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Abstract Background Understanding the relationship between resident microbiota and disease in cultured fish represents an important and emerging area of study. Marine gill disorders in particular are considered an important challenge to Atlantic salmon (Salmo salar) aquaculture, however relatively little is known regarding the role resident gill microbiota might play in providing protection from or potentiating different gill diseases. Here, 16S rRNA sequencing was used to examine the gill microbiome alongside fish health screening in farmed Atlantic salmon. Results were used to explore the relationship between microbial communities and gill disease. Results Microbial community restructuring was observed throughout the sampling period and linked to varied drivers of change, including environmental conditions and severity of gill pathology. Taxa with significantly greater relative abundance on healthier gills included isolates within genus Shewanella, and taxa within family Procabacteriaceae. In contrast, altered abundance of Candidatus Branchiomonas and Rubritalea spp. were associated with damaged gills. Interestingly, more general changes in community richness and diversity were not associated with altered gill health, and thus not apparently deleterious to fish. Gross and histological gill scoring demonstrated seasonal shifts in gill pathology, with increased severity of gill damage in autumn. Specific infectious causes that contributed to observed pathology within the population included the gill disorder amoebic gill disease (AGD), however due to the uncontrolled nature of this study and likely mixed contribution of various causes of gill disease to observed pathology results do not strongly support an association between the microbial community and specific infectious or non-infectious drivers of gill pathology. Conclusions Results suggest that the microbial community of farmed Atlantic salmon gills undergo continual restructuring in the marine environment, with mixed influences upon this change including environmental, host, and pathogenic factors. A significant association of specific taxa with different gill health states suggests these taxa might make meaningful indicators of gill health. Further research with more frequent sampling and deliberate manipulation of gills would provide important advancement of knowledge in this area. Overall, although much is still to be learnt regarding what constitutes a healthy or maladapted gill microbial community, the results of this study provide clear advancement of the field, providing new insight into the microbial community structure of gills during an annual production cycle of marine-stage farmed Atlantic salmon.

【Objective】Trachidermus fasciatus is a nation's second-class protected species, belonging to a migratory fish with eurysalinity. The study aims to explore a suitable desalination pattern for T. fasciatus before the proliferation and release of the fish.【Method】The microstructural changes of the gills of juvenile T. fasciatus under acute freshwater stress, different desalination speeds (the salinity was reduced by 5 concentrations each time, and reduced every 12, 24, 48 h, recorded as A, B, C groups) and different desalination amplitudes (the salinity was reduced once every 24 h, and 2, 3 and 5 salinity concentrations were reduced each time, recorded as D, E and B groups) were investigated by tissue sectioning techniques.【Result】The results showed that, when the juvenile T. fasciatus at salinity of 25 was directly put into freshwater for acute freshwater stress, the morphology of gill tissue structure was normal at 1 h; at 12 h, the gill filaments showed a slight epidermal detachment; at 24 h, the hemocytes appeared to be absorbed and swollen; at 3 d, the number of gill filaments and gill lamellae with vacuoles increased significantly; at 7 d, the gill filaments and gill lamellae showed the most serious epidermal detachment, and the length of gill lamellae was increased while the width was decreased, with bent ends, cytolysis and irregular shape. Vacuoles appeared at the base of the branchial lamella and their positions also increased. The number of blood cells decreased and the number of mitochondria-rich cells increased. At salinity of 25, the gill tissues of group A-C did not change significantly. When salinity was reduced to 15, the structural breakage of gill filaments was more obvious in group A than that in groups B and C; when salinity reached freshwater, the gill tissue structure of group A was severely broken, the length of gill lamellae was increased but the width was reduced, the structure was incomplete, the number of mitochondria-rich cells was increased, and the number of hemocytes was reduced compared with that at salinity of 15; the structures of gill filaments and gill lamellae of group A were significantly different than those of groups B and C, with the degree of incompleteness being more serious, and the difference between groups B and C was not obvious. The differences between groups B and C were not obvious, and the gill tissue structure was broken to a similar extent. When the initial salinity was 25 under freshwater acclimation for 1 h, the differences in gill tissue structure were not obvious among group D, group E and group B; at 12 h, the number of vacuoles in gill tissues of the three groups increased; at 24 h, the number of vacuoles in the gills of group B and group D was more than that of group E; at 3 d, the gill tissues of group B had more detachments compared with the other two groups; at 7 d, the number of mitochondria-rich cells and blood cells was more than that of groups B and D. The degree of incompleteness of gill tissue structure was more serious in group B than that in groups D and E.【Conclusion】With the increase of freshwater acclimation time, T. fasciatus improves its osmotic capacity by increasing the length of gill lamellae and controlling the number of hemocytes and mitochondria-rich cells to reduce the harm of stress on gills. In addition, lowering the desalination rate and amplitude will help T. fasciatus to better adapt to the salinity change, and it is recommended to choose the desalination pattern with the salinity decreasing by 2 per day to reduce the damage of freshwater acclimation on the gill tissues and structures of T. fasciatus.

Abstract The generic term “Gill disease” refers to a wide range of disorders that affect the gills and severely impact salmonid aquaculture systems worldwide. In rainbow trout freshwater aquaculture, various etiological agents causing gill diseases have been described, particularly Flavobacterium and Amoeba species, but research studies suggest a more complex and multifactorial aetiology. Here, a cohort of rainbow trout affected by gill disease is monitored both through standard laboratory techniques and 16S rRNA Next-Generation Sequencing (NGS) analysis during a natural disease outbreak and subsequent antibiotic treatment with Oxytetracycline. NGS results show a clear clustering of the samples between pre- and post-treatment based on the microbial community of the gills. Interestingly, the three main pathogenic bacteria species in rainbow trout (Yersinia ruckeri, Flavobacterium psychrophilum, and Flavobacterium branchiophilum) appear to be weak descriptors of the diversity between pre-treatment and post-treatment groups. In this study, the dynamics of the gill microbiome during the outbreak and subsequent treatment are far more complex than previously reported in the literature, and environmental factors seem of the utmost importance in determining gill disease. These findings present a potential novel perspective on the diagnosis and management of gill diseases, showing the limitations of conventional laboratory methodologies in elucidating the complexity of this disease in rainbow trout. To the authors’ knowledge, this work is the first to describe the microbiome of rainbow trout gills during a natural outbreak and subsequent antibiotic treatment. The results of this study suggest that NGS can play a critical role in the analysis and comprehension of gill pathology. Using NGS in future research is highly recommended to gain deeper insights into such diseases correlating gill’s microbiome with other possible cofactors and establish strong prevention guidelines.

Abstract The aim of this work is to examine the effects of vitamin E addition to water on the structure of the gill tissue and energy metabolism of crucian carp (Carassius auratus) under cooling stress. The crucian carp were chilled using a cold acclimation intelligent chilling equipment from 20 °C to 5 °C. They were divided into three groups: the control group (E1), the negative control group (E2), and the 100 mg/L vitamin E (E3) solution. Three different temperature points (20 °C, 10 °C, and 5 °C) were used to collect, test, and analyze the samples. The findings demonstrated that in the E3 treatment group, phosphoenolpyruvate carboxykinase, acetyl coenzyme A carboxylase, total cholesterol, urea nitrogen, triglyceride, and fatty acid synthase contents were significantly lower under cooling stress than those in the E1 and E2 treatment groups (P

Rising but fluctuating oxygen levels in the Early Palaeozoic provide an environmental context for the radiation of early metazoans, but little is known about how mechanistically early animals satisfied their oxygen requirements. Here we propose that the countercurrent gaseous exchange, a highly efficient respiratory mechanism, was effective in the gills of the Late Ordovician trilobite Triarthrus eatoni. In order to test this, we use computational fluid dynamics to simulate water flow around its gills and show that water velocity decreased distinctly in front of and between the swollen ends, which first encountered the oxygen-charged water, and slowed continuously at the mid-central region, forming a buffer zone with a slight increase of the water volume. In T. eatoni respiratory surface area was maximized by extending filament height and gill shaft length. In comparison with the oxygen capacity of modern fish and crustaceans, a relatively low weight specific area in T. eatoni may indicate its low oxygen uptake, possibly related to a less active life mode. Exceptionally preserved respiratory structures in the Cambrian deuterostome Haikouella are also consistent with a model of countercurrent gaseous exchange, exemplifying the wide adoption of this strategy among early animals.

This study aimed to investigate the protective effects of pomegranate peel (PoP) extract on the gill, liver, and kidney tissues of common carp exposed to cadmium (Cd). For this purpose, 150 common carp weighing 65 ± 0.85 g were randomly divided into five groups with three triplicates for each group (30 fish per group). The control group received a standard diet without cadmium chloride (CdCl2), the Cd group was exposed to 0.5 mg/L CdCl2, and the extract groups received PoP extract with concentrations of 1%, 2%, and 4% (percentage of food weight) along with 0.5 mg/L CdCl2 in the water. After four weeks, tissue samples were collected from the gill, liver, and kidney and stained with hematoxylin and eosin for histopathological examination. In the gills of the Cd group, lesions included congestion, hemorrhage, clubbing or fusion of the secondary lamellae, and telangiectasia of the lamellae. The liver tissue of the Cd group exhibited severe degeneration and necrosis of hepatocytes, while hemorrhage, congestion, cellular degeneration or necrosis, and hyaline cast were visible in the kidney tissue of this group. The severity of the mentioned lesions was significantly reduced in the PoP extract groups, particularly at concentrations of 2% and 4% (p < 0.05). Based on the results, it can be concluded that PoP extract has significant protective effects on the gill, liver, and kidney tissues of common carp exposed to CdCl2.

【Objective】The effects of different mass concentrations of MS-222 and eugenol stresses on gill and liver tissue damage in juvenile Eleutheronema tetradactylum were studied.【Method】Seven different mass concentrations of MS-222 (20, 30, 35, 40, 45, 50 and 60 mg/L) and seven different mass concentrations of eugenol (5, 10, 20, 25, 30, 40 and 50 mg/L) were set to analyze the static anaesthesia effects of different treatments on juvenile E. tetradactylum with an average length of 4.5 (±0.5) cm and an average body weight of 1.5 (±0.5) g.【Result】When the concentration of MS-222 was not less than 35 mg/ L, the juvenile fish began to show stress reactions after 467 s of anesthesia. When the concentration was 50 mg/L, some juvenile fish could be induced into the anesthesia period. When the concentration was 60 mg/L, all juvenile fish could be induced into the anesthesia period after 293 s. When the concentration of eugenol was not less than 25 mg/L, the juvenile fish began to exhibit stress reactions after 291 s of anaesthesia; when the concentration was 30 mg/L, some juvenile fish could be induced into the anaesthesia period; when the concentrations were 40 and 50 mg/L, all juvenile fish could be induced into the anaesthesia period at 194 s and 102 s, respectively. With the increase of MS-222 and eugenol concentrations, the time for juvenile fish to enter the same anaesthesia period was significantly shortened (P < 0.05). At high concentrations, the time for juvenile fish to enter the anaesthesia period was too fast, which exceeded the tolerance value and may have a certain toxic effect on the fish body, leading to fish death. By observing the pathological sections of juvenile fish tissues, it was found that, with the increase of MS-222 and eugenol concentrations, the liver cells of juvenile fish showed vacuoles, disordered arrangement, nuclear dissolution visible in the nucleus and unclear liver plate structure. The gill lamellae were curved with enlarged ends, and they may burst or fall off in severe cases.【Conclusion】A moderate dosage of anaesthetic can reduce the stress response of fish to the external environment and reduce oxygen consumption. Different concentrations of MS-222 and eugenol (except for 5 mg/L) have a sedative effect on E. tetradactylum within 3 minutes. When the concentrations of the two anaesthetics are too high, the anaesthesia time is too long, and the juvenile fish are in a state of stress for a long time, the fish will die.