Osmoregulatory response of Senegalese sole (Solea senegalensis) to changes in environmental salinity

Gulf toadfish (Opsanus beta) urinary bladder ion and water transport is enhanced by acclimation to higher salinity to serve water balance

Marine teleosts experience ion gain and water loss in their natural habitats. Among other tissues, the urinary bladder epithelium of marine fishes has been shown to actively transport ions to facilitate water absorption. However, transport properties of the urinary bladder epithelium of marine fishes and its plasticity in altered ambient salinities is relatively under-investigated. We describe urinary bladder epithelium electrophysiology, water flux, and expressions of ion transporters in urinary bladder tissue of Gulf toadfish (Opsanus beta) acclimated to either 35 ppt or 60 ppt seawater. Water absorption in bladder sac preparations increased ∼350% upon acclimation to 60 ppt. Increases in water transport coincided with a significant ∼137% increase in urinary bladder tissue mucosal-to-serosal short circuit current (Isc) and a ∼56% decrease in tissue membrane resistance. Collectively, these metrics indicate that an active electrogenic system facilitates water absorption via Na+ (and Cl-) transport in urinary bladder tissue. Furthermore, pharmacological inhibition of urinary bladder tissue Isc and expression of a suite of ion transporters and channels previously unidentified in this tissue provide mechanistic insights into the transport processes responsible for water flux. Analysis of water transport to overall Gulf toadfish water balance reveals a modest water conservation role for the urinary bladder of ∼0.5% of total water absorption in 35 ppt and 1.9% in 60 ppt acclimated toadfish. These results emphasize that electrogenic ion transport facilitates water-absorptive properties of the urinary bladder in Gulf toadfish-a process that is regulated to facilitate water homeostasis.NEW & NOTEWORTHY Novel experiments showcasing increased urinary bladder water absorption, ion transport, and altered channel/transporter expression in a marine fish acclimated to high salinities. Our results provide additional and noteworthy mechanistic insight into the ionoregulatory processes controlling water transport at the level of the urinary bladder in marine teleosts. Experimental outcomes are applied to whole organism-level water transport values, and the relative importance of marine teleost urinary bladder function to overall organism water conservatory measures is discussed.

Impact of different levels of handling on Solea senegalensis culture: effects on growth and molecular markers of stress

Aquaculture routine practices may cause stress induction on the fish and compromise their welfare affecting the production. This experiment aimed to evaluate the potential links between handling during culture with stress responses and growth on Senegalese sole (Solea senegalensis). We worked with two fish cohorts in terms of initial body weight and culture stage: Trial 1 included specimens in the fattening stage (226 ± 4.96 g) and Trial 2 animals in the pre-fattening stage (27.20 ± 0.44 g). The tested culture protocol, which lasted 6 and 4 months for Trial 1 and 2, respectively, mainly reduced handling-derived stressors in the experimental tanks via lowering routine samplings to a minimum. This decrease of the handling-derived stress was reflected in both trials with lower concentration of circulating cortisol in blood plasma from the experimental fish when compared to controls. Moreover, the proposed protocol promoted higher growth in the fish cultured in the less disturbing protocol in Trial 2. Higher specific growth rates and mean body weight and length were reported. In order to further explore the potential beneficial effects of our protocol, we studied the musculoskeletal from Trial 2 gene expression of key genes regulating glucocorticoid signaling pathway and apoptosis: glucocorticoid receptors 1 and 2 (gr1, gr2), heat shock protein 90 AA (hsp90aa), and caspase 6 (casp6). In line with the cortisol reduced level in this trial, gr1, hsp90aa, and casp6 genes showed lower expression in the samples coming from the experimental group. The findings of this study provide valuable information to the aquaculture industry for the management of Solea senegalensis stress and welfare.

Environmental salinity modulates olfactory sensitivity in the euryhaline European seabass, Dicentrarchus labrax, acclimated to seawater and brackish water

The olfactory epithelium of fish is - of necessity - in intimate contact with the surrounding water. In euryhaline fish, movement from seawater to freshwater (and vice versa) exposes the epithelium to massive changes in salinity and ionic concentrations. How does the olfactory system function in the face of such changes? The current study compared olfactory sensitivity in seawater- (35‰) and brackish water-adapted seabass (5‰) using extracellular multi-unit recording from the olfactory nerve. Seawater-adapted bass had higher olfactory sensitivity to amino acid odorants when delivered in seawater than in freshwater. Conversely, brackish water-adapted bass had largely similar sensitivities to the same odorants when delivered in seawater or freshwater, although sensitivity was still slightly higher in seawater. The olfactory system of seawater-adapted bass was sensitive to decreases in external [Ca2+], whereas brackish water-adapted bass responded to increases in [Ca2+]; both seawater- and brackish water-adapted bass responded to increases in external [Na+] but the sensitivity was markedly higher in brackish water-adapted bass. In seawater-adapted bass, olfactory sensitivity to l-alanine depended on external Ca2+ ions, but not Na+; brackish water-adapted bass did respond to l-alanine in the absence of Ca2+, albeit with lower sensitivity, whereas sensitivity was unaffected by removal of Na+ ions. A possible adaptation of the olfactory epithelium was the higher number of mucous cells in brackish water-adapted bass. The olfactory system of seabass is able to adapt to low salinities, but this is not immediate; further studies are needed to identify the processes involved.

The Biochemical Composition and Quality of Adult Chinese Mitten Crab Eriocheir sinensis Reared in Carbonate-Alkalinity Water

Saline-alkaline aquaculture has become an important breakthrough in expanding the space available for aquaculture in China. However, the biochemical composition and quality of Eriocheir sinensis reared in carbonate alkalinity water are still unclear. Therefore, this study investigated the edible yield, coloration, and nutritional and flavor quality of Eriocheir sinensis. A significantly lower gonadosomatic index (GSI), meat yield (MY), and total edible yield (TEY) were detected in intensive pond (IP) samples than those in semi-intensive reed wetland (SIWR) (p < 0.05). Six color parameters in the hepatopancreas (p < 0.05) differed between IP and SIRW. The contents of crude protein and fat in the female hepatopancreas of IP were significantly higher than those in SIRW (p < 0.05). The concentrations of total monounsaturated fatty acids (∑MUFA), total essential fatty acids (∑EFA), and hypocholesterolaemic/hypercholesterolaemic ratio (h/H) in the female edible tissues checked were higher in IP than those in SIRW, with significant differences including ∑MUFA in the hepatopancreas and ovary, ∑EFA in the muscle, and h/H in the ovary (p < 0.05). Higher total free amino acid (∑FAA) contents of muscle were detected in SIRW than that in IP samples. Significantly higher K, Ca, Mg, Fe, and Zn contents in the ovary were detected in SIRW samples compared to IP (p < 0.05). Overall, Eriocheir sinensis reared in carbonate-alkalinity water is an important source of nutrients.

Regulation of PGC-1α of the Mitochondrial Energy Metabolism Pathway in the Gills of Indian Medaka (Oryzias dancena) under Hypothermal Stress

Ectothermic fish exposure to hypothermal stress requires adjusting their metabolic molecular machinery, which was investigated using Indian medaka (Oryzias dancena; 10 weeks old, 2.5 ± 0.5 cm) cultured in fresh water (FW) and seawater (SW; 35‱) at room temperature (28 ± 1 °C). The fish were fed twice a day, once in the morning and once in the evening, and the photoperiod was 12 h:12 h light: dark. In this study, we applied two hypothermal treatments to reveal the mechanisms of energy metabolism via pgc-1α regulation in the gills of Indian medaka; cold-stress (18 °C) and cold-tolerance (extreme cold; 15 °C). The branchial ATP content was significantly higher in the cold-stress group, but not in the cold-tolerance group. In FW- and SW-acclimated medaka, the expression of genes related to mitochondrial energy metabolism, including pgc-1α, prc, Nrf2, tfam, and nd5, was analyzed to illustrate differential responses of mitochondrial energy metabolism to cold-stress and cold-tolerance environments. When exposed to cold-stress, the relative mRNA expression of pgc-1α, prc, and Nrf2 increased from 2 h, whereas that of tfam and nd5 increased significantly from 168 h. When exposed to a cold-tolerant environment, prc was significantly upregulated at 2 h post-cooling in the FW and SW groups, and pgc-1α was significantly upregulated at 2 and 12 h post-cooling in the FW group, while tfam and nd5 were downregulated in both FW and SW fish. Hierarchical clustering revealed gene interactions in the cold-stress group, which promoted diverse mitochondrial energy adaptations, causing an increase in ATP production. However, the cold-tolerant group demonstrated limitations in enhancing ATP levels through mitochondrial regulation via the PGC-1α energy metabolism pathway. These findings suggest that ectothermic fish may develop varying degrees of thermal tolerance over time in response to climate change. This study provides insights into the complex ways in which fish adjust their metabolism when exposed to cold stress, contributing to our knowledge of how they adapt.

Activation of the growth-IGF-1 axis, but not appetite, is related to high growth performance in juveniles of the Malabar grouper, Epinephelus malabaricus, under isosmotic condition

Salinity, a determining factor in aquatic environments, influences fish growth. Here, we evaluated the effect of salinity on osmoregulation and growth performance in juveniles of the Malabar grouper, Epinephelus malabaricus, a species of high commercial value in Asian markets; we also identified the salinity that maximized this species' growth rate. Fish were reared at 26 °C and under a 14:10 h photoperiod with a salinity of 5 psu, 11 psu, 22 psu, or 34 psu for 8 weeks. Change in salinity had minimal impact on the plasma Na+ and glucose concentrations, although the Na+/K+-ATPase (nkaα and nkaβ) transcript levels in the gills were significantly lower among fish reared at 11 psu salinity. Concomitantly, oxygen consumption was low in fish reared at 11 psu salinity. The feed conversion ratio (FCR) was lower in fish reared at 5 psu and 11 psu salinities than at 22 psu and 34 psu salinities. However, the specific growth rate (SGR) was higher in fish reared at 11 psu salinity. These results suggest that rearing fish at 11 psu salinity would decrease energy consumption for respiration and improve food-conversion efficiency. Among fish reared at 11 psu salinity, the transcript levels of growth hormone (gh) in the pituitary, as well as its receptor (ghr) and insulin-like growth factor I (igf-1) in the liver, were upregulated; these findings suggested stimulation of the growth axis at low salinity. In contrast, there were minimal differences in the transcript levels of neuropeptide Y (npy) and pro-opiomelanocortin (pomc) in the brains of fish reared at any salinity, suggesting that salinity does not affect appetite. Therefore, growth performance is higher in fish reared at 11 psu salinity because of activation of the GH-IGF system, but not appetite, in Malabar grouper juveniles.

High dietary lipid level promotes low salinity adaptation in the marine euryhaline crab (Scylla paramamosain)

The physiological processes involved in adaptation to osmotic pressure in euryhaline crustaceans are highly energy demanding, but the effects of dietary lipids (fat) on low salinity adaptations have not been well evaluated. In the present study, a total of 120 mud crabs (Scylla paramamosain, BW = 17.87 ± 1.49 g) were fed control and high-fat (HF) diets, at both medium salinity (23‰) and low salinity (4‰) for 6 wk, and each treatment had 3 replicates with each replicate containing 10 crabs. The results indicated that a HF diet significantly mitigated the reduction in survival rate, percent weight gain and feed efficiency induced by low salinity (P < 0.05). Low salinity lowered lipogenesis and activated lipolysis resulting in lipid depletion in the hepatopancreas of mud crabs (P < 0.05). Thus, HF diets enhanced the process of lipolysis to supply more energy. In the gills, low salinity and the HF diet increased the levels of mitochondrial biogenesis markers, the activity of mitochondrial complexes, and the expression levels of genes related to energy metabolism (P < 0.05). Consequently, the positive effects of the HF diet on energy metabolism in mud crabs at low salinity promoted osmotic pressure regulation. Specifically, significantly higher haemolymph osmotic pressure and inorganic ion content, as well as higher osmotic pressure regulatory enzyme activity in gills, and gene and protein expression levels of NaK-ATPase were observed in crabs fed the HF diet at low salinity (P < 0.05). In summary, high dietary lipid levels improved energy provision to facilitate mitochondrial biogenesis, which increased ATP provision for osmotic pressure regulation of mud crabs. This study also illustrates the importance of dietary lipid nutrition supplementation for low salinity adaptations in mud crabs.

Meta-analysis shows environmental contaminants elevate cortisol levels in teleost fish - Effect sizes depend on contaminant class and duration of experimental exposure

Glucocorticoid hormones (GCs) help vertebrates maintain homeostasis during and following challenging events. Short-term elevations in GC levels are necessary for survival, whereas longer-term changes can lead to reduced reproductive output and immunosuppression. Persistent environmental contaminants (ECs) are widespread globally. Experimental exposure of individuals to ECs is associated with varying GC responses, within, and across, species and contaminants. Individuals exposed to ECs over long durations are expected to have prolonged GC elevations, which likely affect their health. We conducted a meta-analysis to test for a relationship between fish GC levels and experimental exposure to ECs, and to explore potential moderators, including duration of exposure, that could help explain the variation in effect sizes within and between studies. We report almost exclusively on cortisol responses of teleost fish to ECs. Although there was much variation in effect sizes, captive-bred fish exposed to ECs had baseline GC levels 1.5× higher than unexposed fish, and fish exposed to pharmaceuticals (estradiols and stimulants being mainly considered) had baseline GC levels approximately 2.5× higher than unexposed fish. We found that captive-bred and wild-caught fish did not differ in GC levels after exposure to the same classes of ECs - studies on captive bred fish may thus enable inferences about GC responses to ECs for wild species. Furthermore, effect sizes did not differ between baseline and challenge-induced GC measures. In different analyses, duration of exposure was negatively correlated to effect size, suggesting that the GC response may acclimate after chronic exposure to some ECs which could potentially alter the GC response of EC-exposed fish to novel stressors. Future studies should explore the effect of multiple stressors on the fish GC response and perform tests on a broader array of contaminant types and vertebrate classes.

Effect of high salinity acclimation on glucose homeostasis in Mozambique tilapia (Oreochromis mossambicus)

During salinity stress, osmoregulatory processes in euryhaline fish need to modify for their survival, and glucose is the preferred mode of extra energy during such conditions. These organisms must have a proper mechanism to maintain glucose homeostasis during such modified osmoregulatory process across different body fluids. Hence, we studied high salinity effect on regulation of glucose homeostasis in Mozambique tilapia. The fish were induced to 15‰ salinity for 21 days. Glucose, glycogen, ion concentrations, Na⁺-K⁺-ATPase, pyruvate kinase, γ-amylase activities and GLUT mRNA expressions were investigated in liver, intestine, gill and white muscle tissues. At the end of experiment, Na⁺ ion concentrations, glucose content and activity of Na⁺-K⁺-ATPase especially in the gill and intestine were increased, while decrease in liver and gill glycogen content was seen. Lower concentration of glycogen decrease was observed in the intestine and white muscle of the treated group. High pyruvate kinase activity was noticed in liver and gill tissues that correlates with high Na⁺-K⁺-ATPase activity. Elevated γ-amylase activity was observed in the liver and intestine suggesting breakdown of glycogen; however, gill and white muscle did not show any increased activity. Increase in GLUT1 and GLUT4 mRNA expressions was observed especially in the gill and intestine, while increase in GLUT2 mRNA expressions was observed in the liver. Upregulations of GLUTs suggest higher influx of glucose into the cell for catabolism to provide energy and further to drive the enhanced osmoregulatory process. These findings suggest glucose homeostasis being regulated in Mozambique tilapia during salinity acclimation.

Sex Associated Effects of Noise Pollution in Stone Sculpin (Paracottus knerii) as a Model Object in the Context of Human-Induced Rapid Environmental Change

Simple Summary

In this comprehensive multidisciplinary study, we applied a novel multilevel approach to stone sculpins Paracottus knerii Dybowski, 1874, as model organisms and test for the first time the hypothesis of sex-dependent differences in response to long-term noise exposure in fish. The results testify that the stone sculpin females appeared to experience excessive stress, while the males showed adaptive recalibrations. These effects may be explained by a unique adaptive strategy of offspring care in the stone sculpin males and their biological role in reproductive behavior within the species. The findings obtained may help to elucidate the links between noise exposure in the context of human-induced rapid environmental change (HIREC), long-term sex-related changes in fishes, and the possible further evolutionary success of a species. Such HIREC modeling not only provides information about the potential consequences under anthropogenic pressure but also can help identify the natural mechanisms of stress resistance in different species, including those related to sex, and also contribute to the development of effective environmental management practices.

Abstract

This work simulates the consequences of HIREC using stone sculpins as model organisms. Sex-dependent effects of long-term noise exposure at mean sound pressure levels of 160–179 dB re 1 μPa (SPL_pk–pk) were measured. We applied a multilevel approach to testing the stress response: a comparative analysis of the macula sacculi and an assessment of hematological and molecular stress responses. Noise exposure resulted in hair cell loss, changes in some cytometric parameters in blood, and an increase in the number of functionally active mitochondria in the red blood cells of males and its decrease in females, demonstrating a mitochondrial allostatic load and depletion of functional reserve. Finally, a statistically significant decrease in the telomerase activity of the auditory epithelium and a shortening of telomere length in the brain as molecular markers of stress were observed after noise exposure only in females. No significant decrease in telomerase activity and shortening of telomere length in nerve target tissues were observed in stressed males. However, we recorded an increase in the telomerase activity in male gonads. This sex-dependent difference in load may be associated with accelerated cellular aging in females and lower stress-related long-term risk in males. In this article, we discuss possible reasons for these noise-induced stress effects.

Sulfate homeostasis in Atlantic salmon is associated with differential regulation of salmonid-specific paralogs in gill and kidney

Sulfate ( SO 4 2 - ) regulation is challenging for euryhaline species as they deal with large fluctuations of SO 4 2 - during migratory transitions between freshwater (FW) and seawater (SW), while maintaining a stable plasma SO 4 2 - concentration. Here, we investigated the regulation and potential role of sulfate transporters in Atlantic salmon during the preparative switch from SO 4 2 - uptake to secretion. A preparatory increase in kidney and gill sodium/potassium ATPase (Nka) enzyme activity during smolt development indicate preparative osmoregulatory changes. In contrast to gill Nka activity a transient decrease in kidney Nka after direct SW exposure was observed and may be a result of reduced glomerular filtration rates and tubular flow through the kidney. In silico analyses revealed that Atlantic salmon genome comprises a single slc13a1 gene and additional salmonid-specific duplications of slc26a1 and slc26a6a, leading to new paralogs, namely the slc26a1a and -b, and slc26a6a1 and -a2. A kidney-specific increase in slc26a6a1 and slc26a1a during smoltification and SW transfer, suggests an important role of these sulfate transporters in the regulatory shift from absorption to secretion in the kidney. Plasma SO 4 2 - in FW smolts was 0.70 mM, followed by a transient increase to 1.14 ± 0.33 mM 2 days post-SW transfer, further decreasing to 0.69 ± 0.041 mM after 1 month in SW. Our findings support the vital role of the kidney in SO 4 2 - excretion through the upregulated slc26a6a1, the most likely secretory transport candidate in fish, which together with the slc26a1a transporter likely removes excess SO 4 2 - , and ultimately enable the regulation of normal plasma SO 4 2 - levels in SW.

Freshening effect on the osmotic response of the Antarctic spiny plunderfish Harpagifer antarcticus

Global warming is having a significant impact around the world, modifying environmental conditions in many areas, including in zones that have been thermally stable for thousands of years, such as Antarctica. Stenothermal sedentary intertidal fish species may suffer due to warming, notably if this causes water freshening from increased freshwater inputs. Acute decreases in salinity, from 33 down to 5, were used to assess osmotic responses to environmental salinity fluctuations in Antarctic spiny plunderfish Harpagifer antarcticus, in particular to evaluate if H. antarcticus is able to cope with freshening and to describe osmoregulatory responses at different levels (haematological variables, muscle water content, gene expression, NKA activity). H. antarcticus were acclimated to a range of salinities (33 as control, 20, 15, 10 and 5) for 1 week. At 5, plasma osmolality and calcium concentration were both at their lowest, while plasma cortisol and percentage muscle water content were at their highest. At the same salinity, gill and intestine Na⁺ -K⁺ -ATPase (NKA) activities were at their lowest and highest, respectively. In kidney, NKA activity was highest at intermediate salinities (15 and 10). The salinity-dependent NKA mRNA expression patterns differed depending on the tissue. Marked changes were also observed in the expression of genes coding membrane proteins associated with ion and water transport, such as NKCC2, CFTR and AQP8, and in the expression of mRNA for the regulatory hormone prolactin (PRL) and its receptor (PRLr). Our results demonstrate that freshening causes osmotic imbalances in H. antarcticus, apparently due to reduced capacity of both transport and regulatory mechanisms of key organs to maintain homeostasis. This has implications for fish species that have evolved in stable environmental conditions in the Antarctic, now threatened by climate change.

Influence of Dietary Lipids and Environmental Salinity on the n-3 Long-Chain Polyunsaturated Fatty Acids Biosynthesis Capacity of the Marine Teleost Solea senegalensis

Fish vary in their ability to biosynthesise long-chain polyunsaturated fatty acids (LC-PUFA) depending upon the complement and function of key enzymes commonly known as fatty acyl desaturases and elongases. It has been reported in Solea senegalensis the existence of a Δ4 desaturase, enabling the biosynthesis of docosahexaenoic acid (DHA) from eicosapentaenoic acid (EPA), which can be modulated by the diet. The present study aims to evaluate the combined effects of the partial replacement of fish oil (FO) with vegetable oils and reduced environmental salinity in the fatty acid composition of relevant body compartments (muscle, hepatocytes and enterocytes), the enzymatic activity over α-linolenic acid (ALA) to form n-3 LC-PUFA through the incubation of isolated hepatocytes and enterocytes with [1-¹⁴C] 18:3 n-3, and the regulation of the S. senegalensis fads2 and elovl5 in the liver and intestine. The presence of radiolabelled products, including 18:4n-3, 20:4n-3 and EPA, provided compelling evidence that a complete pathway enabling the biosynthesis of EPA from ALA, establishing S. senegalensis, has at least one Fads2 with ∆6 activity. Dietary composition prevailed over salinity in regulating the expression of fads2, while salinity did so over dietary composition for elovl5. FO replacement enhanced the proportion of DHA in S. senegalensis muscle and the combination with 20 ppt salinity increased the amount of n-3 LC-PUFA in hepatocytes.

Ion Transporters and Osmoregulation in the Kidney of Teleost Fishes as a Function of Salinity

Euryhaline teleosts exhibit major changes in renal function as they move between freshwater (FW) and seawater (SW) environments, thus tolerating large fluctuations in salinity. In FW, the kidney excretes large volumes of water through high glomerular filtration rates (GFR) and low tubular reabsorption rates, while actively reabsorbing most ions at high rates. The excreted product has a high urine flow rate (UFR) with a dilute composition. In SW, GFR is greatly reduced, and the tubules reabsorb as much water as possible, while actively secreting divalent ions. The excreted product has a low UFR, and is almost isosmotic to the blood plasma, with Mg²⁺, SO₄^2–, and Cl^– as the major ionic components. Early studies at the organismal level have described these basic patterns, while in the last two decades, studies of regulation at the cell and molecular level have been implemented, though only in a few euryhaline groups (salmonids, eels, tilapias, and fugus). There have been few studies combining the two approaches. The aim of the review is to integrate known aspects of renal physiology (reabsorption and secretion) with more recent advances in molecular water and solute physiology (gene and protein function of transporters). The renal transporters addressed include the subunits of the Na⁺, K⁺- ATPase (NKA) enzyme, monovalent ion transporters for Na⁺, Cl^–, and K⁺ (NKCC1, NKCC2, CLC-K, NCC, ROMK2), water transport pathways [aquaporins (AQP), claudins (CLDN)], and divalent ion transporters for SO₄^2–, Mg²⁺, and Ca²⁺ (SLC26A6, SLC26A1, SLC13A1, SLC41A1, CNNM2, CNNM3, NCX1, NCX2, PMCA). For each transport category, we address the current understanding at the molecular level, try to synthesize it with classical knowledge of overall renal function, and highlight knowledge gaps. Future research on the kidney of euryhaline fishes should focus on integrating changes in kidney reabsorption and secretion of ions with changes in transporter function at the cellular and molecular level (gene and protein verification) in different regions of the nephrons. An increased focus on the kidney individually and its functional integration with the other osmoregulatory organs (gills, skin and intestine) in maintaining overall homeostasis will have applied relevance for aquaculture.

The osmotic response capacity of the Antarctic fish Harpagifer antarcticus is insufficient to cope with projected temperature and salinity under climate change

Over the last decades, climate change has intensified. Temperatures have increased and seawater has become "fresher" in Antarctica, affecting fish such as Harpagifer antarcticus. Thus, this study aimed to evaluate changes in the osmoregulatory response of the Antarctic notothenioid fish Harpagifer antarcticus and evaluate how it will cope with the future climate change and environmental conditions in the Antarctic, and in the hypothetical case that its geographical distribution will be extended to the Magellanes region. The present study was undertaken to determine the interaction between temperature and salinity tolerance (2 °C and 33 psu as the control group, the experimental groups were 5, 8, and 11 °C and 28 and 23 psu) and their effect on the osmoregulatory status of H. antarcticus. We evaluated changes in gill-kidney-intestine NKA activity, gene expression of NKAα, NKCC, CFTR, Aquaporins 1 and 8 in the same tissues, muscle water percentage, and plasma osmolality to evaluate osmoregulatory responses. Plasma osmolality decreased with high temperature, also the gill-kidney-intestine NKA activity, gene expression of NKA α, NKCC, CFTR, Aquaporins 1, and 8 were modified by temperature and salinity. We demonstrated that H. antarcticus can not live in the Magallanes region, due to its incapacity to put up with temperatures over 5 °C and with over 8 °C being catastrophic.

Metabolic responses in the gills of tongue sole (Cynoglossus semilaevis) exposed to salinity stress using NMR-based metabolomics

Salinity is an important environmental factor affecting fish physiology. Tongue sole (Cynoglossus semilaevis) is a euryhaline species that can survive in a wide range of salinity, and might be used as a promising model animal for environmental science. In this study, by using the nuclear magnetic resonance (¹H NMR)-based metabolomics, amino acids analysis and real-time quantitative PCR assay, we investigated the metabolic responses in the gills and plasma of tongue sole subjected to hypo- (0 ppt, S0) and hyper-osmotic stress (50 ppt, S50) from isosmotic environment (30 ppt, S30). The results showed that the metabolic profiles of S50 were significantly different from those of S0 and S30 groups, and a clear overlap was found between the latter two groups. Ten metabolites were significantly different between the salt stress groups and the isosmotic group. Taurine and creatine elevated in both S0 and S50 groups. Choline decreased in S50 group while increased in S0 group. Amino acids and energy compounds were higher in the gills of S50 group. The metabolic network showed that ten metabolic pathways were all found in S50 group, while seven pathways were observed in S0 group. Meanwhile, the transcript levels of the Tau-T and ATP synthase in the gills increased with increasing salinity. Aspartate and methionine exhibited significant differences in the plasma among the groups, but did not show differences in the gills. Comparatively, glutamate exhibited significant differences both in the plasma and the gills. Overall, these findings provide a preliminary profile of osmotic regulation in euryhaline fish.

Environmental Salinity Affects Growth and Metabolism in Fingerling Meagre (Argyrosomus Regius)

The meagre (Argyrosomus regius), a farmed fish in Mediterranean countries, seasonally migrates from offshore areas to estuaries for reproduction. During the first two years of life, the meagre evidences a certain grade of euryhalinity by staying in brackish waters close to the shore. The aim of the present study was to establish if fingerling growth in brackish water is improved compared to seawater, where current culture procedures are conducted. Three-month-old fingerlings were maintained for 45 days under two different salinity regimens (12 and 39 ppt). Several growth parameters as well as osmoregulatory and metabolic variables were assessed. Specific growth rate and hepatosomatic index values revealed that fingerlings performed better in brackish waters (12 ppt) compared to 39 ppt. This study contributes to optimizing meagre rearing conditions, thereby supporting the role of A. regius in aquaculture diversification.

Narrowing the Range of Environmental Salinities Where Juvenile Meagre (Argyrosomus regius) Can Be Cultured Based on an Osmoregulatory Pilot Study

Aquaculture in Europe aims to diversify and optimize fish farming. The meagre (Argyrosomus regius) arose as a promising species due to its fast growth rates and flesh quality. Thus, it is currently being produced in several Mediterranean countries, mainly in sea-cages and salt-marshes. However, although meagre naturally spend the first years of life in brackish waters, to date it is cultured in seawater. Here, we show that juveniles may not successfully face either freshwater or hyper-osmotic environments as high as 55 ppt salinity. We found that 55 ppt induced catabolism and mobilization of energy metabolites stored in the liver, probably to maintain its osmotic balance. Furthermore, we found that osmoregulatory tissues such as gills managed to maintain plasma osmolality levels without differences in meagre acclimated at 5, 12 and 39 ppt salinity. Our results demonstrate the euryhaline capacity of this species, highlighting that juveniles may be cultured in a wider range of salinities rather than just at seawater. Future studies should focus on optimal environmental salinities for the growth of A. regius juveniles, including long-term experiments limited to the range of 5 ppt to full-strength seawater. Minimizing fish energy consumption in osmoregulation could be economically beneficial for the aquaculture industry in Europe.

Osmoregulation in the Plotosidae Catfish: Role of the Salt Secreting Dendritic Organ

Unlike other marine teleosts, the Plotosidae catfishes reportedly have an extra-branchial salt secreting dendritic organ (DO). Salinity acclimation [brackishwater (BW) 3aaa, seawater (SWcontrol) 34aaa, and hypersaline water (HSW) 60aaa] for 14 days was used to investigate the osmoregulatory abilities of Plotosus lineatus through measurements of blood chemistry, muscle water content (MWC), Na⁺/K⁺-ATPase (NKA) specific activity and ion transporter expression in gills, DO, kidney and intestine. Ion transporter expression was determined using immunoblotting, immunohistochemistry (IHC) and quantitative polymerase chain reaction (qPCR). HSW elevated mortality, plasma osmolality and ions, and hematocrit, and decreased MWC indicating an osmoregulatory challenge. NKA specific activity and protein levels were significantly higher in DO compared to gill, kidney and intestine at all salinities. NKA specific activity increased in kidney and posterior intestine with HSW but only kidney showed correspondingly higher NKA α-subunit protein levels. Since DO mass was greater in HSW, the total amount of DO NKA activity expressed per gram fish was greater indicating higher overall capacity. Gill NKA and V-ATPase protein levels were greater with HSW acclimation but this was not reflected in NKA activity, mRNA or ionocyte abundance. BW acclimation resulted in lower NKA activity in gill, kidney and DO. Cl^- levels were better regulated and the resulting strong ion ratio in BW suggests a metabolic acidosis. Elevated DO heat shock protein 70 levels in HSW fish indicate a cellular stress. Strong NKA and NKCC1 (Na⁺:K⁺:2Cl^- cotransporter1) co-localization was observed in DO parenchymal cells, which was rare in gill ionocytes. NKCC1 immunoblot expression was only detected in DO, which was highest at HSW. Cystic fibrosis transmembrane regulator Cl^- channel (CFTR) localize apically to DO NKA immunoreactive cells. Taken together, the demonstration of high NKA activity in DO coexpressed with NKCC1 and CFTR indicates the presence of the conserved secondary active Cl^- secretion mechanism found in other ion transporting epithelia suggesting a convergent evolution with other vertebrate salt secreting organs. However, the significant osmoregulatory challenge of HSW indicates that the DO may be of limited use under more extreme salinity conditions in contrast to the gill based ionoregulatory strategy of marine teleosts.

Gene expression of thyrotropin- and corticotrophin-releasing hormones is regulated by environmental salinity in the euryhaline teleost Sparus aurata

In euryhaline teleosts, the hypothalamus-pituitary-thyroid and hypothalamus-pituitary-interrenal axes (HPT and HPI, respectively) are regulated in response to environmental stimuli such as salinity changes. However, the molecular players participating in this physiological process in the gilthead seabream (Sparus aurata), a species of high value for aquaculture, are still not identified and/or fully characterized in terms of gene expression regulation. In this sense, this study identifies and isolates the thyrotropin-releasing hormone (trh) mRNA sequence from S. aurata, encoding prepro-Trh, the putative factor initiating the HPT cascade. In addition, the regulation of trh expression and of key brain genes in the HPI axis, i.e., corticotrophin-releasing hormone (crh) and corticotrophin-releasing hormone-binding protein (crhbp), was studied when the osmoregulatory status of S. aurata was challenged by exposure to different salinities. The deduced amino acid structure of trh showed 65-81% identity with its teleostean orthologs. Analysis of the tissue distribution of gene expression showed that trh mRNA is, though ubiquitously expressed, mainly found in brain. Subsequently, regulation of gene expression of trh, crh, and crhbp was characterized in fish acclimated to 5-, 15-, 40-, and 55-ppt salinities. In this regard, the brain gene expression pattern of trh mRNA was similar to that found for the crh gene, showing an upregulation of gene expression in seabream acclimated to the highest salinity tested. Conversely, crhbp did not change in any of the groups tested. Our results suggest that Trh and Crh play an important role in the acclimation of S. aurata to hypersaline environments.

Effects of ibuprofen and carbamazepine on the ion transport system and fatty acid metabolism of temperature conditioned juveniles of Solea senegalensis

The increasing presence of pharmaceuticals in aquatic environments in the last decades, derived from human and veterinary use, has become an important environmental problem. Previous studies have shown that ibuprofen (IB) and carbamazepine (CBZ) modify physiological and biochemical processes in Senegalese sole (Solea senegalensis) in a temperature-dependent manner. In other vertebrates, there is evidence that both of these pharmaceuticals interfere with the 'arachidonic acid (AA) cascade', which is responsible for the biosynthesis of numerous enzymes that are involved in the osmoregulatory process. The present work aims to study the temperature-dependent effects of these two pharmaceuticals on several biochemical and molecular parameters in Senegalese sole. Regarding osmoregulation, Na⁺, K⁺ -ATPase enzyme activity was determined in the gills, kidney and intestine, and the expressions of both Na⁺, K⁺ -ATPase 1α-subunit isoforms (ATP1A1a and ATP1A1b) were quantified in gills. Gill prostaglandin-endoperoxide synthase-2 (PTGS2) gene expression and fatty acid composition were selected to determine the interference of both pharmaceuticals with the AA cascade. Senegalese sole juveniles, acclimatised at 15°C or 20°C, were exposed through intraperitoneal injection to IB (10mg/kg) and CBZ (1mg/kg) for 48h. Non-injected fish (Control) and those injected with the carrier (sunflower oil; S.O.), acclimated at each of the two temperatures, were used for comparison. The results show that IB directly affected the osmoregulatory mechanisms that alter gill and intestine Na⁺, K⁺ -ATPase activities. In addition, the copy number of ATP1A1a was higher at 20°C than at 15°C, which could be a direct response to the temperature variation. The gene expression of PTGS2 was affected by neither drug administration nor acclimation temperature. Nevertheless, detailed analysis of AA and eicosapentaenoic acid (EPA) percentages revealed a CBZ-derived effect in the fatty acid composition of the gills.

NMR-based metabolomic study on the toxicological effects of pesticide, diazinon on adaptation to sea water by endangered Persian sturgeon, Acipenser persicus fingerlings

NMR-based metabolomics was applied to explore metabolic impacts of diazinon on sea water adaptation of Persian sturgeon fingerlings, Acipenser persicus. Fingerlings were exposed to sub-lethal concentrations of diazinon in freshwater (FW) for 96 h (short-term trial) and 12 days (long-term trial) and then exposed in brackish water (BW) (12 mg L^-1 salinity) for 24 h. After 96 h and 12 days exposure in FW, identified metabolites (amino acids, osmolytes, energy metabolites) showed different change-patterns compared to control group (P < 0.05) as follow: (A) short-term trial: higher plasma levels of glucose, lactate (in all diazinon-exposed fish), acetate and acetoacetate (in 0.9 mg L^-1diazinon treatment); lower levels of creatine (in all diazinon-exposed fish), trimethylamine-N-oxide, choline, taurine, betaine, N,N-dimethylglycine and almost all amino acids in fish exposed to high concentrations of diazinon (0.54 and 0.9 mg L^-1 diazinon). (B) Long-term trial: higher plasma levels of lipid oxidation metabolites and almost all amino acids in fish exposed to 0.54 and 0.9 mg L^-1 diazinon; lower levels of creatine, trimethylamine-N-oxide, N,N-dimethylglycine, betaine, choline (in all diazinon-exposed fish), glucose (in 0.54 and 0.9 mg L^-1diazinon treatments) and taurine (in 0.9 mg L^-1 diazinon treatment). When fish were exposed in BW for 24 h, the plasma levels of osmolytes decreased significantly in almost all experimental groups of short-term and long-term trial (P < 0.05). In short-term trial, the plasma levels of glucose in all groups and lactate in 0.18 and 0.54 mg L^-1 diazinon treatments increased after salinity challenge (P < 0.05). However, a significant decrease was observed in lactate levels in 0.9 mg L^-1 diazinon treatment (P < 0.05). Also, the plasma levels of amino acids decreased mostly in fish of control group than exposed fish (P < 0.05). The plasma glycerol concentration showed a significant decrease only in fish of 0.54 mg L^-1 diazinon treatment (P < 0.05). In long term trial, the energetic metabolites (acetate, acetoacetate, glycerol) showed significant increases mostly in fish exposed to high concentrations of diazinon (P < 0.05). Phosphocreatine was detected only in groups exposed to 0.54 and 0.9 mg L^-1 diazinon. Some amino acids decreased in control and diazinon-exposed groups while glycine (in control and 0.18 mg L^-1 diazinon treatment), glutamine and alanine (in 0.9 mg L^-1 diazinon treatment) elevated significantly after 24 h acclimation in BW (P < 0.05). Our results may help to understand the effects of pesticides on fish osmoregulation from a metabolic approach.

The effects of environmental salinity on the growth and physiology of totoaba Totoaba macdonaldi and shortfin corvina Cynoscion parvipinnis

Totoaba Totoaba macdonaldi and shortfin corvina Cynoscion parvipinnis, were acclimated and reared together at salinities of 0, 2, 5, 10, 20 and 35 for 56 days. Initial overall mean ± s.d. body masses of 67·6 ± 7·1 g T. macdonaldi and 37·3 ± 3·1 g C. parvipinnis increased to final overall masses of 217·4 ± 30·3 and 96·5 ± 16·5 g, respectively, at the end of the study. Totoaba macdonaldi was not able to tolerate salinities of 0 and 2 and C. parvipinnis of 0. In contrast, both species had 100% survival at salinities ≥ 10. Somatic growth was highest not at natural seawater salinity of 35, but at 10. Plasma osmolality ranged from 172·5 to 417·0 mOsmol kg^-1 for T. macdonaldi and from 207·0 to 439·5 mOsmol kg^-1 for C. parvipinnis and varied in direct proportion to salinity. The estimated isosmotic salinities of T. macdonaldi and C. parvipinnis were 12·3 and 13·4, respectively. Cynoscion parvipinnis reared at two had significantly lower plasma lysozyme activity (95·0 Units ml^-1 ) than fish held at salinities from 5 to 35 (ranging from 215·0 to 355·0 Units ml^-1 ), but without clear trends over this range. Blood neutrophil oxidative radical production (NBT) (ranging from 3·9 to 6·7 mg ml^-1 ) had some significant differences among salinities, but these did not follow a clear pattern. For T. macdonaldi, neither lysozyme activity nor NBT was affected by salinity. Ash content of whole fishes varied directly and moisture content inversely, with salinity for both species.

Effects of different salinities on growth performance, survival, digestive enzyme activity, immune response, and muscle fatty acid composition in juvenile American shad (Alosa sapidissima)

The effects of salinity on survival, growth, special activity of digestive enzymes, nonspecific immune response, and muscle fatty acid composition were evaluated in the American shad (Alosa sapidissima). Juveniles of 35 days after hatching were reared at 0 (control), 7, 14, 21, and 28 ppt for 60 days. At the end of the experiment, juvenile American shad presented higher survival and specific growth rate (SGR) in salinity group (7, 14, and 21 ppt) than control group (P < 0.05). The special activity of trypsin and chymotrypsin was highest in fish reared at 21 ppt, while the highest lipase special activity was obtained in control group (P < 0.05). The special activity of alkaline phosphatase (ALP), lysozyme (LZM), superoxide dismutase (SOD), and catalase (CAT) showed significant increases in salinity group (14 and 21 ppt) compared to control group (P < 0.05). Lower muscle ash contents were detected in salinity group (14, 21, and 28 ppt) than control group (P < 0.05), while the contents of crude lipid and crude protein were significantly higher than control group (P < 0.05). The level of monounsaturated fatty acids (MUFA) exhibited a decreasing trend, while an increased level of polyunsaturated fatty acids (PUFA) was detected with the increase of salinity. Among the PUFA, the content of n-3 fatty acids in muscle tissue was found to be increasing with the increasing salinity, especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Results indicate that appropriate increase in salinity was reasonable and beneficial for juvenile American shad culture after a comprehensive consideration, especially salinity range from 14 to 21 ppt.

Intestinal response to salinity challenge in the Senegalese sole (Solea senegalensis)

Fish are continuously forced to actively absorb or expel water and ions through epithelia. Most studies have focused on the gill due to its role in Na⁺ and Cl^- trafficking. However, comparatively few studies have focused on the changing function of the intestine in response to external salinity. Therefore, the present study investigated the main intestinal changes of long-term acclimation of the Senegalese sole (Solea senegalensis) to 5, 15, 38 and 55ppt. Through the measurement of short-circuit current (Isc) in Ussing chambers and biochemical approaches, we described a clear anterior/posterior functional regionalization of the intestine in response to salinity. The use of specific inhibitors in Ussing chamber experiments, revealed that the bumetanide-sensitive Na⁺/K⁺/Cl^- co-transporters are the main effectors of Cl^- uptake in both anterior intestine and rectum. Additionally, the use of the anion exchanger specific inhibitor, DIDS, showed a salinity/region dependency of anion exchanger function. Moreover, we also described ouabain-sensitive Na⁺/K⁺-ATPase (NKA) and Bafilomycin A1-sensitive H⁺-ATPase activities (HA), which displayed changes related to salinity and intestinal region. However, the most striking result of the present study is the description of an omeprazole-sensitive H⁺/K⁺-ATPase (HKA) in the rectum of Senegalese sole. Its activity was consistently measurable and increased at lower salinities, reaching rates even higher than those of the NKA. Together our results provide new insights into the changing role of the intestine in response to external salinity in teleost fish. The rectal activity of HKA offers an alternative/cooperative mechanism with the HA in the final processing of intestinal water absorption by apical titration of secreted bicarbonate.

Plasma Stress Responses in Juvenile Red-Spotted Grouper (Epinephelus akaara) exposed to Abrupt Salinity Decrease

The objective of the current study was to determine acute plasma stress responses in two size groups of juvenile Epinephelus akaara (average body weight: 8.4±2.1 and 3.3±0.6 g; 150 and 120 days after hatch, respectively) exposed to abrupt salinity drops (from 34 practical salinity unit, PSU seawater to 18, 10 PSU (experiment 1) or 26, 18, 10 PSU (experiment 2), respectively). Plasma glucose, glutamic oxalate transaminase, glutamic pyruvate transaminase, red blood cell counts, and gill histology were determined during 72 h exposure. Significantly increased plasma glucose, glutamic oxalate transaminase levels, and red blood cell counts were observed in fish exposed to 18 or 10 PSU. Histological changes, such as hyperplasia and lifting of epithelium in the gill secondary lamellae, were also observed in fish exposed to 18 or 10 PSU at 72 h post-drop. E. akaara exposed to sudden salinity drops to 18 or 10 PSU still seems to undergo the primary adjustment phase before fish reaches a new homeostasis, whereas fish exposed to 26 PSU seems to mount osmotic changes. Therefore, the no observed adverse effect levels for 72 h acute salinity challenge was 26 PSU in our study, and salinity drop to 18 PSU and below can possibly cause acute adverse effect, in which fish could be vulnerable to additional stresses such as a temperature changes or handling stress.

Different expression patterns of renal Na+/K+-ATPase α-isoform-like proteins between tilapia and milkfish following salinity challenges

Euryhaline teleosts can survive in a broad range of salinity via alteration of the molecular mechanisms in certain osmoregulatory organs, including in the gill and kidney. Among these mechanisms, Na⁺/K⁺-ATPase (NKA) plays a crucial role in triggering ion-transporting systems. The switch of NKA isoforms in euryhaline fish gills substantially contributes to salinity adaptation. However, there is little information about switches in the kidneys of euryhaline teleosts. Therefore, the responses of the renal NKA α-isoform protein switch to salinity challenge in euryhaline tilapia (Oreochromis mossambicus) and milkfish (Chanos chanos) with different salinity preferences were examined and compared in this study. Immunohistochemical staining in tilapia kidneys revealed the localization of NKA in renal tubules rather than in the glomeruli, similar to our previous findings in milkfish kidneys. Protein abundance in the renal NKA pan α-subunit-like, α1-, and α3-isoform-like proteins in seawater-acclimated tilapia was significantly higher than in the freshwater group, whereas the α2-isoform-like protein exhibited the opposite pattern of expression. In the milkfish, higher protein abundance in the renal NKA pan α-subunit-like and α1-isoform-like proteins was found in freshwater-acclimated fish, whereas no difference was found in the protein abundance of α2- and α3-isoform-like proteins between groups. These findings suggested that switches for renal NKA α-isoforms, especially the α1-isoform, were involved in renal osmoregulatory mechanisms of euryhaline teleosts. Moreover, differences in regulatory responses of the renal NKA α-subunit to salinity acclimation between tilapia and milkfish revealed that divergent mechanisms for maintaining osmotic balance might be employed by euryhaline teleosts with different salinity preferences.

Isolation Driven Divergence in Osmoregulation in Galaxias maculatus (Jenyns, 1848) (Actinopterygii: Osmeriformes)

Background

Marine species have colonized extreme environments during evolution such as freshwater habitats. The amphidromous teleost fish, Galaxias maculatus is found mainly migrating between estuaries and rivers, but some landlocked populations have been described in lakes formed during the last deglaciation process in the Andes. In the present study we use mtDNA sequences to reconstruct the historical scenario of colonization of such a lake and evaluated the osmoregulatory shift associated to changes in habitat and life cycle between amphidromous and landlocked populations.

Results

Standard diversity indices including the average number of nucleotide differences (Π) and the haplotype diversity index (H) indicated that both populations were, as expected, genetically distinctive, being the landlocked population less diverse than the diadromous one. Similarly, pairwise G_ST and N_ST comparison detected statistically significant differences between both populations, while genealogy of haplotypes evidenced a recent founder effect from the diadromous stock, followed by an expansion process in the lake. To test for physiological differences, individuals of both populations were challenged with a range of salinities from 0 to 30 ppt for 8 days following a period of progressive acclimation. The results showed that the landlocked population had a surprisingly wider tolerance to salinity, as landlocked fish survival was 100% from 0 to 20 ppt, whereas diadromous fish survival was 100% only from 10 to 15 ppt. The activity of ATPase enzymes, including Na⁺/K⁺-ATPase (NKA), and H⁺-ATPase (HA) was measured in gills and intestine. Activity differences were detected between the populations at the lowest salinities, including differences in ATPases other than NKA and HA. Population differences in mortality are not reflected in enzyme activity differences, suggesting divergence in other processes.

Conclusions

These results clearly demonstrate the striking adaptive changes of G. maculatus osmoregulatory system, especially at hyposmotic environments, associated to a drastic shift in habitat and life cycle at a scale of a few thousand years.

Different Modulatory Mechanisms of Renal FXYD12 for Na(+)-K(+)-ATPase between Two Closely Related Medakas upon Salinity Challenge

Upon salinity challenge, the Na⁺-K⁺-ATPase (NKA) of fish kidney plays a crucial role in maintaining ion and water balance. Moreover, the FXYD protein family was found to be a regulator of NKA. Our preliminary results revealed that fxyd12 was highly expressed in the kidneys of the two closely related euryhaline medaka species (Oryzias dancena and O. latipes) from different natural habitats (brackish water and fresh water). In this study, we investigated the expression and association of renal FXYD12 and NKA α-subunit as well as potential functions of FXYD12 in the two medakas. These findings illustrated and compared the regulatory roles of FXYD12 for NKA in kidneys of the two medakas in response to salinity changes. In this study, at the mRNA and/or protein level, the expression patterns were similar for renal FXYD12 and NKA in the two medakas. However, different patterns of NKA activities and different interaction levels between FXYD12 and NKA were found in the kidneys of these two medakas. The results revealed that different strategies were used in the kidneys of the two medaka species upon salinity challenge. On the other hand, gene knockdown experiments demonstrated that the function of O. dancena FXYD12 allowed maintenance of a high level of NKA activity. The results of the present study indicated that the kidneys of the examined euryhaline medakas originating from brackish water and fresh water exhibited different modulatory mechanisms through which renal FXYD12 enhanced NKA activity to maintain internal homeostasis. Our findings broadened the knowledge of expression and functions of FXYD proteins, the modulators of NKA, in vertebrates.

Unraveling the Tissue-Specific Gene Signatures of Gilthead Sea Bream (Sparus aurata L.) after Hyper- and Hypo-Osmotic Challenges

A custom microarray was used for the transcriptomic profiling of liver, gills and hypothalamus in response to hypo- (38‰ → 5‰) or hyper- (38‰ → 55‰) osmotic challenges (7 days after salinity transfer) in gilthead sea bream (Sparus aurata) juveniles. The total number of differentially expressed genes was 777. Among them, 341 and 310 were differentially expressed in liver after hypo- and hyper-osmotic challenges, respectively. The magnitude of changes was lower in gills and hypothalamus with around 131 and 160 responsive genes in at least one osmotic stress condition, respectively. Regardless of tissue, a number of genes were equally regulated in either hypo- and hyper-osmotic challenges: 127 out of 524 in liver, 11 out of 131 in gills and 19 out of 160 in hypothalamus. In liver and gills, functional analysis of differentially expressed genes recognized two major clusters of overlapping canonical pathways that were mostly related to “Energy Metabolism” and “Oxidative Stress”. The later cluster was represented in all the analyzed tissues, including the hypothalamus, where differentially expressed genes related to “Cell and tissue architecture” were also over-represented. Overall the response for “Energy Metabolism” was the up-regulation, whereas for oxidative stress-related genes the type of response was highly dependent of tissue. These results support common and different osmoregulatory responses in the three analyzed tissues, helping to load new allostatic conditions or even to return to basal levels after hypo- or hyper-osmotic challenges according to the different physiological role of each tissue.

Effects on the metabolism, growth, digestive capacity and osmoregulation of juvenile of Sub-Antarctic Notothenioid fish Eleginops maclovinus acclimated at different salinities

In this study we assessed the influence of three different environmental salinities (5, 15 and 31 psu during 90 days) on growth, osmoregulation, energy metabolism and digestive capacity in juveniles of the Notothenioid fish Eleginops maclovinus. At the end of experimental time samples of plasma, liver, gill, intestine, kidney, skeletal muscle, stomach and pyloric caeca were obtained. Growth, weight gain, hepatosomatic index and specific growth rate increased at 15 and 31 psu and were lower at 5 psu salinity. Gill Na(+), K(+)-ATPase (NKA) activity presented a "U-shaped" relationship respect to salinity, with its minimum rates at 15 psu, while this activity correlated negatively with salinity at both anterior and posterior intestinal portions. No significant changes in NKA activity were observed in kidney or mid intestine. Large changes in plasma, metabolite levels and enzymatic activities related to energy metabolism in liver, gill, intestine, kidney and muscle were generally found in the groups exposed to 5 and 31 psu compared to the 15 psu group. Only the pepsin activity (digestive enzymes) assessed enhanced with environmental salinity, while pyloric caeca trypsin/chymotrypsin ratio decreased. This study suggests that juvenile of E. maclovinus presents greater growth near its iso-osmotic point (15 psu) and hyperosmotic environment (31 psu). Acclimation to low salinity increased the osmoregulatory expenditure as seen by the gill and anterior intestine results, while at high salinity, branchial osmoregulatory activity was also enhanced. This requires the mobilization of lipid stores and amino acids, thereby holding the growth of fish back. The subsequent reallocation of energy sources was not sufficient to maintain the growth rate of fish exposed to 5 psu. Thus, E. maclovinus juveniles present better growth efficiencies in salinities above the iso-osmotic point and hyperosmotic environment of this species, showing their best performance at 15 psu as seen by the main osmoregulatory and energy metabolism enzymatic activities.

Exploration of the mechanisms of protein quality control and osmoregulation in gills of Chromis viridis in response to reduced salinity

Fish gills are the vital multifunctional organ in direct contact with external environment. Therefore, activation of the cytoprotective mechanisms to maintain branchial cell viability is important for fish upon stresses. Salinity is one of the major factors strongly affecting cellular and organismal functions. Reduction of ambient salinity may occur in coral reef and leads to osmotic stress for reef-associated stenohaline fish. However, the physiological responses to salinity stress in reef-associated fish were not examined substantially. With this regard, the physiological parameters and the responses of protein quality control (PQC) and osmoregulatory mechanisms in gills of seawater (SW; 33-35 ‰)- and brackish water (BW; 20 ‰)-acclimated blue-green damselfish (Chromis viridis) were explored. The results showed that the examined physiological parameters were maintained within certain physiological ranges in C. viridis acclimated to different salinities. In PQC mechanism, expression of heat-shock protein (HSP) 90, 70, and 60 elevated in response to BW acclimation while the levels of ubiquitin-conjugated proteins were similar between the two groups. Thus, it was presumed that upregulation of HSPs was sufficient to prevent the accumulation of aggregated proteins for maintaining the protein quality and viability of gill cells when C. viridis were acclimated to BW. Moreover, gill Na(+)/K(+)-ATPase expression and protein amounts of basolaterally located Na(+)/K(+)/2Cl(-) cotransporter were higher in SW fish than in BW fish. Taken together, this study showed that the cytoprotective and osmoregulatory mechanisms of blue-green damselfish were functionally activated and modulated to withstand the challenge of reduction in salinity for maintaining physiological homeostasis.

Molecular characterization and transcriptional regulation of the Na +/K+ ATPase α subunit isoforms during development and salinity challenge in a teleost fish, the Senegalese sole (Solea senegalensis)

In the present work, five genes encoding different Na(+),K(+) ATPase (NKA) α-isoforms in the teleost Solea senegalensis are described for the first time. Sequence analysis of predicted polypeptides revealed a high degree of conservation across teleosts and mammals. Phylogenetic analysis clustered the five genes into three main clades: α1 (designated atp1a1a and atp1a1b), α2 (designated atp1a2) and α3 (designated atp1a3a and atp1a3b) isoforms. Transcriptional analysis in larvae showed distinct expression profiles during development. In juvenile tissues, the atp1a1a gene was highly expressed in osmoregulatory organs, atp1a2 in skeletal muscle, atp1a1b in brain and heart and atp1a3a and atp1a3b mainly in brain. Quantification of mRNA abundance after a salinity challenge showed that atp1a1a transcript levels increased significantly in the gill of soles transferred to high salinity water (60 ppt). In contrast, atp1a3a transcripts increased at low salinity (5 ppt). In situ hybridization (ISH) analysis revealed that the number of ionocytes expressing atp1a1a transcripts in the primary gill filaments was higher at 35 and 60 ppt than at 5 ppt and remained undetectable or at very low levels in the lamellae at 5 and 35 ppt but increased at 60 ppt. Immunohistochemistry showed a higher number of positive cells in the lamellae. Whole-mount analysis of atp1a1a mRNA in young sole larvae revealed that it was localized in gut, pronephric tubule, gill, otic vesicle, yolk sac ionocytes and chordacentrum. Moreover, atp1a1a mRNAs increased at mouth opening (3 DPH) in larvae incubated at 36 ppt with a greater signal in gills.

Effects of salinity acclimation on the endocrine disruption and acute toxicity of bifenthrin in freshwater and euryhaline strains of Oncorhynchus mykiss

The pyrethroid insecticide bifenthrin is frequently detected at ng/L concentrations in tributaries of the San Francisco Bay Delta. The estuary is also experiencing increasing salinity through climate change and water redirection. To evaluate the impacts of hypersaline conditions on bifenthrin toxicity in anadromous salmonids of the San Francisco Bay Delta (CA, USA), a 14-d laboratory exposure was performed using 2 strains of Oncorhynchus mykiss (rainbow trout and steelhead) acclimated to freshwater and to 8 g/L and 17 g/L salinity. The fish were then exposed to nominal concentrations of 0 µg/L, 0.1 µg/L, and 1.5 µg/L bifenthrin. Rainbow trout exhibited significant mortality following exposure to 1.5 µg/L (1.07 ± 0.35 µg/L measured) bifenthrin in freshwater. Elevated levels of Na⁺ /K⁺ adenosine triphosphatase α1A mRNA subunit expression was observed in the gill of rainbow trout acclimated to hypersaline conditions relative to freshwater animals. No significant difference was noted in Na⁺ /K⁺ adenosine triphosphatase subunit levels in brains of either strain in freshwater or hypersaline conditions. Likewise, significant differences were not observed in plasma vitellogenin or steroid hormone concentrations in either strain whether maintained in freshwater or saltwater. Saltwater acclimation significantly reduced nicotinamide adenine dinucleotide phosphate-catalyzed biotransformation of bifenthrin in liver microsomes of rainbow trout but not of steelhead. The present study showed that, relative to steelhead, rainbow trout have different responses to bifenthrin acute toxicity as well as different rates of hepatic bifenthrin biotransformation and regulation of Na⁺ /K⁺ adenosine triphosphatase subunits in gills. These data indicate that significant differences exist between the strains and that animal life history may have important effects on the susceptibility of each strain to environmental contaminants.

Anesthesia and transport of fat snook Centropomus parallelus with the essential oil of Nectandra megapotamica(Spreng.) Mez

This study analyzed the chemical composition and anesthetic potential of essential oil (EO) of Nectandra megapotamica in fat snook (Centropomus parallelus). For the extraction of EO by hydrodistillation, leaves were separated in young (EO-Y) or old (EO-O), and the chemical composition of the EOs was determined by CG-MS. The anesthetic potential was assessed by the evaluation of induction and recovery time of anesthesia and stress response from anesthesia and transport. Three experiments were carried out: i) four different concentrations of each EO were tested to evaluate anesthesia induction and recovery time; ii) two concentrations of EO-O were tested for the evaluation of its effects on stress parameters (glucose, lactate, and Na+ and K+ plasma levels) caused by anesthesia; and iii) fish were transported in plastic bags, supplied with two concentrations of EO-O for the evaluation of water quality and mortality. All experiments were performed on fish acclimated to 0 and 33 ppt salinity. The main constituents of the Y and O-EOs were bicyclogermacrene (46.5/34.6%), α-pinene (26.8/26.2%), β-pinene (7.9/12.3%), and germacrene D (9.6/9.1%). Mild sedation was achieved at 30 °L L-1(1.3-3.2 min) and deep anesthesia at 150 °L L-1(5.6-8.0 min) with both EOs. The recovery time ranged from 1-10 min. The EO-O was not able to avoid the stress of anesthesia evidenced by elevated glucose and lactate plasma levels observed in all groups. Plasma levels of Na+ and K+ were not significantly affected by treatments. During transport, the use of EO-O did not prevent deterioration in water quality and the post-transport mortality. In conclusion, the EO of N. megapotamica has anesthetic activity in fat snook, but it was not able to prevent the stress of anesthesia and transport.

Physiological short-term response to sudden salinity change in the Senegalese sole (Solea senegalensis)

The physiological responses of Senegalese sole to a sudden salinity change were investigated. The fish were first acclimated to an initial salinity of 37.5 ppt for 4 h. Then, one group was subjected to increased salinity (55 ppt) while another group was subjected to decreased salinity (5 ppt). The third group (control group) remained at 37.5 ppt. We measured the oxygen consumption rate, osmoregulatory (plasma osmolality, gill and kidney Na(+),K(+)-ATPase activities) and stress (plasma cortisol and metabolites) parameters 0.5 and 3 h after transfer. Oxygen consumption at both salinities was higher than for the control at both sampling times. Gill Na(+),K(+)-ATPase activity was significantly higher for the 55 ppt salinity at 0.5 h. Plasma osmolality decreased in the fish exposed to 5 ppt at the two sampling times but no changes were detected for high salinities. Plasma cortisol levels significantly increased at both salinities, although these values declined in the low-salinity group 3 h after transfer. Plasma glucose at 5 ppt salinity did not vary significantly at 0.5 h but decreased at 3 h, while lactate increased for both treatments at the first sampling time and returned to the control levels at 3 h. Overall, the physiological response of S. senegalensis was immediate and involved a rise in oxygen consumption and plasma cortisol values as well as greater metabolite mobilization at both salinities.

Dexamethasone modulates expression of genes involved in the innate immune system, growth and stress and increases susceptibility to bacterial disease in Senegalese sole (Solea senegalensis Kaup, 1858)

Cortisol, the main glucocorticoid in fish, undertakes pleiotropic biological effects in response to stressors to maintain homeostasis. It can exert several actions on the immune system, growth and cellular metabolism, establishing a fine-tune regulation stress response and cross-talk interactions with other regulatory pathways. In this study, we investigated a causal relationship between high levels of glucocorticoids and susceptibility to pathogens and modification of gene expression profiles in Senegalese sole. For this purpose, we carried out two experiments using post-metamorphic individuals (21 days after hatching) that were exposed to dexamethasone (DXM), a potent glucocorticoid, in order to mimic cortisol effects. We quantified transcript levels of a wide set of genes involved in innate immune system (g-type lysozyme and hepcidin (hamp1)), HPI axis (crf, crfbp, pomcα, pomcβ, gr1 and gr2), HPT axis (tgb), cellular stress defense system (hsp70 and hsp90aa), GH/IGF axis (igf-I and igf-Ir) and the neuropeptide trh. Short-term exposure to 0.1, 1 and 10 ppm DXM provoked a reduction of pomcβ transcripts and an increase of crfbp mRNAs in a dose-dependent manner at 48 and 72 h after treatment. Moreover, g-type lysozyme transcript levels decreased significantly at 72 h whereas hamp1 mRNA levels increased at 48 h after exposure. Long-term DXM treatment (10 ppm DXM) affected negatively weight of soles (~20% lower than controls). Moreover, reduced mRNA levels were observed for pomcβ after 1 week and igf-I and hamp1 after 2 weeks. In contrast, crfbp and crf increased mRNA levels after 2 weeks. hsp70 exhibited a dual response increasing transcript levels at 1 week after treatment and reducing thereafter. No significant changes in gene expression were observed at any time during this study for tgb, trh, hsp90aa, pomcα, gr1 and gr2. Finally, a challenge experiment using the pathogen Photobacterium damselae subsp piscicida confirmed earlier and higher mortalities in DXM-treated animals. Taken together, these data indicate that a prolonged exposure to DXM increases the susceptibility to pathogens and reduces growth. Moreover, DXM can trigger a wide cellular response modulating the expression of genes involved in the innate immune system, HPI and GH/IGF axes as well as cellular stress defense. These results are highly valuable to evaluate responses associated to aquaculture stressful conditions and discriminate specific glucocorticoid-mediated effects.

Feed deprivation in Senegalese sole (Solea senegalensis Kaup, 1858) juveniles: effects on blood plasma metabolites and free amino acid levels

The effects of prolonged feed deprivation were assessed on blood plasma metabolites and free amino acid levels in Solea senegalensis. Juvenile specimens were maintained at two experimental conditions (24 h fasted and 21 days feed-deprived). In feed-deprived fish, relative growth rate and hepatosomatic index as well as plasma triglycerides and proteins levels were significantly lower. However, plasma cortisol levels were higher in feed-deprived fish, while plasma glucose and lactate values were not significantly different between treatments. Furthermore, feed-deprived fish showed higher levels of total plasma free amino acids than 24 h fasted fish. In 21 days feed-deprived sole, an increase in plasma cortisol levels may suggest a functional role in mobilizing energy due to the catabolic action of this hormone in teleosts. Higher levels of glutamine, arginine and ornithine in 21 days feed-deprived fish may be indicative of a dual role for these amino acids: ammonia detoxification and carbon source for gluconeogenesis. The increased plasma glucogenic and branched-chain amino acid levels, together with the maintenance of plasma glucose in 21 days feed-deprived sole, suggest active liver gluconeogenic processes supported by tissue proteolysis.

Ambient salinity modifies the action of triiodothyronine in the air-breathing fish Anabas testudineus Bloch: effects on mitochondria-rich cell distribution, osmotic and metabolic regulations

The hydromineral and metabolic actions of thyroid hormone on osmotic acclimation in fish is less understood. We, therefore, studied the short-term action of triiodothyronine (T(3)), the potent thyroid hormone, on the distribution and the function of gill mitochondria-rich (MR) cells and on the whole body hydromineral and metabolic regulations of air-breathing fish (Anabas testudineus) adapted to either freshwater (FW) or acclimated to seawater (SA; 30 g L(-1)). As expected, 24 h T(3) injection (100 ng g(-1)) elevated (P<0.05) plasma T(3) but classically reduced (P<0.05) plasma T(4). The higher Na(+), K(+)-ATPase immunoreactivity and the varied distribution pattern of MR cells in the gills of T(3)-treated FW and SA fish, suggest an action of T(3) on gill MR cell migration, though the density of these cells remained unchanged after T(3) treatment. The ouabain-sensitive Na(+), K(+)-ATPase activity, a measure of hydromineral competence, showed increases (P<0.05) in the gills of both FW and SA fish after T(3) administration, but inhibited (P<0.05) in the kidney of the FW fish and not in the SA fish. Exogenous T(3) reduced glucose (P<0.05) and urea (P<0.05) in the plasma of FW fish, whereas these metabolites were elevated (P<0.05) in the SA fish, suggesting a modulatory effect of ambient salinity on the T(3)-driven metabolic actions. Our data identify gill MR cell as a target for T(3) action as it promotes the spatial distribution and the osmotic function of these cells in both fresh water and in seawater. The results besides confirming the metabolic and osmotic actions of T(3) in fish support the hypothesis that the differential actions of T(3) may be due to the direct influence of ambient salinity, a major environmental determinant that alters the osmotic and metabolic strategies of fish.