Seasonal effects on hematological and innate immune parameters in sea bass Dicentrarchus labrax

Hepatic melanomacrophage centers in the arctic cultured fish Cyclopterus lumpus are not indicative of its health state

The lumpfish, Cyclopterus lumpus, holds significant promise as a candidate for large-scale aquaculture production, particularly in its role as a cleaner fish used to manage sea lice infestations in Atlantic salmon Salmo salar farming. Melanomacrophage centers (MMCs) represent polymorphic structures present in the hemolymphopoietic organs of various vertebrates, serving as a widely applicable histological indicator of the fish immune and health status. This study aims to investigate the histochemical characteristics of MMCs within lumpfish livers and to compare MMC density between hatchery-produced (farmed) and wild individuals. Liver samples were collected from 34 lumpfish and subjected to a range of staining techniques, including haematoxylin-eosin, Azan-Mallory's trichrome, Masson-Fontana, Perls-Van Geison, Mallory's hemofuscin, immunohistochemical detection of cytochrome P450 monooxygenase 1 A (CYP1A), and the terminal deoxynucleotidyl transferase-mediated d'UTP nick-end labelling (TUNEL) method. Hepatocytes from hatchery-produced males exhibited notably high lipid content. Additionally, cells showing positive staining with Masson-Fontana, likely associated with the monocyte/macrophage lineage, were identified. Furthermore, small MMCs containing melanin, lipofuscin-ceroids, and ferric ions were detected. While the density of single monocytes/macrophages was markedly higher in hatchery-produced males, no significant discrepancies in MMCs density were observed between wild and hatchery-produced fish, or between males and females of the same origin. The study also revealed the presence of necrotic foci, characterized by hypertrophic hepatocytes positive for both TUNEL and CYP1A staining. These hypertrophic hepatocytes displayed large lipid droplets and pycnotic nuclei, with hatchery-produced males showing a higher numerical density of such foci. In contrast to findings in other fish species, the study found that MMCs did not appear to serve as reliable markers of health status in lumpfish. This conclusion was reached as MMCs density did not exhibit a correlation with necrotic foci or hepatocyte lipid content.

Links between reproduction and immunity in two sympatric wild marine fishes

According to life-history theory, limited resources can result in trade-offs between costly physiological functions. Particularly, it can be expected that individuals present lower immune function, or an alternative immunological strategy, during their reproductive compared to their non-reproductive season. Here we investigate the link between reproduction and immunity in two sympatric marine fish species, the rockfish Sebastes oculatus and the sandperch Pinguipes brasilianus. The results showed lower values of total white blood cells and spleen index, but higher levels of natural antibodies (only in females) in reproductive rockfish compared to non-reproductive ones. On the other hand, reproductively active sandperch showed lower levels of natural antibodies and a higher neutrophil to lymphocyte ratio and spleen index (only in males), compared to non-reproductive ones. Also, negative correlations between reproductive and immune parameters were observed in female rockfish at the individual level, but not in sandperch. Our results are consistent with the presence of different immunological strategies in reproductive and non-reproductive periods, with patterns that appear to be species-specific. This specificity suggests that various aspects of immunity might respond differentially to resource limitation, which could be associated with the disparate life-history strategies of the studied species. Alternatively, though not exclusively, the observed patterns could be driven by abiotic factors that characterize the reproductive season of each species (i.e., winter for rockfish, summer for sandperch). Our study contributes to ecoimmunological knowledge on free-living fish and highlights that detection of trade-offs can depend on the combination of study species, season, sex, and specific immune components measured.

Growth performance, gut microbiota composition, health and welfare of European sea bass (Dicentrarchus labrax) fed an environmentally and economically sustainable low marine protein diet in sea cages

The large use of fish meal/fish oil in carnivorous fish feeds is the main concern regarding environmental sustainability of aquaculture. Here, we evaluated the effects of an innovative diet, designed to be (1) environmentally sustainable by lowering the marine protein content while being (2) cost effective by using sustainable alternative raw materials with acceptable cost and produced on an industrial scale, on growth performance, gut microbiota composition, health and welfare of European sea bass (Dicentrarchus labrax), a key species of the Mediterranean marine aquaculture, reared in sea cages. Results show that the specific growth rate of fish fed the low marine protein diet was significantly lower than those fed conventional diet (0.67% vs 0.69%). Fatty acid profile of fillets from fish fed a low marine protein diet presented significant lower n-6 and higher n-3 content when compared to conventional ones. Then, a significant increase in the abundance of Vibrio and reduction of Photobacterium were found in the gut of fish fed with the low marine protein diet but effects on sea bass health needs further investigation. Finally, no major health and welfare alterations for fish fed the low marine protein diet were observed, combined with a potential slight benefit related to humoral immunity. Overall, these results suggest that despite the low marine protein diet moderately affects growth performance, it nevertheless may enhance environmental and economic sustainability of the sea bass aquaculture.

Exploring gut microbiota in adult Atlantic salmon (Salmo salar L.): Associations with gut health and dietary prebiotics

BACKGROUND

The importance of the gut microbiota for physiological processes in mammals is well established, but the knowledge of their functional roles in fish is still limited. The aims of this study were to investigate associations between variation in taxonomical composition of the gut microbiota and gut health status in Atlantic salmon and to explore possible modulatory effects of dietary prebiotics in one net-pen farm in open water. The fish with initial mean body weight of around 240 g were fed diets based on the same basal composition, either without (Ref diet) or with (Test diet) yeast cell wall based-prebiotics, during the marine production phase from December to September the following year. Sampling was conducted at three sampling time points: January, April, and September, with average water temperature of 3.9 ℃, 3.4 ℃ and 9.6 ℃, respectively.

RESULTS

As the fish progressed towards September, growth, brush border membrane enzyme activities, and the expression in the gut of most of the observed genes involved in immune (e.g., il8, cd4a, myd88, il1b, gilt, tgfb, cd8b and cd3), barrier (e.g., zo1, occludin, ecad, claudin25b and claudin15), and metabolism increased significantly. Lipid accumulation in pyloric enterocytes decreased remarkably, suggesting improvement of gut health condition. The growth of the fish did not differ between dietary treatments. Further, dietary prebiotics affected the gut health only marginally regardless of duration of administration. Regarding gut microbiota composition, a decrease in alpha diversity (Observed species, Pielou and Shannon) over time was observed, which was significantly associated with an increase in the relative abundance of genus Mycoplasma and decrease in 32 different taxa in genus level including lactic acid bacteria (LAB), such as Lactobacillus, Leuconostoc, and Lactococcus. This indicates that developmental stage of Atlantic salmon is a determinant for microbial composition. Multivariate association analysis revealed that the relative abundance of Mycoplasma was positively correlated with gut barrier gene expression, negatively correlated with plasma glucose levels, and that its relative abundance slightly increased by exposure to prebiotics. Furthermore, certain LAB (e.g., Leuconostoc), belonging to the core microbiota, showed a negative development with time, and significant associations with plasma nutrients levels (e.g., triglyceride and cholesterol) and gene expression related to gut immune and barrier function.

CONCLUSIONS

As Atlantic salmon grew older under large-scale, commercial farm settings, the Mycoplasma became more prominent with a concomitant decline in LAB. Mycoplasma abundance correlated positively with time and gut barrier genes, while LAB abundance negatively correlated to time. Dietary prebiotics affected gut health status only marginally.

A Systematic Review and Meta-Analysis of Basal and Post-Stress Circulating Cortisol Concentration in an Important Marine Aquaculture Fish Species, European Sea Bass, Dicentrarchus labrax

BACKGROUND

European sea bass is a species characterized by high and dispersed cortisol levels. The aim of the present study was to analyze all published data on basal and post-acute stress cortisol levels in this species.

METHODS

For this systematic review and meta-analysis the Web of Science and Scopus databases were searched for papers reporting plasma or serum cortisol levels in E. sea bass, without language or date restrictions. Data were extracted directly for the reported results and were analyzed separately for basal and post-acute stress levels, as well their standardized mean differences (SMD) using random-effects meta-analyses.

RESULTS

Of 407 unique records identified, 69 were eligible. Basal cortisol levels had a pooled effect of 88.7 ng mL^-1 (n = 57), while post-acute stress levels were 385.9 ng mL^-1 (n = 34). The average SMD between basal and post-stress was calculated to be 3.02 (n = 22). All analyses had a high between-study heterogeneity. Results for basal and post-stress levels were affected by the assay type and anesthesia prior to blood sampling.

CONCLUSIONS

Cortisol levels in E. sea bass are higher than most studied fish species and display large heterogeneity. Application of stress led to elevated cortisol levels in all studies examined. In all cases, sources of between-studies heterogeneity were identified.

Solid-State Fermentation of Plant Feedstuff Mixture Affected the Physiological Responses of European Seabass (Dicentrarchus labrax) Reared at Different Temperatures and Subjected to Salinity Oscillation

This study aimed to evaluate the effects of dietary inclusion of plant feedstuff mixture (PFM) pre-treated by solid-state fermentation (SSF) on the physiological responses of European seabass. For that purpose, two diets were formulated to contain: 20% inclusion level of non-fermented plant ingredients mixture (20Mix) and 20Mix fermented by A. niger in SSF conditions (20Mix-SSF). Seabass juveniles (initial body weight: 20.9 ± 3.3 g) were fed the experimental diets, reared at two different temperatures (21 and 26 °C) and subjected to weekly salinity oscillations for six weeks. Growth performance, digestive enzyme activities, humoral immune parameters, and oxidative stress indicators were evaluated. A reduction in weight gain, feed intake, and thermal growth coefficient was observed in fish fed the fermented diet (20Mix-SSF). Salinity oscillation led to an increase in weight gain, feed efficiency, daily growth index, and thermal growth coefficient, regardless of dietary treatment. Higher rearing temperatures also increased daily growth index. No dietary effect was observed on digestive enzymes activities, whereas rearing temperature and salinity oscillation modulated digestive enzyme activities. Oxidative stress responses were significantly affected by experimental diets, temperature, and salinity conditions. Catalase and glutathione peroxidase activities showed an interactive effect. Fish reared at 21 °C showed higher enzymatic activity when fed the 20Mix-SSF. Conversely, fish reared at 26 °C showed higher GPx activity when fed the 20Mix diet. Fish reared at 26 °C showed reduced peroxidase and lysozyme activities, while salinity fluctuation led to increased lysozyme activity and decreased ACH50 activity. ACH50 activity increased in fish fed the 20Mix-SSF. Overall, the dietary inclusion of PFM fermented by A. niger was unable to mitigate the impact of environmental stress on physiological performance in European seabass. In fact, fermented feed caused an inhibition of growth performances and an alteration of some physiological stress indicators.

Haematology reference values for Dicentrarchus labrax and Sparus aurata: A systematic review and meta-analysis

Haematological parameters are frequently used as physiological indicators in aquaculture studies. These parameters also have extended applications in clinical evaluation, diagnosis and prognosis in fish health status. However, no normal reference range of values has been demonstrated in depth for any of these haematological parameters for the European sea bass (Dicentrarchus labrax) or gilthead seabream (Sparus aurata). The main objective of this article is to present for the first time through extended literature review, the haematological parameters normal range values for the two most important aquaculture fish species farmed in Mediterranean Sea, D. labrax and S. aurata, and to demonstrate their similarities and their differences. In this article, we also discuss the environmental and external factors affecting their normal blood parameters values and we propose fundamental guidelines on the reporting units.

Temperature effects on teleost immunity in the light of climate change

Temperature is an important environmental modulator of teleost immune activity. Susceptibility of teleosts to temperature variation depends on the species-specific adaptive temperature range, and the activity of the teleost immune system is generally temperature-dependent. Similar to many physiological and metabolic traits of ectotherms, temperature modulates the activity of immune traits. At low temperatures, acquired immunity of many teleost species is down-modulated, and their immuno-competence mainly depends on innate immunity. At intermediate temperatures, both innate and acquired immunity are fully active and provide optimal protection, including long-lasting immunological memory. When temperatures increase and reach the upper permissive range, teleost immunity is compromised. Moreover, temperature shifts may have negative effects on teleost immune functions, in particular if shifts occur rapidly with high amplitudes. On the contrary, short-term temperature increase may help teleost immunity to fight against pathogens transiently. A major challenge to teleosts therefore is to maintain immuno-competence throughout the temperature range they are exposed to. Climate change coincides with rising temperatures, and more frequent and more extreme temperature shifts. Both are likely to influence the immuno-competence of teleosts. Nonetheless, teleosts exist in habitats that differ substantially in temperature, ranging from below zero in the Arctic's to above 40°C in warm springs, illustrating their enormous potential to adapt to different temperature regimes. The present review seeks to discuss how changes in temperature variation, induced by climate change, might influence teleost immunity.

Aquaponics as a Promising Strategy to Mitigate Impacts of Climate Change on Rainbow Trout Culture

Simple Summary

Climate change and overexploitation of natural resources drive the need for innovative food production within a sustainability corridor. Aquaponics, combining the technology of recirculation aquaculture systems (RAS) and hydroponics in a closed-loop network, could contribute to addressing these problems. Aquaponic systems have lower freshwater demands than agriculture, greater land use efficiency, and decreased environmental impact combined with higher fish productivity. Rainbow trout is one of the major freshwater fish cultured worldwide, which, however, has not yet been commercially developed in aquaponics. Nevertheless, research conducted so far indicates that the trout species represents a good candidate for aquaponics.

Abstract

The impact of climate change on both terrestrial and aquatic ecosystems tends to become more progressively pronounced and devastating over the years. The sector of aquaculture is severely affected by natural abiotic factors, on account of climate change, that lead to various undesirable phenomena, including aquatic species mortalities and decreased productivity owing to oxidative and thermal stress of the reared organisms. Novel innovative technologies, such as aquaponics that are based on the co-cultivation of freshwater fish with plants in a sustainable manner under the context of controlled abiotic factors, represent a promising tool for mitigating the effect of climate change on reared fish. The rainbow trout (Oncorhynchus mykiss) constitutes one of the major freshwater-reared fish species, contributing to the national economies of numerous countries, and more specifically, to regional development, supporting mountainous areas of low productivity. However, it is highly vulnerable to climate change effects, mainly due to the concrete raceways, in which it is reared, that are constructed on the flow-through of rivers and are, therefore, dependent on water’s physical properties. The current review study evaluates the suitability, progress, and challenges of developing innovative and sustainable aquaponic systems to rear rainbow trout in combination with the cultivation of plants. Although not commercially developed to a great extent yet, research has shown that the rainbow trout is a valuable experimental model for aquaponics that may be also commercially exploited in the future. In particular, abiotic factors required in rainbow trout farming along, with the high protein proportion required in the ratios due to the strict carnivorous feeding behavior, result in high nitrate production that can be utilized by plants as a source of nitrogen in an aquaponic system. Intensive farming of rainbow trout in aquaponic systems can be controlled using digital monitoring of the system parameters, mitigating the obstacles originating from extreme temperature fluctuations.

Seasonal immune rhythm of leukocytes in the freshwater snakehead fish, Channa punctatus

Annual rhythms are observed in many physiological processes and are an important approach to cope with seasonal stressors. The use of lower vertebrates as an experimental model is crucial to understand the evolution of this biological clock. This study aims to characterize the seasonal variability in the leukocyte immune responses in Channa punctatus. Leukocytes were harvested from peripheral blood and respiratory burst activity, leukocyte phagocytosis, and nitrite production were assessed to study innate immunity. Peripheral blood lymphocytes were segregated by centrifugation (density gradient) and proliferative responses of lymphocytes, in the presence of mitogens, were used to study cell-mediated immunity. Annual rhythms were validated in superoxide anion production, nitrite release and phagocytosis. Cosinor analysis revealed a differential pattern of lymphocyte proliferation which was dependent upon season and mitogen used. It was concluded that seasonal variation in immune activity might be associated with annual adaptation against diseases and the optimum immune status of seasonal breeders like fish helps them fight seasonal changes.

Seasonal variations in the skin epidermal structure and mucosal immune parameters of rainbow trout skin (Oncorhynchus mykiss) at different stages of farming

The aim of this study was to investigate the seasonal changes in the epidermal structure and the innate immunity parameters of skin mucus in rainbow trout. The skin epidermis and mucus samples were collected over three consecutive seasons including winter, spring and late summer from three different weight groups i.e., 2-20 g (W1), 100-200 g (W2) and 400-600 g (W3) fish. The skin mucosal immunity analysis of rainbow trout showed that the haemagglutination activity increased significantly with increasing fish size from W1 to W3 in all three seasons, while no significant seasonal changes occurred in haemagglutination activity. Moreover, the bactericidal activity against fish pathogens increased significantly with increasing water bacterial load in late summer. The SDS-PAGE analysis of mucus showed a high amount of low molecular weight proteins (<35 kDa) in the late summer that was correlated with the increase in bactericidal activity. Histological analysis of the epidermis structure of rainbow trout skin showed that the density and size of goblet cells and consequently the mucus secretion significantly increased in W3 group in all seasons. In all three weight groups of fish, the density of goblet cells significantly increased from winter to spring and late summer along with increasing water temperature. Moreover, the goblet cell density showed a significant positive relationship with the soluble protein concentration and haemagglutination activity (p < 0.01). The results of this study demonstrated the more active immune role of the skin epidermal cells and mucus in rainbow trout during summer to protect fish against the pathogenic microorganisms. Given its potent bactericidal properties and the lack of haemolytic activity, the rainbow trout mucus might be used as a safe and inexpensive source for developing antimicrobial agents to prevent and treat some bacterial diseases in human and fish.

Mediterranean Aquaculture in a Changing Climate: Temperature Effects on Pathogens and Diseases of Three Farmed Fish Species

Climate change is expected to have a drastic effect on aquaculture worldwide. As we move forward with the agenda to increase and diversify aquaculture production, rising temperatures will have a progressively relevant impact on fish farming, linked to a multitude of issues associated with fish welfare. Temperature affects the physiology of both fish and pathogens, and has the potential to lead to significant increases in disease outbreaks within aquaculture systems, resulting in severe financial impacts. Significant shifts in future temperature regimes are projected for the Mediterranean Sea. We therefore aim to review and discuss the existing knowledge relating to disease outbreaks in the context of climate change in Mediterranean finfish aquaculture. The objective is to describe the effects of temperature on the physiology of both fish and pathogens, and moreover to list and discuss the principal diseases of the three main fish species farmed in the Mediterranean, namely gilthead seabream (Sparus aurata), European seabass (Dicentrarchus labrax), and meagre (Argyrosomus regius). We will attempt to link the pathology of each disease to a specific temperature range, while discussing potential future disease threats associated with the available climate change trends for the Mediterranean Sea.

Extreme winter cold-induced osmoregulatory, metabolic, and physiological responses in European seabass (Dicentrarchus labrax) acclimatized at different salinities

Despite climate-change challenges, for most aquaculture species, physiological responses to different salinities during ambient extreme cold events remain unknown. Here, European seabass acclimatized at 3, 6, 12, and 30 PSU were subjected to 20 days of an ambient extreme winter cold event (8 °C), and monitored for growth and physiological performance. Growth performance decreased significantly (p < 0.05) in fish exposed at 3 and 30 PSU compared to 6 and 12 PSU. During cold stress exposure, serum Na+, Cl-, and K+ concentrations were significantly (p < 0.05) increased in fish exposed at 30 PSU. Serum cortisol, glucose, and blood urea nitrogen (BUN) were increased significantly (p < 0.05) in fish exposed at 3 and 30 PSU. In contrast, opposite trends were observed for serum protein, lactate, and triglycerides content during cold exposure. Transaminase activities [glutamic-pyruvate transaminase (GPT), glutamic oxaloacetic transaminase (GOT), lactic acid dehydrogenase (LDH), gamma-glutamyl-transaminase (γGGT)] were significantly higher in fish exposed at 3 and 30 PSU on days 10 and 20. The abundance of heat shock protein 70 (HSP70), tumor necrosis factor-α (TNF-α), cystic fibrosis transmembrane conductance (CFTR) were significantly (p < 0.05) increased in fish exposed at 3 and 30 PSU during cold shock exposure. In contrast, insulin-like growth factor 1 (Igf1) expression was significantly lower in fish exposed at 3 and 30 PSU. Whereas, on day 20, Na+/K+ ATPase α1 and Na+/K+/Cl- cotransporter-1 (NKCC1) were significantly upregulated in fish exposed at 30 PSU, followed by 12, 6, and 3 PSU. Results demonstrated that ambient extreme winter cold events induce metabolic and physiological stress responses and provide a conceivable mechanism by which growth and physiological fitness are limited at cold thermal events. However, during ambient extreme cold (8 °C) exposure, European seabass exhibited better physiological fitness at 12 and 6 PSU water, providing possible insight into future aquaculture management options.

Prepubertal gonad investment modulates thymus function: evidence in a teleost fish

Thymus plasticity following gonadectomy or sex hormone replacement has long since exemplified sex hormone effects on the immune system in mammals and, to a lesser extent, in 'lower vertebrates', including amphibians and fish. Nevertheless, the underlying physiological significances as well as the ontogenetic establishment of this crosstalk remain largely unknown. Here, we used a teleost fish, the European sea bass, Dicentrarchus labrax, to investigate: (1) whether the regulation of thymus plasticity relies on resource trade-off with somatic growth and reproductive investment and (2) if the gonad-thymus interaction takes place during gonadal differentiation and development. Because gonadal development and, supposedly, thymus function in sea bass depend on environmental changes associated with the winter season, we evaluated thymus changes (foxn1 expression, and thymocyte and T cell content) in juvenile D. labrax raised for 1 year under either constant or fluctuating photoperiod and temperature. Importantly, in both conditions, intensive gonadal development following sex differentiation coincided with a halt of thymus growth, while somatic growth continued. To the best of our knowledge, this is the first study showing that gonadal development during prepuberty regulates thymus plasticity. This finding may provide an explanation for the initiation of the thymus involution related to ageing in mammals. Comparing fixed and variable environmental conditions, our work also demonstrates that the extent of the effects on the thymus, which are related to reproduction, depend on ecophysiological conditions, rather than being directly related to sexual maturity and sex hormone levels.

Genome-wide analysis of Toll-like receptors in zebrafish and the effect of rearing temperature on the receptors in response to stimulated pathogen infection

Water temperature has a major influence on the host innate immune defence and the infectivity of pathogens in ectothermic teleosts. Toll-like receptors (TLRs) are the first and well-characterized innate immune receptors that are conserved in vertebrates. However, little is known about the effect of temperature variation on TLRs in fish species. In this study, we used adult zebrafish as a research model to investigate the effect of water temperature on TLRs. Whole genome searches identified 20 TLR homologue genes in zebrafish. Multiple sequence alignment and protein structure analysis revealed the conserved domains for these TLR proteins. To identify TLR genes related to temperature variation, TLR family genes from 12 species with different body temperatures were assigned to conduct phylogenetic analyses. Based on the phylogenetic relationships, TLR3, TLR4, TLR5 and TLR20~21 were selected as candidate genes. Immunostimulation data indicated that TLR3, TLR5, and TLR21 were more sensitive to temperature variation and their expression levels were affected in response to pathogen stimulation. Taken together, our results provide a new opportunity to understand the roles of temperature on host innate immune response in fishes and have broader implications for disease prevention in aquaculture.

What metabolic, osmotic and molecular stress responses tell us about extreme ambient heatwave impacts in fish at low salinities: The case of European seabass, Dicentrarchus labrax

Unprecedented shifts in temperature and precipitation patterns in recent decades place multiple abiotic stressors on the fish. In teleosts, metabolic, osmoregulatory, and molecular potential as tolerance responses to extreme ambient heatwave events at different salinities are poorly understood. The study was performed to evaluate the physio-biochemical stress responses and acclimation potential of European seabass, Dicentrarchus labrax maintained at four different salinities followed by an extreme ambient heatwave exposure. Fish were kept at 32, 12, 6, and 2 psu for 35 days followed by a simulated extreme ambient heatwave (33 °C) exposure for 10 days. Fish growth performances, physio-biochemical and molecular responses were recorded. Fish acclimated at 32 and 2 psu exhibited significantly (p < 0.05) decreased growth performance. Serum [Na⁺] and [Cl^-] ions were significantly lowered (p < 0.05) in 32 psu fish on day 10 of heatwave exposure. While serum glucose, triglycerides, and protein tended to decrease during the extreme ambient heatwave exposure, lactate content increased significantly (p < 0.05) in 32 psu fish on day 10. In 32 and 2 psu fish, serum metabolic enzymes, and cortisol levels increased significantly (p < 0.05) during the extreme heatwave exposure. On days 5 and 10, HSP70 mRNA was significantly (p < 0.05) upregulated in kidneys and gills of 32 and 2 psu fish, while Igf1 showed downregulation. In gills of 2 psu fish, ATPase Na⁺/K⁺-α1 and NKCC1 expression decreased significantly (p < 0.05) in 2 psu, in contrast, significant upregulation was observed at 32 psu fish during extreme ambient heatwave exposure. On days 5 and 10, cystic fibrosis transmembrane conductance (CFTR) upregulation was significantly lower (p < 0.05) in 32 and 2 psu fish. Results suggest that European seabass held at 12 and 6 psu water fare better physiological fitness during the tested extreme ambient heatwave event (33 °C), providing possible insights into options for future aquaculture management in a warming environment.

Effects of extreme ambient temperature in European seabass, Dicentrarchus labrax acclimated at different salinities: Growth performance, metabolic and molecular stress responses

Extreme weather events are becoming more intense and frequent as a result of climate change. The modulation of hemato-physiological potential as a compensatory response to extreme warm events combined with different salinities is poorly understood. This study aimed to assess the hemato-physiological and molecular response of European seabass, Dicentrarchus labrax exposed to extreme warm temperature (33 °C) after prior acclimatization at 32 psu, 12 psu, 6 psu, and 2 psu water. Fish were acclimated to 32 psu, 12 psu, 6 psu, and 2 psu followed by 10 days extreme warm (33 °C) exposure. Along with growth performance and survival, hemato-physiological response and molecular response of fish were recorded. Fish held at 32 psu and 2 psu exhibited significantly lower growth performance and survival than those at 12 psu and 6 psu (p < 0.05). Red blood cells (RBC), hematocrit, and hemoglobin content were significantly decreased, while white blood cells (WBC), erythrocytic cellular abnormalities (ECA) and erythrocytic nuclear abnormalities (ENA) were found to increase significantly in 32 psu and 2 psu fish (p < 0.05). Plasma lactate was found to increase significantly in 32 psu fish on day 10 (p < 0.05). Activities of glutathione peroxidase (GPx), glutathione reductase (GR), superoxide dismutase (SOD), catalase (CAT), and TNF-α expression increased significantly in 32 psu and 2 psu fish (p < 0.05). Most of the repeated measured parameters indicated limited acclimation capacity during the extreme warm exposure at all four salinity groups. However, overall results indicate that European seabass acclimatized at 12 psu and 6 psu salinities, can cope better during extreme warm exposure (33 °C).

Productive Results, Oxidative Stress and Contaminant Markers in European Sea Bass: Conventional vs. Organic Feeding

In the present study European sea bass (Dicentrarchus labrax) subjected to two different diets (organic vs. conventional) were evaluated in terms of growing performances, oxidative stress, and contaminant markers. Growing performances were evaluated using biometric measures and condition factor (K), whereas insulin-like growth factor (IGF-I and IGF-II) levels were assessed trough Real-Time PCR analysis. For oxidative stress, immunohistochemical staining for 8-hydroxy-2'-deoxyguanosine (8-OHdG) and 4-hydroxy-2-nonenal (HNE) was performed, whereas total glutathione (GSH) in blood serum was determined by an enzymatic method adapted. Cytochrome P4501A (CYP1A) and melanomacrophage centers (MMCs) were evaluated as contaminant markers trough immunohistochemical and histochemical approaches, respectively. The growing performances showed a positive trend in both groups but a greater productivity in conventional fed fish compared to the organic ones. A significant higher expression of MMCs was observed in organic vs. conventional diet fed fish. Fillet analysis showed a higher MUFA content and a lower PUFAs n-6 content in organically fed sea bass indicating that diets with a content in fatty acids closer to that of wild fish will definitely affect the fatty acid profile of the fish flesh. On the other hand, the diet composition did not seem to affect neither the oxidative stress parameters (GSH, 8-OHdG, HNE) nor the CYP1A expression.

Immunity and health of two wild marine fishes naturally exposed to anthropogenic pollution

There are increasing global concerns of the alarming pollution impacts on marine life, thus it is becoming essential to generate reliable tools to monitor and understand the effects of these impacts on aquatic organisms. We performed a field study assessing how exposure to anthropogenic pollution impacts immunological and health-state parameters and parasite infection of a wild marine fish, the Brazilian sandperch Pinguipes brasilianus. Then we compared this information to previously published data of a sympatric species, the Patagonian rockfish Sebastes oculatus inhabiting the same polluted and pristine areas. The field study revealed that exposed P. brasilianus showed chronic stress, poor immune condition and higher prevalence and abundance of acanthocephalan parasites. By comparing these former results with already published in S. oculatus, we concluded that, although both species exhibited physiological alterations associate to inhabiting sites exposed to pollution, their specific immunological and health-state responses differed. Our results demonstrate that Patagonian reef-fish assemblages inhabiting sites exposed to pollutant are being affected in their immune and heath condition, which could potentially result in higher susceptibility to disease and in turn population decline. These findings highlight the necessity of more studies incorporating interspecific comparisons to assess variation in fish susceptibility in an ecoimmunotoxicological context and get a more profound understanding of anthropogenic impacts on wildlife.

Seasonal simulated photoperiods influence melatonin release and immune markers of pike perch Sander lucioperca

Melatonin is considered as the time-keeping hormone acting on important physiological functions of teleosts. While the influence of melatonin on reproduction and development is well described, its potential role on immune functions has little been considered. In order to better define an immune modulation by the melatonin hormone, we hypothesized that natural variations of photoperiod and subsequent changes in melatonin release profile may act on immune status of pikeperch. Therefore, we investigated during 70 days the effects of two photoperiod regimes simulating the fall and spring in western Europe, on pikeperch physiological and immune responses. Samples were collected at 04:00 and 15:00 at days 1, 37 and 70. Growth, plasma melatonin levels, innate immune markers and expression of immune-relevant genes in head kidney tissue were assessed. While growth and stress level were not affected by the seasonal simulated photoperiods, nocturnal levels of plasma melatonin were photoperiod-dependent. Innate immune markers, including lysozyme, complement, peroxidase and phagocytic activities, were stimulated by the fall-simulated photoperiod and a significant correlation was made with plasma melatonin. In addition to bring the first evidence of changes in fish immunocompetence related to photoperiod, our results provide an additional indication supporting the immunomodulatory action of melatonin in teleosts.

Seasonal changes of hydrolytic enzyme activities in the skin mucus of rainbow trout, Oncorhynchus mykiss at different body sizes

The innate immune factors in the skin mucus of fish are affected by the ecological and physiological conditions such as developmental stage and seasonal cycle. The aim of this study was to investigate the seasonal changes in soluble protein and the hydrolytic enzyme activities of the skin mucus of rainbow trout including lysozyme, alkaline phosphatase (ALP) and proteases at different body sizes. Skin mucus samples were collected over three consecutive season periods including winter, spring and late summer. In each season, sampling was performed separately from three different weight groups including 2-20 g (W1), 100-200 g (W2) and 400-600 g (W3) fish. Our results showed a significant increase of soluble protein in all three weight groups from winter to spring when water temperature elevated from 9 °C to 14 °C. Moreover lysozyme activity was remarkably elevated in W1 fish from winter to late summer. In all three seasons, the activity of lysozyme was significantly decreased along with increasing the fish size. Contrary to lysozyme, the activity of proteases and ALP showed a decreasing trend from winter to late summer. A significant positive correlation was found between the proteases and ALP activity, proposing that both proteases and ALP might have important synergic roles in the mucosal innate immune function of rainbow trout. Moreover, using reverse transcription PCR (RT-PCR) analysis of some proteases genes including cathepsin-L and cathepsin-D, we demonstrated that the proteases are transcribed and likely synthesized in epidermal mucus cells of rainbow trout. The present study confirmed seasonal changes of hydrolytic enzyme activities in the skin mucus of rainbow trout across all three weight groups, with the highest variation in juvenile fish.

Season and sex have different effects on hematology and cytokines in striped hamsters (Cricetulus barabensis)

Animals in the temperate zones face seasonal variations in environments and hence their immune responses change seasonally. In the current study, seasonal changes in hematological parameters and cytokines in striped hamsters (Cricetulus barabensis) were examined to test the winter immunoenhancement hypothesis, which states that immune function tends to increase in fall and winter compared with other seasons. Male and female hamsters were captured from the wild in the fall and winter of 2014 and in the spring and summer of 2015. Maximum body mass in both sexes and relative fatness in female hamsters occurred in the summer, indicating that body condition was the best during this season. All hematological parameters were not different between male and female hamsters, and were also not affected by the interaction of season and sex except neutrophil granulocytes (GRAN). Red blood cells (RBC) and haematocrit (PCV) were higher in the fall and winter, and hemoglobin concentration (HGB) was the highest in winter in hamsters compared with the spring and summer, implying that their oxygen-carrying capacity and oxygen affinity of the blood increased during these seasons. Compared with other seasons, the number of white blood cells (WBC) was higher in winter than in summer, intermediate granulocytes (MID), the percent of MID (MID%), GRAN and the percent of GRAN (GRAN%) were the highest in winter, which all supported the winter immunoenhancement hypothesis. However, the count of lymphocytes (LYMF) was the highest in spring, being inconsistent with this hypothesis. IL-2 levels, but not TNF-α, were influenced by seasons, sex and their interaction in hamsters. Regardless of sex, IL-4 titres were higher in spring and summer than in fall and winter in hamsters. INF-γ titres in male hamsters did not differ between the spring and summer, while its titres in female hamsters was lower in spring in contrast with winter and summer. Higher IL-2 and IL-4 levels during the breeding seasons might be crucial in controlling the increased possibilities of infections in these seasons. In summary, season and sex had disparate effects on different hematological profiles and the levels of cytokines in hamsters.

Varieties of immunity activities and gut contents in tilapia with seasonal changes

We performed 16S rDNA sequencing of tilapia fecal samples to analyze changes in tilapia gut contents after cultivation of the fish in the presence of sandwich-like floating beds of Chinese medicinal herbs (5 and 10% planting-areas; 5% Polygonum cuspidatum). The interactive effects between water quality and blood and hepatic pro- and anti-inflammatory concentrations were also assessed. Our results showed that the water quality (i.e., NO₃^--N, NO₂^--N, TP removal rates) improved, and the abundance of Chloroflexi and Cyanobacteria increased. The abundance of Bacteroidetes, Verrucomicrobia, Saccharibacteria, and Actinobacteria showed both significant seasonal decreases and increases in the presence of P. cuspidatum (increases in August and decreases in July). Fish blood and hepatic IL-10 and IFN-γ levels (together with fish sampled in September) significantly increased in the P. cuspidatum group sampled in August, while those of TNF-α (10% sandwich-like, P. cuspidatum), IL-1β (P. cuspidatum), IL-8 (5% sandwich-like in September, S905S) significantly decreased. Heat shock proteins 60 and 70 levels significantly increased in the P. cuspidatum group, and complement C3 and C4 concentrations significantly increased in S905S. This study demonstrated that enhanced immunity through the regulation of pro- and anti-inflammatory proteins was sustained throughout development until harvest, particularly in fish grown with P. cuspidatum.

Multidisciplinary haematology as prognostic device in environmental and xenobiotic stress-induced response in fish

The variations of haematological parameters hematocrit, hemoglobin concentration, leukocyte and erythrocyte count have been used as pollution and physiological indicators of organic dysfunction in both environmental and aquaculture studies. These parameters are commonly applied as prognostic and diagnostic tools in fish health status. However, there are both extrinsic and intrinsic factors to consider when performing a blood test, because a major limitation for field researchers is that the "rules" for animal or human haematology do not always apply to wildlife. The main objective of this review is to show how some environmental and xenobiotic factors are capable to modulating the haematic cells. Visualizing the strengths and limitations of a haematological analysis in the health assessment of wild and culture fish. Finally, we point out the importance of the use of mitochondrial activities as part of haematological evaluations associated to environment or aquaculture stress.

Estrogen-dependent seasonal adaptations in the immune response of fish

Clinical and experimental evidence shows that estrogens affect immunity in mammals. Less is known about this interaction in the evolutionary older, non-mammalian, vertebrates. Fish form an excellent model to identify evolutionary conserved neuroendocrine-immune interactions: i) they are the earliest vertebrates with fully developed innate and adaptive immunity, ii) immune and endocrine parameters vary with season, and iii) physiology is constantly disrupted by increasing contamination of the aquatic environment. Neuro-immuno-endocrine interactions enable adaption to changing internal and external environment and are based on shared signaling molecules and receptors. The presence of specific estrogen receptors on/in fish leukocytes, implies direct estrogen-mediated immunoregulation. Fish leukocytes most probably are also capable to produce estrogens as they express the cyp19a and cyp19b - genes, encoding aromatase cytochrome P450, the enzyme critical for conversion of C19 steroids to estrogens. Immunoregulatory actions of estrogens, vary among animal species, and also with dose, target cell type, or physiological condition (e.g., infected/non-infected, reproductive status). They moreover are multifaceted. Interestingly, season-dependent changes in immune status correlate with changes in the levels of circulating sex hormones. Whereas E2 circulating in the bloodstream is perhaps the most likely candidate to be the physiological mediator of systemic immune-reproductive trade-offs, leukocyte-derived hormones are hypothesized to be mainly involved in local tuning of the immune response. Contamination of the aquatic environment with estrogenic EDCs may violate the delicate and precise allostatic interactions between the endogenous estrogen system and the immune system. This has negative effects on fish health, but will also affect the physiology of its consumers.

Watershed-Scale Land Use Activities Influence the Physiological Condition of Stream Fish

Land use changes within watersheds can have large effects on stream ecosystems, but the mechanistic basis of those effects remains poorly understood. While changes to population size presumably reflect underlying variation in organismal health and condition, such individual-level metrics are rarely evaluated in the context of ecosystem disturbance. To address this deficiency, we combined physiological sampling with geographic information systems to quantify the effects of land use on physiological indicators of health in largemouth bass. More specifically, we first quantified blood metrics relating to nutrition, oxidative stress, and the glucocorticoid stress response from largemouth bass residing in eight watersheds. We then used Akaike's information criterion to define relationships between these blood metrics and land cover, including forests, agricultural areas, urban areas, and wetlands. The proportion of forest cover in a watershed was the best predictor of blood metrics representing recent feeding and resistance to oxidative stress, whereas the proportion of wetlands was the best predictor of glucocorticoid function; however, further investigation is needed, as the explanatory power of the models was relatively low. Patterns in energy reserves were not influenced by any land use practices. Interestingly, anthropogenic land use categories, such as urban and agricultural areas, were not the best predictor for any blood metrics. Together, our results indicate that fish health is most related to natural features of a landscape rather than anthropogenic land uses. Furthermore, these findings suggest that physiological methods could supplement traditional population and community assessments to develop a more comprehensive understanding of ecosystem interactions and improve stream management.

Effect of temperature and short chain fructooligosaccharides supplementation on the hepatic oxidative status and immune response of turbot (Scophthalmus maximus)

In this study the effect of diet supplementation with different levels of short-chain fructooligosaccharides (scFOS) on the hepatic oxidative status, hematology and innate immune parameters was evaluated in turbot reared at 15 °C and 20 °C. Four practical diets containing half of the protein provided by plant ingredients and the other half by fish meal were supplemented with scFOS at 0%, 0.5%, 1.0% and 2.0% and fed to turbot juveniles for 9 weeks. Independently of the rearing temperature, diet with 1% scFOS increased the haematocrit (Ht) while 2% scFOS augmented the mean corpuscular haemoglobin concentration (MCHC). Mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH), white blood cells (%) and lysozyme were higher in fish reared at 15 °C, whereas red blood cells and neutrophil numbers increased in fish reared at 20 °C. Catalase (CAT) and glutathione peroxidase (GPX) activities were affected by rearing temperature being lower in fish reared at 20 °C. Compared to the control diet, at 15 °C, turbot fed 0.5 or 1% scFOS presented lower activities of CAT and glutathione reductase (GR). At 20 °C turbot fed the 2% scFOS diet presented lower activities of CAT and GPX. Lipid peroxidation (LPO) and glucose 6-phosphate dehydrogenase (G6PDH) activity were not affected by temperature nor dietary prebiotic incorporation. Results of this study suggest scFOS has no effect on innate immunology or hematology. High temperature (20 °C) does not induce turbot oxidative stress, but the recommended dietary scFOS incorporation level for counteracting oxidative stress may differ with other rearing temperature.

Seasonal variations of the humoral immune parameters of European sea bass (Dicentrarchus labrax L.)

Seasonal cycles, mainly due to great variations in the light duration and temperature, are important and modulate several aspects of the animal behavior. In the case of poikilotherms animals such as fish this is very relevant. Thus, temperature changes fish immunity and affects disease resistance. We evaluate in this work the season variations of the European sea bass (Dicentrarchus labrax) humoral innate parameters focusing on winter months, at which the culture of this specie is more difficult. Our results showed that not all the innate immune parameters are depressed by low temperatures. Moreover, some of them are more dependent than others to the season and both temperature and photoperiod are operating together.

Seasonality influence on biochemical and hematological indicators of stress and growth of pirarucu (Arapaima gigas), an Amazonian air-breathing fish

Environmental factors such as seasonal cycles are the main chronic stress cause in fish increasing incidence of disease and mortality and affecting productive performance. Arapaima gigas (pirarucu) is an Amazonian air-breathing and largest freshwater fish with scales in the world. The captivity development of pirarucu is expanding since it can fatten up over 1 kg per month reaching 10 kg body mass in the first year of fattening. This work was conducted in three periods (April to July 2010, August to November 2010, and December 2010 to March 2011) defined according to rainfall and medium temperatures. Seasonality effect analysis was performed on biochemical (lectin activity, lactate dehydrogenase, and alkaline phosphatase activities) and hematological (total count of red blood cells, hematocrit, hemoglobin, and hematimetric Wintrobe indexes) stress indicators, as well as on growth and wellbeing degree expressed by pirarucu condition factor developed in captivity. All biochemical and hematological stress indicators showed seasonal variations. However, the fish growth was allometrically positive; condition factor high values indicated good state of healthiness in cultivation. These results reinforce the robust feature of pirarucu and represent a starting point for understanding stress physiology and environmental changes during cultivation enabling identification and prevention of fish adverse health conditions.

Protective effect of hydroferrate fluid, MRN-100, against lethality and hematopoietic tissue damage in γ-radiated Nile tilapia, Oreochromis niloticus

Hydroferrate fluid, MRN-100, an iron-based compound derived from bivalent and trivalent ferrates, is a potent antioxidant compound. Therefore, we examined the protective effect of MRN-100 against γ-radiation-induced lethality and damage to hematopoietic tissues in fish. A total of 216 Nile tilapia fish (Oreochromis niloticus) were randomly divided into four groups. Group 1 served as a control that was administered no radiation and no MRN-100 treatment. Group 2 was exposed only to γ-radiation (15 Gy). Groups 3 and 4 were pre-treated with MRN-100 at doses of either 1 ml/l or 3 ml/l in water for 1 week, and subsequently exposed to radiation while continuing to receive MRN-100 for 27 days. The survival rate was measured, and biochemical and histopathological analyses of hematopoietic tissues were performed for the different treatment groups at 1 and 4 weeks post-radiation. Exposure to radiation reduced the survival rate to 27.7%, while treatment with MRN-100 maintained the survival rate at 87.2%. In addition, fish exposed to γ-radiation for 1 week showed a significant decrease in the total number of white blood cells (WBCs) and red blood cells (RBCs) series. However, treatment with MRN-100 protected the total WBC count and the RBCs series when compared with irradiated fish. Furthermore, significant histological lesions were observed in the hepatopancreas, spleen and gills of irradiated fish. However, treatment with MRN-100 protected the histopathology of various organs. We conclude that MRN-100 is a radioprotective agent in fish and may be useful as an adjuvant treatment to counteract the adverse side effects associated with radiation exposure.

Interacting effects of water temperature and dietary protein level on hematological parameters in Nile tilapia juveniles, Oreochromis niloticus (L.) and mortality under Streptococcus iniae infection

Based on central composite rotatable experimental design and response surface method, the interacting effects of temperature (20 °C-34 °C) and dietary protein level (25%-50%) on hematological parameters including red blood cell (RBC), white blood cell (WBC) and hemoglobin (Hb) of juvenile Oreochromis niloticus were studied under laboratory conditions. The experiment lasted for 7 weeks. After the feeding trial, fish were challenged with Streptococcus iniae and mortality was recorded for within 8 days. Results showed that the linear and quadratic effects of temperature on RBC, WBC and Hb were highly significant (P < 0.01). When the dietary protein level was 25%-50%, the RBC, WBC and Hb were increased firstly and then decreased, but the linear and quadratic effects of protein level were insignificant (P > 0.05). The interacting effects of temperature and protein level on RBC and Hb were significant (P < 0.05). The regression equations of RBC, WBC and Hb toward the two factors of interest were established, with the coefficients of determination being 0.870, 0.836 and 0.881, respectively (P < 0.01). These equations could be used for prediction in practice. After the challenge, the mortalities for the combinations of 22.1 °C/28.7% and 20.0 °C/37.5% were significantly higher than 27.0 °C/37.5% (P < 0.05). The optimal temperature/dietary protein level combination was obtained at 27.9 °C/38.1% at which the lowest mortality (13.76%) was attained. This value was close to the optimal temperature/dietary protein level combination (29.4 °C/41.9%) for the greatest levels of RBC (2.560 × 10(6) μL(-1)), WBC (270.648 × 10(3) μL(-1)) and Hb (92.851 g L(-1)). The results of this study indicated that preferred temperature/dietary protein level combination might strengthen the non-specific immunity and reduce susceptibility to S. iniae.

Hematological parameters and phagocytic activity in fat snook (Centropomus parallelus) bred in captivity

The objective of this work was to determine the hematological parameters and the phagocytic capacity of peritoneal macrophages of fat snook related to sex, stage of gonadal maturation and seasonal cycle. Blood was collected from 135 animals (78 females and 57 males) and used for determinations of: erythrocyte number, hematocrit, hemoglobin, erythrocyte indices mean corpuscular volume (MCV) and mean corpuscular hemoglobin concentration (MCHC), total and differential leukocyte counts, and thrombocyte count. The phagocytic capacity and phagocytic index were determined after Saccharomyces cerevisiae inoculation in the peritoneal cavity of the animals. The hematological results according to sex showed that the erythrocyte, total leukocyte and thrombocyte counts were statistically higher in males than females, with the latter showing a higher MCV. Concerning to erythrocyte count, hematocrit and hemoglobin concentration analyzed separately by sex and stage of gonadal maturation, males were found to have significantly elevated values in the mature stage and decreased levels in the resting stage. The results of the erythrocyte and leukocyte series, thrombocytes and phagocytic activity related to seasonal cycle showed significant differences in both sexes, where hematocrit and hemoglobin concentration were lower in winter and higher in the other seasons, mean corpuscular volume was higher in the summer and lower in the winter and fall, total leukocytes and thrombocytes lower in the spring and higher in the fall, lymphocytes low in the winter and summer and high in the spring and phagocytic capacity and phagocytic index high in the summer and low in the winter and fall. The results showed that the hematological values in males are statistically higher than those in females, the erythrocyte values in males increase with the progression of gonadal maturation and that winter is the season of the year least favorable for hematological and phagocytic responses for survival of fat snook kept in captivity. The parameters studied could be utilized in the evaluation of the health status of this species in captivity.