Alternative responses to environmental change can buffer populations from climate variation. Variation in behavioural responses to thermal stress may mediate population-level responses to warming, but relatively little is known about the trade-offs associated with alternative strategies. In freshwater ecosystems cold-water fish exhibit intra-population variation in refuge seeking behaviour during periods of thermal stress. Moving to refuge may present a conflict between temperature and food, since consumption rates should decline with increasing fish densities (and potentially lower food production) in refuges. In comparison, fish that do not seek refuge incur higher metabolic demands but may also have higher foraging rates. While this trade-off is fundamentally important for cold-water fish in a warming world, it is difficult to quantify, and the relative fitness of alternative strategies remains largely unknown. We integrated field measurements of adult coastal cutthroat trout diet, fish growth and water temperature within a bioenergetics framework to quantify the energetic trade-offs associated with alternative responses to summer maximum temperatures. While the energetic costs and benefits of each strategy were markedly different, both yielded comparable growth (i.e. net energy balance), suggesting that both strategies represent evolutionarily stable strategies at present. However, warming temperatures are expected to penalize fish remaining in non-refuge habitat, making this strategy less energetically profitable in the future. Our findings underscore the importance of considering individual variation in response to climate change and floodplain heterogeneity in climate adaptation planning.

Freshwater ecosystems are increasingly imperiled by the dual pressures of biological invasions and climate change, necessitating robust predictive frameworks for effective management. This study integrates advanced ensemble machine learning (EML) within a species distribution modeling (SDM) framework to assess the current and future invasion risk of Carassius species (C. auratus, C. gibelio, and C. langsdorfii) across Iranian inland waters. A comprehensive dataset of 486 occurrence records was analyzed alongside eight rigorously selected environmental predictors encompassing climatic, topographical, hydrological, and anthropogenic variables. The BIOMOD2 R package facilitated the construction of an EML-based SDM, leveraging six algorithms weighted by AUC to maximize predictive accuracy. Model performance, evaluated via AUC and true skill statistic (TSS), demonstrated high discriminatory power. Projections under two CMIP6 climate scenarios (SSP 126 and SSP 585) reveal significant potential for range expansion and spatial redistribution of Carassius species, particularly under high-emission trajectories, highlighting increased invasion risks in ecologically sensitive basins. Variable importance analysis underscored the primacy of temperature, precipitation, terrain ruggedness, and human footprint in shaping invasion potential. Additionally, using kernel density estimation (KDE) analysis, the Caspian basin emerged as a critical invasion region for Carassius populations. These findings underscore the urgent need for targeted monitoring and management strategies and demonstrate the utility of EML-SDMs in anticipating biological invasions under global change. The integrative approach presented here provides a scalable framework for proactive biodiversity conservation and policy development in freshwater systems facing multifaceted anthropogenic threats and provides a replicable framework for forecasting biological invasions in other vulnerable freshwater systems.Supplementary InformationThe online version contains supplementary material available at 10.1038/s41598-025-21958-8.

This study analyzed microplastics (MPs; target size: 0.1-5mm) in freshwater fish and aquatic plants in an urban lake in Kumamoto, Japan. MPs were detected in 82% of the collected sample of herbivorous fish species, Nile tilapia (Oreochromis niloticus) and Redbelly tilapia (Tilapia zillii) at mean level of 10 ± 13 items ind-1, which was more than 10 times higher than the levels found in two carnivorous fish species, largemouth bass (Micropterus salmoides) and snakehead (Channa argus). Fish samples near the outlets of stormwater runoff and a wastewater treatment plant showed higher abundances of MPs than fish collected from other locations in the lake. Polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET) were dominant polymers in fish, accounting for 83%. These three polymers were also commonly detected in three species of aquatic plants, which retained MPs at 4.6-35itemsg-1 dry weight. A comparison of the characteristics of the MPs detected in the herbivorous fish and aquatic plant samples collected at the same station showed similar profiles in terms of polymer types, shape, and colors. This indicates that herbivorous fish are exposed to MPs adsorbed onto the surface of aquatic plants through their feeding activity. Our findings highlighted that aquatic plants act as a mediator to increase the bioavailability of MPs in freshwater fish by trapping MPs on the plant surface.

Correction: The biological relevance of potentially toxic metals in freshwater fish

Climate change is expected to increase the invasiveness of non-native aquarium fish, yet national assessments rarely consider future warming conditions. We screened 46 popular ornamental freshwater fish imported into Türkiye for invasion risk. Each species received a Basic Risk Assessment (BRA) score under current conditions and an adjusted Climate Change Assessment (CCA) score under regional warming scenarios. The BRA classified 12 species (26.1%) as high risk, while under the CCA, this rose to 16 species (34.8%). Four species, namely goldfish Carassius auratus, common carp Cyprinus carpio, vermiculated sailfin catfish Pterygoplichthys disjunctivus, and leopard pleco P. gibbiceps, reached very high-risk status. Climate change adjustment resulted in an increase in the basic scores for 34 species, a decrease for three, and no change for nine. Seven species achieved the maximum increment of + 12 under predicted climate change conditions. Our results show that marginal habitats, such as thermal springs and effluent canals, could become suitable for these species, even as water stress in Central Anatolia threatens both native and invasive species. We recommend dynamic, climate-aware risk protocols, combined with empirical validation via field and eDNA monitoring, and a precautionary ‘whitelist’ import system based on robust risk screening protocols as applied in this study.

ABSTRACTAquatic ecosystems have their abiotic and biotic factors constantly altered by various factors. Among them, water temperature is an abiotic factor that can significantly affect fish physiology, increasing energy demand, which can impact homeostasis and survival. Endocrine and metabolic changes and enzymatic modulation are referred to as stress responses, which can lead to oxidative stress, generating negative physiological effects when temperature limits are exceeded. Oxidative stress biomarkers used in combination can highlight the effects of a stressful condition. Here, we seek to understand how the species Astyanax lacustris, which is native to Brazil and has ecological and economic importance, as well as remarkable research potential, responds to changes in water temperature. Thus, we evaluated the effects of high (31°C ± 1°C) and low (15°C ± 1°C) thermal stress on the antioxidant defense system in the heart and kidneys of A. lacustris. Specimens were collected from artificial lakes in União da Vitória (PR) and exposed to different temperatures for periods of 2, 6, 12, 24, 48, 72, or 96 h, with a control group mantained at 23°C ± 1°C. The results indicated that in the heart exposed to 31°C, there was modulation in the biomarkers superoxide dismutase (SOD), glutathione peroxidase (GPx), and glutathione (GSH), while at 15°C only GPx activity was altered. In the kidneys of fish exposed to 31°C, there was a change in the activity of the biomarkers catalase (CAT), glutathione‐S‐transferase (GST), and lipid peroxidation (LPO), while at 15°C there was modulation of the glutathione reductase (GR) biomarker and changes in the levels of reactive oxygen species (ROS). Responses to heat stress were organ‐specific, influenced by temperature and exposure time. Principal component analysis (PCA) indicated an association of glutathione‐dependent biomarkers at high temperatures in the kidneys, while responses in the heart were similar across temperatures. Overall, A. lacustris exhibited distinct antioxidant responses in different tissues under thermal stress, with kidney response being more sensitive to heat, while cardiac responses were less variable across treatments.

Coal mining activities release various contaminants into aquatic ecosystems, including polycyclic aromatic hydrocarbons (PAHs), such as benzo[a]pyrene (B[a]P), which are of particular concern because of their persistence and toxicity. This review examines B[a]P as a model toxicant, detailing its environmental entry routes and effects on freshwater fish. We emphasize the current knowledge on the molecular mechanisms of B[a]P-induced toxicity, focusing on oxidative stress, DNA damage, and histopathological alterations across multiple organ systems. Species- and tissue-specific responses of various fish are discussed, highlighting the complex interplay between the exposure conditions and biological outcomes. This review also explores the application of diverse biomarkers, including biochemical, histological, and molecular endpoints, for assessing fish health in coal-impacted ecosystems. Emerging technologies, such as omics approaches and non-invasive sampling methods, have been evaluated for their potential to enhance environmental monitoring capabilities. By integrating the findings from laboratory and field studies, this review provides a comprehensive overview of the sublethal physiological effects and ecological consequences of acute and chronic B[a]P exposure in aquatic ecosystems affected by mining. This analysis offers insights into evidence-based management strategies and underscores the need for ongoing monitoring to protect freshwater biodiversity in coal mining regions worldwide.

Patient: Female, 38-year-oldFinal Diagnosis: Parasitic acute cholecystitusSymptoms: Abdominal painClinical Procedure: —Specialty: SurgeryObjective: Rare diseaseBackgroundClonorchis sinensis is a common liver fluke that typically inhabits the bile ducts, with infections primarily reported in endemic regions such as Northeast China, Korea, Taiwan, and Vietnam. Diagnosing parasitic infections of the biliary system can be challenging, particularly because their clinical presentation often overlaps with that of more common conditions such as acute or chronic cholecystitis. In regions where such parasites are not endemic, these infections are often overlooked, potentially leading to delayed or inappropriate treatment. Increased awareness is therefore essential.Case ReportWe present a case of a 38-year-old woman from Irbid, Jordan, with no prior medical or surgical history, who presented with symptoms consistent with acute cholecystitis. Imaging showed a distended gallbladder with pericholecystic fluid, and laparoscopic cholecystectomy was performed. Histopathological examination revealed features of acute-on-top-of-chronic cholecystitis with a mixed inflammatory infiltrate including eosinophils. A fluke-shaped parasite, measuring approximately 2×1 millimeters in size and containing eggs, was morphologically consistent with Clonorchis sinensis. This finding was unexpected given the patient’s lack of travel to endemic areas or known dietary exposure risks, such as consumption of raw freshwater fish. She was treated successfully with anti-helminthic medication and recovered completely without complications. Post-treatment follow-up confirmed full resolution without recurrence.ConclusionsAlthough Clonorchis sinensis is not native to Jordan, this case highlights the importance of considering parasitic infections in the differential diagnosis of cholecystitis, even in atypical settings. As global travel and food importation increase, healthcare providers must remain vigilant for uncommon infectious etiologies presenting in non-endemic areas.