Physiological and Respiratory Responses of the Mozambique Tilapia (Oreochromis mossambicus) to Salinity Acclimation

Environmental conditions elicit a slow but enduring response of histone post-translational modifications in Mozambique tilapia

This study sheds new light on the timescale through which histone post-translational modifications (PTMs) respond to environmental stimuli, demonstrating that the histone PTM response does not necessarily precede the proteomic response or acclimation. After a variety of salinity treatments were administered to Mozambique tilapia (Oreochromis mossambicus) throughout their lifetimes, we quantified 343 histone PTMs in the gills of each fish. We show here that histone PTMs differ dramatically between fish exposed to distinct environmental conditions for 18 months, and that the majority of these histone PTM alterations persist for at least 4 weeks, irrespective of further salinity changes. However, histone PTMs respond minimally to 4-week-long periods of salinity acclimation during adulthood. The results of this study altogether signify that patterns of histone PTMs in individuals reflect their prolonged exposure to environmental conditions.

Salinity stress in the black-chinned tilapia Sarotherodon melanotheron

Physiological and morphological acclimation capacities of black-chinned tilapia, Sarotherodon melanotheron were studied from fish to gill cell level when fish are maintained in freshwater, seawater, and hypersaline conditions. Fish osmoregulatory capacity, gill ionocyte morphology, osmo-respiratory compromise, O2 consumption rate, branchial antioxidative defense, and cell apoptosis were considered. Captive juvenile tilapias were maintained in controlled freshwater conditions (FW: 0.4 ppt; 12 mOsm kg-1) or gradually transferred to seawater (SW: 32 ppt; 958 mOsm kg-1) and concentrated SW (cSW: 65 ppt; 1920 mOsm kg-1). After 15 days in these conditions, blood osmolality and chloride ion concentration were determined. Gill ionocyte density and morphology were measured using immunolabelled histological sections to specifically detect the sodium pump (NKA). Gill osmo-respiratory compromise was also calculated along with oxygen consumption rates from normoxic to hypoxic conditions from excised gills (indirect respirometry). Finally, catalase and caspase 3/7activities were recorded from gill extracts. Results indicate that elevated salinity induces an osmotic imbalance and a profound morphological change with proliferating and hypertrophied ionocytes. This thickening of the gill interlamellar cell mass and the shortening of the lamellae induce a reduced osmo-respiratory ratio and reduced respiratory capacity under both normoxic and hypoxic conditions. Although salinity changes do not affect one of the major antioxidative defense mechanism, it strongly affects apoptosis that appears the most elevated in SW. However, in freshwater condition, fish can maintain their osmotic balance with a low ionocyte density, a low apoptotic level and a drastically reduced O2 consumption in normoxic condition that is maintained in hypoxia. Therefore, S. melanotheron presents the typical functional remodeling due to environmental salinity changes ranging from FW to SW. However, elevated seawater induces major cellular stress inducing a profound gill morphofunctional dysfunctioning. While cell apoptosis is reduced, ionocyte proliferation is massively increased with impaired osmotic regulation and reduced O2 consumption both in normoxic and hypoxic conditions.

Twenty five years of world heritage status: Show us the benefits!

In the Solomon Islands and around the Pacific there is commonly a disconnect between government priorities for economic development through resource extraction and community aspirations for local resource management, conservation, and alternative pathways to livelihoods development, which includes tourism. Nowhere is this disconnect more stark than in communities on Rennell Island, within the region's oldest inscribed World Heritage area. These communities have so far resisted extractive industry development but have not yet benefited from inscription. Alternative livelihood opportunities compatible with a conservation economy are a priority. Our research objective was to explore community aspirations and priorities. We used Q-methodology to reveal discourses associated with conservation, livelihoods generation, and drivers of well-being and then evaluated these aspirations in scenarios in a socio-ecological system. We revealed three factors, each aligned with conservation and tourism development with clear opposition to extractive industries. Key differences focussed on immediate personal circumstances, attachment to kastom, and food and water security. Our research points to clear support for the area's continued conservation and for livelihood pathways that might secure this but low capacity to pursue this. Notwithstanding, the communities' patience is wearing thin and there is growing cynicism about the role of World Heritage protection as a route towards livelihoods development.

Modulatory effects of dietary prickly pear (Opuntia ficus-indica) peel on high salinity tolerance, growth rate, immunity and antioxidant capacity of Nile tilapia (Oreochromis niloticus)

This study evaluated the effects of prickly pear (Opuntia ficus-indica) peel (PPP) on salinity tolerance, growth, feed utilization, digestive enzymes, antioxidant capacity, and immunity of Nile tilapia (Oreochromis niloticus). PPP was incorporated into four iso-nitrogenous (280 g kg-1 protein) and iso-energetic (18.62 MJ kg-1) diets at 0 (PPP0), 1 (PPP1), 2 (PPP2), and 4 (PPP4) g kg-1. Fish (9.69 ± 0.2 g) (mean ± SD) were fed the diets for 75 days. Following the feeding experiment, fish were exposed to a salinity challenge (25‰) for 24 h. Fish survival was not affected by the dietary PPP inclusion either before or after the salinity challenge. Fish fed the PPP-supplemented diets showed lower aspartate aminotransferase, alanine aminotransferase, cortisol, and glucose levels compared to PPP0, with the lowest values being observed in PPP1. Fish fed dietary PPP had higher growth rates and feed utilization than PPP0. Quadratic regression analysis revealed that the best weight gain was obtained at 2.13 g PPP kg-1 diet. The highest activities of protease and lipase enzymes were recorded in PPP1, while the best value of amylase was recorded in PPP2, and all PPP values were higher than PPP0. Similarly, PPP1 showed higher activities of lysozyme, alternative complement, phagocytic cells, respiratory burst, superoxide dismutase, glutathione peroxidase and catalase, and lower activity of malondialdehyde than in PPP0. Further increases in PPP levels above 2 g kg-1 diet led to significant retardation in the immune and antioxidant parameters. Thus, the inclusion of PPP at about 1 to or 2 g kg-1 diet can improve stress tolerance, immunity, and antioxidant capacity in Nile tilapia.

Female tilapia, Oreochromis sp. mobilised energy differently for growth and reproduction according to living environment

This study was conducted to investigate the energy mobilisation preference and ionoregulation pattern of female tilapia, Oreochromis sp. living in different environments. Three different treatments of tilapia as physiology compromising model were compared; tilapia cultured in recirculating aquaculture system (RAS as Treatment I-RAS), tilapia cultured in open water cage (Treatment II-Cage) and tilapia transferred from cage and cultured in RAS (Treatment III-Compensation). Results revealed that tilapia from Treatment I and III mobilised lipid to support gonadogenesis, whilst Treatment II tilapia mobilised glycogen as primary energy for daily exercise activity and reserved protein for growth. The gills and kidney Na+/K+ ATPase (NKA) activities remained relatively stable to maintain homeostasis with a stable Na+ and K+ levels. As a remark, this study revealed that tilapia strategized their energy mobilisation preference in accessing glycogen as an easy energy to support exercise metabolism and protein somatogenesis in cage culture condition, while tilapia cultured in RAS mobilised lipid for gonadagenesis purposes.

Gene expression profiling and physiological adaptations of pearl spot (Etroplus suratensis) under varying salinity conditions

Eutroplus suratensis (Pearl spot) is naturally found in estuarine environments and has been noted to have a high salinity tolerance. By examining the impact of various salinity levels on the growth and survival of pearl spot, the present study aims to enhance aquaculture profitability by assessing their adaptability and physiological adjustments to changes in salinity and determining their potential to acclimate to a broad range of salinity regimes. Results revealed no mortality in the control group (0 ppt), and in 15, 25 and 35 ppt treatment groups. However, the remaining groups (45, 60, and 75 ppt) showed differing levels of mortality with 44 % mortality observed in the 45 ppt group and 100 % mortality in both the 60 and 75 ppt groups. The expression analysis showed that liver IGF-1 mRNA expression increased by 2.6-fold at 15 ppt, and HSP70 mRNA expression in the liver also showed a significant increase with rising salinity levels. In addition, OSTF1 expression exhibited an increase at 15 ppt, whereas SOD and CAT expression reached their highest levels at 25 ppt. At 15 ppt, the expression of NKA mRNA increased significantly by 2.8-fold. The study's overall findings suggested that utilizing a salinity level of 15 ppt for pearl spot production could be viable for profitable aquaculture.

The GH/IGF axis in the sea lamprey during metamorphosis and seawater acclimation

How the growth hormone (GH)/insulin-like growth factor (IGF) system affects osmoregulation in basal vertebrates remains unknown. We examined changes in the expression of components of the GH/IGF axis and gill ion transporters during metamorphosis and following seawater (SW) exposure of sea lamprey. During metamorphosis, increases in gill nka and nkcc1 and salinity tolerance were accompanied by increases in pituitary gh, liver igf1, gill ghr and igf1, but not liver ghr. SW exposure of fully metamorphosed sea lamprey resulted in slight increases in plasma chloride concentrations after SW exposure, indicating a high level of SW tolerance, but no major changes in mRNA levels of gill ion transporters or components of the GH/IGF axis. Our results indicate that metamorphosis is a critical point in the lifecycle of sea lamprey for stimulation of the GH/IGF axis and is temporally associated with and likely promotes metamorphosis and SW tolerance.

Accumulation and Elimination of Cadmium by the Nile Tilapia (Oreochromis niloticus) in differing Temperatures and Responses of Oxidative Stress Biomarkers

The aims of this study were to investigate the accumulation (15 days) and elimination (15 and 30 days) of cadmium (Cd) in the liver, gill, kidney and muscle of Oreochromis niloticus following exposures to different concentrations (1 and 2 mg/L) of Cd at different water temperatures (20, 25, 30 ^oC). Additionally, responses of oxidative stress biomarkers (superoxide dismutase, SOD; catalase CAT; glutathione peroxidase, GPx and malondialdehyde, MDA) of the liver were determined following Cd exposures. In accumulation period, Cd levels increased significantly in all the tissues at all temperatures and tissue accumulation order was kidney > liver > gill. All tissues, except the muscle, accumulated Cd in relation to exposure concentrations and water temperatures. There was no measurable level of Cd accumulation in the muscle, except in fish exposed to 2 mg Cd/L at 30 ^oC. Likewise, elimination of Cd from the tissues also increased in depends on periods and water temperatures. The order of Cd elimination from the tissues was gill > liver > kidney. The oxidative stress biomarkers also responded to both Cd exposure and temperature increases. The activities of antioxidant enzymes such as SOD, CAT, GPx and MDA levels in the liver increased in relation to increases in Cd concentrations and water temperatures.

Metabolic costs associated with seawater acclimation in a euryhaline teleost, the fourspine stickleback (Apeltes quadracus)

The cost of osmoregulation in teleosts has been debated for decades, with estimates ranging from one to 30 % of routine metabolic rate. The variation in the energy budget appears to be greater for euryhaline fish due to their ability to withstand dynamic salinity levels. In this study, a time course of metabolic and physiological responses of the euryhaline fourspine stickleback (Apeltes quadracus) acclimated to freshwater (FW) and then exposed to seawater (SW) was examined. There was 18% mortality in the first 3 days following exposure to SW, with no mortalities in the FW control group. Gill Na⁺/K⁺-ATPase (NKA) activity, an index of osmoregulatory capacity, increased 2.6-fold in SW fish peaking on days 7 and 14. Gill citrate synthase activity, an index of aerobic capacity, was 50-62% greater in SW than FW fish and peaked on day 7. Tissue water content was significantly lower in the SW fish on day 1 only, returning to FW levels by day 3. Routine metabolic rate was decreased within 24 h of SW exposure and was maintained slightly (8-22%) but significantly lower in SW compared to FW water controls throughout the 2-week experiment. These results indicate that elevated salinity resulted in increased SW osmoregulatory and aerobic capacity in the gill, but with a reduced whole animal metabolic rate to this euryhaline species.

Correlation between Metabolic Rate and Salinity Tolerance and Metabolic Response to Salinity in Grass Carp (Ctenopharyngodon idella)

Simple Summary

The association between the metabolic rate and salinity tolerance in stenohaline freshwater fish could affect how fish adapt to changes in environmental salinity. In Experiment I, the metabolic rates and upper salinity tolerance limit of the grass carp were determined individually, and we aimed to test whether an association existed between the salinity tolerance capacity and both the resting metabolic rate and maximum metabolic rate. In Experiment II, the effects of increasing salinity on metabolic rates, gill histology, and Na⁺-K⁺-ATPase activities were determined in grass carp. The results suggest that a lower metabolic rate may not necessarily allow for a better salinity tolerance capacity of grass carp. Salinity-induced changes in the gill surface contribute more to ion exchange capacity than to oxygen uptake capacity.

Abstract

The metabolic rate could be one of the factors affecting the salinity tolerance capacity of fish. Experiment I tested whether metabolic rates correlate with the upper salinity tolerance limit among individual grass carp by daily increasing salinity (1 g kg⁻¹ day⁻¹). The feeding dropped sharply as the salinity reached 10 g kg⁻¹ and ceased when salinities exceeded 11 g kg⁻¹. The ventilation frequency decreased weakly as salinity increased from 0 to 12 g kg⁻¹ and then increased rapidly as salinity reached 14 g kg⁻¹. The fish survived at salinities lower than 14 g kg⁻¹, and all fish died when salinity reached 17 g kg⁻¹. The upper salinity tolerance limit was not correlated with metabolic rates. Therefore, a lower metabolic rate may not necessarily allow for better salinity tolerance capacity. Experiment II tested how different salinities (0, 0.375, 0.75, 1.5, 3, and 6 g kg⁻¹ for 2 weeks) affect the metabolic parameters of grass carp. The changes in the resting metabolic rate with increasing salinity could be explained by the relative changes in interlamellar cell mass and protruding lamellae. The maximum metabolic rate remained constant, suggesting that the salinity-induced changes in the gill surface had a minor effect on oxygen uptake capacity.

Effects of salinity on gills' chloride cells, stress indices, and gene expression of Asian seabass (Lates calcarifer, Bloch, 1790)

A 2-week research was carried out to assess water salinity (WS) effects including 0, 15, 35, and 50‰ on osmoregulatory mechanisms and stress indices in Asian sea bass (34.4 g) juveniles. Except for fish reared at 50‰, in the other treatments, it gradually decreased to the prescribed WS during a 10-day period (- 5‰ a day). After a 10-day acclimation period, fish were reared at the prescribed WS for 2 weeks. Fish reared at 15 and 35‰ had higher chloride cell (CC) counts in the interlamellar region. The number of CC in the interlamellar region elevated with increment of WS up to 35‰, but they were pronouncedly reduced in 50‰ group. The diameter of CC in the interlamellar region was not affected by WS. The smallest nucleus diameter of CC in the interlamellar region was observed in fish reared at 15‰ (P < 0.05). The largest and the smallest amounts of serum aspartate aminotransferase were observed in fish reared at freshwater and 15‰, respectively. Fish reared at 35‰ had the highest serum sodium and potassium contents. Serum chloride content and total osmolality increased with increment of WS (P < 0.05). Serum cortisol and glucose contents gradually increased with elevation of WS up to 35‰; then, their contents remarkably decreased. The relative expression of insulin like growth factor-1 in the liver of fish reared at 35‰ was strikingly higher than that in the other groups. The relative expression of HSP70 gene in fresh water group was pronouncedly elevated compared to other treatments. The relative expression of interleukin-1β in 15 and 35‰ groups was higher than that in the other groups; however, the relative expression of lysozyme gene in the liver of fish reared at fresh water was pronouncedly lower than that in the other treatments. The results of this study suggested rearing L. calcarifer at 15‰ closer to the isosmotic point and better provide its welfare.

Functional Feeds to Tackle Meagre (Argyrosomus regius) Stress: Physiological Responses under Acute Stressful Handling Conditions

Marine algae are recognised sources of bioactive compounds that have attracted great interest as nutritional supplements for aquaculture fish. Intensive rearing conditions often expose fish to husbandry-related stressors, rendering fish more susceptible to disease and reducing production yields. The present work evaluated the potential of two marine algae extracts (Fucus vesiculosus and Nannochloropsis gaditana) as nutritional supplements to mitigate stress effects in meagre (Argyrosomus regius) exposed to an acute handling stress (AS). A plant-based diet was used as a control, and three other diets were prepared, which were similar to the control diet but supplemented with 1% of each algal extract or a combination of the two extracts (0.5% each). The effects of supplemented diets on stress biomarkers, antioxidant enzyme activities, and immune response were analysed in fish exposed to AS after 4 weeks of feeding. Supplemented diets did not affect growth performance but the inclusion of F. vesiculosus promoted higher feed efficiency, as compared to the control group. Dietary algal extracts supplementation reduced plasma glucose levels, increased white blood cell counts, and reduced the expression of pro-inflammatory genes when compared with the control. N. gaditana supplementation led to a reduction in hepatic antioxidant enzyme activity and glutathione levels, while F. vesiculosus supplementation increased muscle glutathione reductase activity and reduced lipid peroxidation. These findings support the potential of algal extracts as nutraceuticals in aquafeeds to enhance the ability of fish to cope with husbandry-related stressful conditions and ultimately improve fish health and welfare.

Environmental Salinity Modifies Mucus Exudation and Energy Use in European Sea Bass Juveniles

The European sea bass (Dicentrarchus labrax) is a euryhaline marine teleost that can often be found in brackish and freshwater or even in hypersaline environments. Here, we exposed sea bass juveniles to sustained salinity challenges for 15 days, simulating one hypoosmotic (3‰), one isosmotic (12‰) and one hyperosmotic (50‰) environment, in addition to control (35‰). We analyzed parameters of skin mucus exudation and mucus biomarkers, as a minimally invasive tool, and plasma biomarkers. Additionally, Na⁺/K⁺-ATPase activity was measured, as well as the gill mucous cell distribution, type and shape. The volume of exuded mucus increased significantly under all the salinity challenges, increasing by 130% at 50‰ condition. Significantly greater amounts of soluble protein (3.9 ± 0.6 mg at 50‰ vs. 1.1 ± 0.2 mg at 35‰, p < 0.05) and lactate (4.0 ± 1.0 µg at 50‰ vs. 1.2 ± 0.3 µg at 35‰, p < 0.05) were released, with clear energy expenditure. Gill ATPase activity was significantly higher at the extreme salinities, and the gill mucous cell distribution was rearranged, with more acid and neutral mucin mucous cells at 50‰. Skin mucus osmolality suggested an osmoregulatory function as an ion-trap layer in hypoosmotic conditions, retaining osmosis-related ions. Overall, when sea bass cope with different salinities, the hyperosmotic condition (50‰) demanded more energy than the extreme hypoosmotic condition.

Morphological and physiological traits of Mediterranean sticklebacks living in the Camargue wetland (Rhone river delta)

Three-spined sticklebacks (Gasterosteus aculeatus L.) living at the southern limit of the species distribution range could possess specific morphological and physiological traits that enable these fish to live at the threshold of their physiological capacities. Morphological analysis was carried out on samples of sticklebacks living in different saline habitats of the Camargue area (Rhone delta, northern Mediterranean coast) obtained from 1993 to 2017. Salinity acclimation capacities were also investigated using individuals from freshwater-low salinity drainage canals and from mesohaline-euryhaline lagoons. Fish were maintained in laboratory conditions at salinity values close to those of their respective habitats: low salinity (LS, 5‰) or seawater (SW, 30‰). Fish obtained from a mesohaline brackish water lagoon (BW, 15‰) were acclimated to SW or LS. Oxygen consumption rates and branchial Na⁺ /K⁺ -ATPase (NKA) activity (indicator of fish osmoregulatory capacity) were measured in these LS or SW control fish and in individuals subjected to abrupt SW or LS transfers. At all the studied locations, only the low-plated "leiurus" morphotype showed no spatial or temporal variations in their body morphology. Gill rakers were only longer and denser in fish sampled from the LS-freshwater (FW) drainage canals. All fish presented similar physiological capacities. Oxygen consumption rates were not influenced by salinity challenge except in SW fish transferred to LS immediately and 1 h after transfer. However, and as expected, gill NKA activity was salinity dependent. Sticklebacks of the Camargue area sampled from habitats with contrasted saline conditions are homogenously euryhaline, have low oxygen consumption rates and do not appear to experience significantly greater metabolic costs when challenged with salinity. However, an observed difference in gill raker length and density is most probably related to the nutritional condition of their habitat, indicating that individuals can rapidly acclimatize to different diets.

Plasticity of Respiratory Function Accommodates High Oxygen Demand in Breeding Sea Cucumbers

Physiological plasticity allows animals to adjust their physiological function to abiotic and biotic variations. It has been mostly studied in the context of response to external factors and not much is known on how animals adjust their physiology to cope with variations in internal conditions. The process of reproduction implies gonadal maturation and other internal changes, bringing various challenges to the animal such as an increased demand for energy and oxygen. Here, the capacity of the sea cucumber, Apostichopus japonicus to adjust its respiratory function and physiological mechanisms during reproduction was studied using a time-lapse videography and metabolomics approach. The results showed that reproduction caused a significant increase in oxygen consumption in A. japonicus. Interestingly, breeding sea cucumbers can accommodate the high oxygen demand by accelerating respiratory rate. However, to maintain a necessary high level of respiratory activity during reproduction, sea cucumbers need consume large amounts of adenosine triphosphate (ATP). In addition, the metabolomic data suggests that oxidative stress and hormone regulation are the physiological mechanisms linking reproduction and respiratory function. Altogether, these findings suggest that plasticity of respiratory function is an effective tactic to cope with high oxygen demand during reproduction.

Intra-Specific Difference in the Effect of Salinity on Physiological Performance in European Perch (Perca fluviatilis) and Its Ecological Importance for Fish in Estuaries

Changes in environmental salinity challenge fish homeostasis and may affect physiological performance, such as swimming capacity and metabolism, which are important for foraging, migration, and escaping predators in the wild. The effects of salinity stress on physiological performance are largely species specific, but may also depend on intra-specific differences in physiological capabilities of sub-populations. We measured critical swimming speed (U_crit) and metabolic rates during swimming and at rest at salinities of 0 and 10 in European perch (Perca fluviatilis) from a low salinity tolerance population (LSTP) and a high salinity tolerance population (HSTP). U_crit of LSTP was significantly reduced at a salinity of 10 yet was unaffected by salinity change in HSTP. We did not detect a significant cost of osmoregulation, which should theoretically be apparent from the metabolic rates during swimming and at rest at a salinity of 0 compared to at a salinity of 10 (iso-osmotic). Maximum metabolic rates were also not affected by salinity, indicating a modest tradeoff between respiration and osmoregulation (osmo-respiratory compromise). Intra-specific differences in effects of salinity on physiological performance are important for fish species to maintain ecological compatibility in estuarine environments, yet render these sub-populations vulnerable to fisheries. The findings of the present study are therefore valuable knowledge in conservation and management of estuarine fish populations.

Developmental changes in oxygen consumption and hypoxia tolerance in the heat and hypoxia-adapted tabasco line of the Nile tilapia Oreochromis niloticus, with a survey of the metabolic literature for the genus Oreochromis

The genus Oreochromis is among the most popular of the tilapiine cichlid tribe for aquaculture. However, their temperature and hypoxia tolerance, if tested at all, is usually tested at temperatures of 20-25°C, rather than at the considerably higher temperatures of 30-35°C typical of tropical aquaculture. We hypothesized that both larvae and adults of the heat and hypoxia-adapted Tabasco-line of the Nile tilapia Oreochromis niloticus would be relatively hypoxia-tolerant. Oxygen consumption rate ( M ˙ O 2 ), Q₁₀ and aquatic surface respiration (ASR) was measured using closed respirometry at 2 (c. 0.2 g), 30 (c. 2-5 g), 105 c. (10-15 g) and 240 (c. 250 g) days of development, at 25°C, 30°C and 35°C. M ˙ O 2 at 30°C was inversely related to body mass: c. 90 μM O₂ g^-1 /h in larvae down to c. 1 μM O₂ g^-1 /h in young adults. Q₁₀ for M ˙ O 2 was typical for fish over the range 25-35°C of 1.5-2.0. ASR was exhibited by 50% of the fish at pO₂ of 15-50 mmHg in a temperature-dependent fashion. However, the largest adults showed notable ASR only when pO₂ fell to below 10 mmHg. Remarkably, p_crit for M ˙ O 2 was 12-17 mmHg at 25-30°C and still only 20-25 mmHg across development at 35°C. These values are among the lowest measured for teleost fishes. Noteworthy is that all fish maintain equilibrium, ventilated their gills and showed routine locomotor action for 10-20 min after M ˙ O 2 ceased at near anoxia and when then returned to oxygenated waters, all fish survived, further indicating a remarkable hypoxic tolerance. Remarkably, data assembled for M ˙ O 2 from >30 studies showed a > x2000 difference, which we attribute to calculation or conversion errors. Nonetheless, p_crit was very low for all Oreochromis sp. and lowest in the heat and hypoxia-adapted Tabasco line.

Salinity tolerance of a rare and endangered unionid mussel, Popenaias popeii (Texas Hornshell) and its implications for conservation and water management

Unionid mussels are considered sensitive to salinity and there is growing concern in arid and semi-arid regions that declining flows coupled with anthropogenic impacts are amplifying natural salinity levels. In this study, we tested the effects of varying salinity concentrations (3.0, 4.0, 5.0, 6.0, 7.0 and 10.0 ppt NaCl) on survival of adult Popenaias popeii, (Texas Hornshell). This species occurs in the Rio Grande basin of Texas and northern Mexico, an arid to semi-arid stream plagued by salinization, and was recently listed as Endangered under the U.S. Endangered Species Act. We performed 2, 4, and 10-day toxicity tests on individuals from two disjunct populations: Laredo, TX, and the Lower Canyons of the Rio Grande near Big Bend National Park. We found no significant differences in LC50 estimates between populations at 96-hrs or 10-days but significant differences in TUD50s at 5 ppt between populations, which indicates that tolerance does not vary but sensitivity may between these populations. Overlaying LC50 estimates at 10-days for both populations on plots of salinity (ppt) measured over time, we show parts of the Rio Grande periodically approach or exceed 4.0 ppt, indicating these reaches are becoming unsuitable for P. popeii and populations within them at risk.

Leptin Stimulates Cellular Glycolysis Through a STAT3 Dependent Mechanism in Tilapia

We assessed if leptin, a cytokine hormone known to enhance energy expenditure by promoting lipid and carbohydrate catabolism in response to physiologic stress, might directly regulate cellular glycolysis. A transcriptomic analysis of prolactin cells in the tilapia (Oreochromis mossambicus) pituitary rostral pars distalis (RPD) revealed that recombinant leptin (rtLep) differentially regulates 1,995 genes, in vitro. Machine learning algorithms and clustering analyses show leptin influences numerous cellular gene networks including metabolism; protein processing, transport, and metabolism; cell cycle and the hypoxia response. Leptin stimulates transcript abundance of the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (gapdh) in a covariate manner to the hypoxic stress gene network. Orthogonal tests confirm that rtLepA dose-dependently increases gapdh gene expression in the RPD along with transcript abundance of 6-phosphofructo-1-kinase (pfk1), the rate limiting glycolytic enzyme. Functional testing demonstrated that leptin stimulates PFK activity and glycolytic output, while Stattic (a STAT3 blocker) was sufficient to suppress these responses, indicating leptin stimulates glycolysis through a STAT3-dependent mechanism. Leptin also stimulated pfk1 gene expression and lactate production in primary hepatocyte incubations in a similar manner to those shown for the pituitary RPD. This work characterizes a critical metabolic action of leptin to directly stimulate glycolysis across tissue types in a teleost model system, and suggest that leptin may promote energy expenditure, in part, by stimulating glycolysis. These data in a teleost fish, suggest that one of leptin's ancient, highly-conserved functions among vertebrates may be stimulation of glycolysis to facilitate the energetic needs associated with various stressors.

Somatotropic Axis Regulation Unravels the Differential Effects of Nutritional and Environmental Factors in Growth Performance of Marine Farmed Fishes

The Gh/Prl/Sl family has evolved differentially through evolution, resulting in varying relationships between the somatotropic axis and growth rates within and across fish species. This is due to a wide range of endogenous and exogenous factors that make this association variable throughout season and life cycle, and the present minireview aims to better define the nutritional and environmental regulation of the endocrine growth cascade over precisely defined groups of fishes, focusing on Mediterranean farmed fishes. As a result, circulating Gh and Igf-i are revitalized as reliable growth markers, with a close association with growth rates of gilthead sea bream juveniles with deficiency signs in both macro- or micro-nutrients. This, together with other regulated responses, promotes the use of Gh and Igf-i as key performance indicators of growth, aerobic scope, and nutritional condition in gilthead sea bream. Moreover, the sirtuin-energy sensors might modulate the growth-promoting action of somatotropic axis. In this scenario, transcripts of igf-i and gh receptors mirror changes in plasma Gh and Igf-i levels, with the ghr-i/ghr-ii expression ratio mostly unaltered over season. However, this ratio is nutritionally regulated, and enriched plant-based diets or diets with specific nutrient deficiencies downregulate hepatic ghr-i, decreasing the ghr-i/ghr-ii ratio. The same trend, due to a ghr-ii increase, is found in skeletal muscle, whereas impaired growth during overwintering is related to increase in the ghr-i/ghr-ii and igf-ii/igf-i ratios in liver and skeletal muscle, respectively. Overall, expression of insulin receptors and igf receptors is less regulated, though the expression quotient is especially high in the liver and muscle of sea bream. Nutritional and environmental regulation of the full Igf binding protein 1-6 repertoire remains to be understood. However, tissue-specific expression profiling highlights an enhanced and nutritionally regulated expression of the igfbp-1/-2/-4 clade in liver, whereas the igfbp-3/-5/-6 clade is overexpressed and regulated in skeletal muscle. The somatotropic axis is, therefore, highly informative of a wide-range of growth-disturbing and stressful stimuli, and multivariate analysis supports its use as a reliable toolset for the assessment of growth potentiality and nutrient deficiencies and requirements, especially in combination with selected panels of other nutritionally regulated metabolic biomarkers.

The effects of acclimation temperature, salinity, and behavior on the thermal tolerance of Mozambique tilapia (Oreochromis mossambicus)

Mozambique tilapia have been shown to be incredibly stress tolerant with respect to environmental salinity, hypoxia, and ammonia concentrations. Temperature challenges to this species have shown that they have difficulty with cold acclimation. The purpose of this study was to measure the effects of acclimation temperature and salinity on the thermal tolerance of Mozambique tilapia as assessed by critical thermal maxima (CT_Max) and critical thermal minima (CT_Min). We also monitored fish behavior and quantified ventilation rate. To our knowledge, this study was the first to investigate upper and lower thermal tolerances, and the effect of environmental salinity in this physiologically impressive species. Using predictive regression analyses of the thermal limits, thermal tolerance polygons were constructed and total areas were calculated 678.9°C² for freshwater (FW)-acclimated tilapia, and 739.4°C² seawater (SW)-acclimated tilapia. During the thermal challenges, we observed two novel behaviors in response to thermal challenge, ventilation cessation behavior (VCB) and aquatic surface respiration (ASR), and we conclude that the use of these behaviors extended the thermal limits of these fish in both FW and two-thirds SW by limiting the exposure of the gill epithelium to the changing environment.

The effects of elevated temperature and ocean acidification on the metabolic pathways of notothenioid fish

The adaptations used by notothenioid fish to combat extreme cold may have left these fish poorly poised to deal with a changing environment. As such, the expected environmental perturbations brought on by global climate change have the potential to significantly affect the energetic demands and subsequent cellular processes necessary for survival. Despite recent lines of evidence demonstrating that notothenioid fish retain the ability to acclimate to elevated temperatures, the underlying mechanisms responsible for temperature acclimation in these fish remain largely unknown. Furthermore, little information exists on the capacity of Antarctic fish to respond to changes in multiple environmental variables. We have examined the effects of increased temperature and pCO₂ on the rate of oxygen consumption in three notothenioid species, Trematomus bernacchii, Pagothenia borchgrevinki, and Trematomus newnesi. We combined these measurements with analysis of changes in aerobic and anaerobic capacity, lipid reserves, fish condition, and growth rates to gain insight into the metabolic cost associated with acclimation to this dual stress. Our findings indicated that temperature is the major driver of the metabolic responses observed in these fish and that increased pCO₂ plays a small, contributing role to the energetic costs of the acclimation response. All three species displayed varying levels of energetic compensation in response to the combination of elevated temperature and pCO₂. While P. borchgrevinki showed nearly complete compensation of whole animal oxygen consumption rates and aerobic capacity, T. newnesi and T. bernacchii displayed only partial compensation in these metrics, suggesting that at least some notothenioids may require physiological trade-offs to fully offset the energetic costs of long-term acclimation to climate change related stressors.

Plasma osmolality and oxygen consumption of perch Perca fluviatilis in response to different salinities and temperatures

The present study determined the blood plasma osmolality and oxygen consumption of the perch Perca fluviatilis at different salinities (0, 10 and 15) and temperatures (5, 10 and 20° C). Blood plasma osmolality increased with salinity at all temperatures. Standard metabolic rate (SMR) increased with salinity at 10 and 20° C. Maximum metabolic rate (MMR) and aerobic scope was lowest at salinity of 15 at 5° C, yet at 20° C, they were lowest at a salinity of 0. A cost of osmoregulation (SMR at a salinity of 0 and 15 compared with SMR at a salinity of 10) could only be detected at a salinity of 15 at 20° C, where it was 28%. The results show that P. fluviatilis have capacity to osmoregulate in hyper-osmotic environments. This contradicts previous studies and indicates intraspecific variability in osmoregulatory capabilities among P. fluviatilis populations or habitat origins. An apparent cost of osmoregulation (28%) at a salinity of 15 at 20° C indicates that the cost of osmoregulation in P. fluviatilis increases with temperature under hyperosmotic conditions and a power analysis showed that the cost of osmoregulation could be lower than 12·5% under other environmental conditions. The effect of salinity on MMR is possibly due to a reduction in gill permeability, initiated to reduce osmotic stress. An interaction between salinity and temperature on aerobic scope shows that high salinity habitats are energetically beneficial during warm periods (summer), whereas low salinity habitats are energetically beneficial during cold periods (winter). It is suggested, therefore, that the seasonal migrations of P. fluviatilis between brackish and fresh water is to select an environment that is optimal for metabolism and aerobic scope.

Assessing the Functional Role of Leptin in Energy Homeostasis and the Stress Response in Vertebrates

Leptin is a pleiotropic hormone that plays a critical role in regulating appetite, energy metabolism, growth, stress, and immune function across vertebrate groups. In mammals, it has been classically described as an adipostat, relaying information regarding energy status to the brain. While retaining poor sequence conservation with mammalian leptins, teleostean leptins elicit a number of similar regulatory properties, although current evidence suggests that it does not function as an adipostat in this group of vertebrates. Teleostean leptin also exhibits functionally divergent properties, however, possibly playing a role in glucoregulation similar to what is observed in lizards. Further, leptin has been recently implicated as a mediator of immune function and the endocrine stress response in teleosts. Here, we provide a review of leptin physiology in vertebrates, with a particular focus on its actions and regulatory properties in the context of stress and the regulation of energy homeostasis.

Metabolic cost of osmoregulation in a hypertonic environment in the invasive African clawed frog Xenopus laevis

Studies of aquatic invertebrates reveal that salinity affects feeding and growth rates, reproduction, survival, and diversity. Little is known, however, about how salinity impacts the energy budget of vertebrates and amphibians in particular. The few studies focused on this topic in vertebrates suggest that the ingestion of salts and the resulting osmoregulatory activity is energetically expensive. We analyzed the effect of saline acclimation on standard metabolic rates (SMR) and the activities of metabolic enzymes of internal organs and osmoregulatory variables (plasma osmolality and urea plasma level) in females of Xenopus laevis by means of acclimating individuals to an isosmotic (235 mOsm NaCl; ISO group) and hyper-osmotic (340 mOsm NaCl; HYP group) environment for 40 days. After acclimation, we found that total and mass-specific SMR was approximately 80% higher in the HYP group than those found in the ISO group. These changes were accompanied by higher citrate synthase activities in liver and heart in the HYP group than in the ISO group. Furthermore, we found a significant and positive correlation between metabolic rates and plasma urea, and citrate synthase activity in liver and heart. These results support the notion that the cost of osmoregulation is probably common in most animal species and suggest the existence of a functional association between metabolic rates and the adjustments in osmoregulatory physiology, such as blood distribution and urea synthesis.

Physiological effects of salinity on Delta Smelt, Hypomesus transpacificus

Abiotic factors like salinity are relevant to survival of pelagic fishes of the San Francisco Bay Estuary. We tested the effects of 4 parts per thousand (ppt) salinity increases on Delta Smelt (DS) in a laboratory experiment simulating salinity increases that might occur around the low-salinity zone (LSZ) (<6 ppt). Adult DS, fed 2% body mass per day, starting at 0.5 ppt [freshwater (FW)], were exposed to weekly step-increases of 4 ppt to a maximum of 10 ppt saltwater (SW) over 19 days, and compared to FW controls. DS (n = 12/treatment per sampling) were sampled at 24, 72, and 96 h (1, 3, and 4 days) post-salinity increase for analyses of hematocrit, plasma osmolality, muscle water content, gill chloride cell (CC) Na(+)/K(+)-ATPase (NKA) and apoptosis after being weighed and measured (n = 3 tanks per treatment). No apparent increase in length or weight occurred nor did a difference in survival. Following step-increases in SW, hematocrit increased over time. Other fish responses generally showed a pattern; specifically plasma osmolality became elevated at 1 day and diminished over 4 days in SW. Percent muscle water content (%) did not show significant changes. CCs showed increased NKA, cell size and apoptosis over time in SW, indicating that CCs turnover in DS. The cell renewal process takes days, at least over 19 days. In summary, DS are affected by salinities of the LSZ and ≤10 ppt, though they employ physiological strategies to acclimate.

Osmoregulatory adaptations of freshwater air-breathing snakehead fish (Channa striata) after exposure to brackish water

NaCl-rich rock salt dissolved in natural water source leads to salinity fluctuation that profoundly affects freshwater ecosystem and aquatic fauna. The snakehead (Channa striata) can live in saline water, but the osmoregulatory mechanisms underlying this ability remain unclear. Herein, we found that exposure to salinities ≥ 10‰ NaCl markedly elevated plasma cortisol and glucose levels, and caused muscle dehydration. In a study of time-dependent response after being transferred from fresh water (0‰ NaCl, FW) to salt-dissolved brackish water (10‰ NaCl, SW), FW-SW, cortisol increased rapidly along with elevations of plasma glucose and lactate. Interestingly, plasma cortisol returned to baseline after prolonged exposure, followed by a second peak that probably enhanced the branchial Na(+)/K(+)-ATPase activity. Under SW-FW condition, Na(+)/K(+)-ATPase activity was not altered as compared to SW-adapted fish. In conclusion, salinity change, especially FW-SW, induced a stress response and hence cortisol release in C. striata, which might increase plasma glucose and lactate to energize the branchial Na(+)/K(+)-ATPase.

Effects of salinity on metabolic rate and branchial expression of genes involved in ion transport and metabolism in Mozambique tilapia (Oreochromis mossambicus)

This study investigated the effects of two rearing salinities, and acute salinity transfer, on the energetic costs of osmoregulation and the expression of metabolic and osmoregulatory genes in the gill of Mozambique tilapia. Using automated, intermittent-flow respirometry, measured standard metabolic rates (SMRs) of tilapia reared in seawater (SW, 130 mg O₂ kg⁻¹ h⁻¹) were greater than those reared in fresh water (FW, 103 mg O₂ kg⁻¹ h⁻¹), when normalized to a common mass of 0.05 kg and at 25±1°C. Transfer from FW to 75% SW increased SMR within 18h, to levels similar to SW-reared fish, while transfer from SW to FW decreased SMR to levels similar to FW-reared fish. Branchial gene expression of Na⁺-K⁺-2Cl⁻ cotransporter (NKCC), an indicator of SW-type mitochondria-rich (MR) cells, was positively correlated with SMR, while Na⁺-Cl⁻ cotransporter (NCC), an indicator of FW-type MR cells, was negatively correlated. Principal Components Analysis also revealed that branchial expression of cytochrome c oxidase subunit IV (COX-IV), glycogen phosphorylase (GP), and a putative mitochondrial biogenesis regulator in fish, peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), were correlated with a higher SMR, plasma osmolality, and environmental salinity, while expression of glycogen synthase (GS), PGC-1β, and nuclear respiratory factor 1 (NRF-1) had negative correlations. These results suggest that the energetic costs of osmoregulation are higher in SW than in FW, which may be related to the salinity-dependent differences in osmoregulatory mechanisms found in the gills of Mozambique tilapia.

Effect of salinity on oxygen consumption in fishes: a review

The effect of salinity on resting oxygen uptake was measured in the perch Perca fluviatilis and available information on oxygen uptake in teleost species at a variety of salinities was reviewed. Trans-epithelial ion transport against a concentration gradient requires energy and exposure to salinities osmotically different from the body fluids therefore imposes an energetic demand that is expected to be lowest in brackish water compared to fresh and sea water. Across species, there is no clear trend between oxygen uptake and salinity, and estimates of cost of osmotic and ionic regulation vary from a few per cent to >30% of standard metabolism.

Role for leptin in promoting glucose mobilization during acute hyperosmotic stress in teleost fishes

Osmoregulation is critical for survival in all vertebrates, yet the endocrine regulation of this metabolically expensive process is not fully understood. Specifically, the function of leptin in the regulation of energy expenditure in fishes, and among ectotherms, in general, remains unresolved. In this study, we examined the effects of acute salinity transfer (72  h) and the effects of leptin and cortisol on plasma metabolites and hepatic energy reserves in the euryhaline fish, the tilapia (Oreochromis mossambicus). Transfer to 2/3 seawater (23  ppt) significantly increased plasma glucose, amino acid, and lactate levels relative to those in the control fish. Plasma glucose levels were positively correlated with amino acid levels (R2=0.614), but not with lactate levels. The mRNA expression of liver leptin A (lepa), leptin receptor (lepr), and hormone-sensitive and lipoprotein lipases (hsl and lpl) as well as triglyceride content increased during salinity transfer, but plasma free fatty acid and triglyceride levels remained unchanged. Both leptin and cortisol significantly increased plasma glucose levels in vivo, but only leptin decreased liver glycogen levels. Leptin decreased the expression of liver hsl and lpl mRNAs, whereas cortisol significantly increased the expression of these lipases. These findings suggest that hepatic glucose mobilization into the blood following an acute salinity challenge involves both glycogenolysis, induced by leptin, and subsequent gluconeogenesis of free amino acids. This is the first study to report that teleost leptin A has actions that are functionally distinct from those described in mammals acting as a potent hyperglycemic factor during osmotic stress, possibly in synergism with cortisol. These results suggest that the function of leptin may have diverged during the evolution of vertebrates, possibly reflecting differences in metabolic regulation between poikilotherms and homeotherms.

Rapid decreases in salinity, but not increases, lead to immune dysregulation in Nile tilapia, Oreochromis niloticus (L.)

Rapid changes in salinity, as with other environmental stressors, can have detrimental effects on fish and may trigger increased susceptibility to disease. However, the precise mechanisms of these effects are not well understood. We examined the effects of sudden increases or decreases in salinity on teleost immune function using Nile tilapia, Oreochromis niloticus (L.), as the fish model in a battery of bioassays of increasing immune system specificity. Two different salinity experiments were performed: one of increasing salinity (0 to 5, 10 and 20 g L(-1) ) and one of decreasing salinity (20 to 15, 10 and 5 g L(-1) ). Histopathology of anterior kidney, gills, gonads, intestines and liver of exposed fish was performed, but no remarkable lesions were found that were attributable to the salinity treatment regimes. The spleen was removed from each fish for analysis of cytokine expression, and peripheral blood was used for haematology, cortisol and phagocytosis assays. In the increasing salinity experiments, no significant changes were observed in any immune system assays. However, in the decreasing salinity experiments, lymphopenia, neutrophilia and monocytosis were observed in the peripheral blood without modification of the packed cell volume, plasma protein or plasma cortisol levels. Phagocytosis was increased in response to decreases in salinity from 20 g L(-1) to 15 g L(-1) , 10 g L(-1) and 5 g L(-1) , whereas phagocytic index was not significantly altered. Transforming growth factor-β (TGF-β) transcription increased during the same decreases in salinity. However, the TGF-β value at 5 g L(-1) was less than those in the 15 and 10 g L(-1) salinity treatments. Interleukin-1β (IL-1β) transcription did not significantly respond to either salinity regime. In total, acute salinity changes appeared to trigger reactive dysregulation of the immune response in tilapia, a situation which, when combined with additional co-occurring stressors such as sudden changes in temperature and/or dissolved oxygen, could make fish more susceptible to infectious diseases. Accordingly, these findings may help to explain how sudden environmental changes may initiate disease outbreaks and lead to critical declines in cultured or wild fish populations.

The effects of arachidonic acid on the endocrine and osmoregulatory response of tilapia (Oreochromis mossambicus) acclimated to seawater and subjected to confinement stress

In previous studies in freshwater tilapia (Oreochromis mossambicus), dietary supplementation with arachidonic acid (ArA; 20:4n - 6) had considerable, opposing effects on the main ion-transporting enzyme Na(+)/K(+)-ATPase in gills and kidneys and changed the release of osmoregulatory hormones, such as cortisol. The present study was performed to assess the influence of dietary ArA on (1) the osmoregulatory capacity of tilapia acclimated to seawater (SW) (34‰) and (2) the osmoregulatory imbalance associated with acute stress. The increased ambient salinity was associated with significant alterations in the tissue fatty acid composition, particularly the n - 6 polyunsaturated fatty acids (PUFAs). Tissue levels of ArA were further increased as a result of dietary supplementation, whereas docosahexaenoic acid (DHA, 22:6n - 3) and eicosapentaenoic acid (EPA, 20:5n - 3) decreased in gills and kidneys. Basal plasma cortisol as well as lactate levels were elevated in the ArA-supplemented SW-acclimated tilapia compared with the control group. The 5 min of confinement (transient stress) increased plasma cortisol, glucose, and lactate levels with significantly higher levels in ArA-supplemented tilapia. Confinement was also associated with significantly elevated plasma osmolality, sodium, chloride, and potassium levels. ArA-supplemented tilapia showed markedly lower ionic disturbances after confinement, suggesting that dietary ArA can attenuate the hydromineral imbalance associated with acute stress. These results emphasize the involvement of ArA and/or its metabolites in the endocrine and osmoregulatory processes and the response to confinement stress.

Changes of glycogen metabolism in the gills and hepatic tissue of tilapia (Oreochromis mossambicus) during short-term Cd exposure

The aim of the study was to test the hypothesis that the mechanism of glycogen metabolism has taken place in gills rather than in liver during Cd exposure. Male tilapia were exposed to 44.45 μM ambient Cd for 12h, and we found blood glucose significantly increased, however, lactate levels showed no significant changes. The glycogen phosphorylase (GP) activity increased immediately after 0.75 to 3h of Cd exposure in the gills, and after 1 to 6h in the liver, respectively. In addition, the glycogen level depleted faster in the gills than in the liver. Plasma cortisol level increased from 0.25 to 1h and recovered after 3h, while the glucagon did not significantly change during Cd exposure. Glucocorticoid receptor (GR) mRNA expression decreased after 0.75 h in the gills, while it significantly increased after 6h in the liver. Ca(2+), Na(+), Cl(-), and K(+) significantly decreased upon Cd exposure within 6h following Cd-induced toxic stress. We suggested that the cortisol is the spontaneous stimulation of glycogen metabolism in the gills, and it triggers a subsequent energy supply later in the liver. Taken together, the profile of glycogen metabolism between gills and liver during Cd-exposure stress provide good support to our hypothesis.

Metabolic and osmoregulatory changes and cell proliferation in gilthead sea bream (Sparus aurata) exposed to cadmium

The impact of cadmium on metabolism and osmoregulation was assessed in gilthead sea bream (Sparus aurata). Seawater acclimated fish were injected intraperitoneally with a sublethal dose of cadmium (1.25 mg Cd/kg body wt). After 7 days, half of the injected fish were sampled. The remaining fish were transferred to hypersaline water and sampled 4 days later. Gill and kidney Na(+)/K(+)-ATPase activities, plasma levels of cortisol, several metabolites and osmolytes, as well as osmolality were measured. Hepatosomatic index and condition factor were calculated. The expression levels of Na(+)/K(+)-ATPase, heat shock proteins (HSP70, HSP90) and proliferating cell nuclear antigen was assessed by western blotting. Cadmium treatment adversely affected the Na(+)/K(+)-ATPase activity, although, there was no perturbation in ion homeostasis and the animals were not compromised following transfer to hypersaline water. Increased cell proliferation and Hsp90 expression likely contributed to the attenuation of the deleterious effects of cadmium exposure.

Rapid changes in plasma cortisol, osmolality, and respiration in response to salinity stress in tilapia (Oreochromis mossambicus)

We elucidated a time course for cortisol release in tilapia as it corresponds to changes in plasma osmolytes and respiration. Following exposure of freshwater (FW) tilapia to 25 per thousand seawater (SW), we measured plasma osmolality, [Na(+)], [K(+)], [Cl(-)], hematocrit, cortisol concentration, oxygen-consumption rate (MO2), and ventilation frequency over 5days and compared them to FW control fish. Cortisol increased rapidly by 3h and remained elevated for 3days. Plasma osmolality, [Na(+)], and [Cl(-)] were elevated at 6-8h, peaked 24h following SW exposure, and then decreased to near-FW levels by 3days. MO2 increased at 24h post-SW exposure relative to FW, while ventilation frequency increased by 3h. Overall, we interpret changes in cortisol as resulting from a change in salinity, in contrast to changes in plasma solute concentrations that could be due to adjustments resulting from the fish's cortisol response as it faces osmoregulatory distress. Increases in oxygen-consumption rate at 24h and ventilation frequency at 3h are likely as a result of the cellular stress response occurring during salinity stress. No significant changes in blood hematocrit were observed, which suggests that tilapia are capable of rapidly counteracting dehydration during acute hyperosmotic stress.

Hyperosmotic shock adaptation by cortisol involves upregulation of branchial osmotic stress transcription factor 1 gene expression in Mozambique Tilapia

The Mozambique tilapia (Oreochromis mossambicus) is a euryhaline species that does not survive direct seawater exposure. Cortisol is involved in re-establishing electrolyte homeostasis in seawater and is thought to play a role in allowing tilapia to cope with abrupt seawater exposure, but the mechanism(s) are far from clear. Recently, osmotic stress transcription factor 1 (OSTF1) was identified as a key signaling molecule involved in hyperosmotic stress adaptation in tilapia. Consequently, we tested the hypothesis that upregulation of OSTF1 expression by cortisol is a key response for hyperosmotic stress adaptation in tilapia. Fish were exposed to different salinities over a 24h period, while a major electrolyte disturbance and mortality was observed only with full-strength seawater exposure. Therefore, we administered cocoa butter implants of cortisol (50mg/kg) intraperitoneally to tilapia maintained in fresh water and after three days exposed these fish to full-strength seawater. There was 50% mortality in the control fish upon seawater exposure, but this was abolished by cortisol treatment. Abrupt seawater exposure did not affect plasma cortisol levels, while, as expected, exogenous administration of this steroid elevated plasma cortisol levels both in fresh water and seawater. Cortisol treatment significantly induced OSTF1 gene expression in fresh water tilapia, and also enhanced further the seawater-induced OSTF1 mRNA abundance. Plasma osmolality decreased, while gill Na(+)/K(+)-ATPase activity was suppressed in the cortisol group in seawater compared to the sham group. This corresponded with a significant reduction in gill ionocyte size and Na(+)/K(+)-ATPase activity and protein expression after seawater exposure. Cortisol did not modify liver metabolism, but significantly suppressed gill metabolic capacity in seawater. Overall, cortisol adapts tilapia to a hyperosmotic shock associated with abrupt seawater exposure. This involves upregulation of OSTF1 gene expression and a concomitant suppression of branchial metabolism in tilapia.

Differential responses in gills of euryhaline tilapia, Oreochromis mossambicus, to various hyperosmotic shocks

Euryhaline tilapia (Oreochromis mossambicus) survived in brackish water (BW; 20 per thousand) but died in seawater (SW; 35 per thousand) within 6 h when transferred directly from fresh water (FW). The purpose of this study was to clarify responses in gills of FW tilapia to various hyperosmotic shocks induced by BW or SW. In FW-acclimated tilapia, scanning electron micrographs of gills revealed three subtypes of MR cell apical surfaces: wavy-convex (subtype I), shallow-basin (subtype II), and deep-hole (subtype III). Density of apical surfaces of mitochondrion-rich (MR) cell in gills of the BW-transfer tilapia decreased significantly within 3 h post-transfer due to disappearance of subtype I cells, but increased from 48 h post-transfer because of increasing density of subtype III cells. SW-transfer individuals, however, showed decreased density of MR cell openings after 1 h post-transfer because subtype I MR cell disappeared. On the other hand, relative branchial Na+/K+-ATPase (NKA) alpha1-subunit mRNA levels, protein abundance, and NKA activity of the BW-transfer group increased significantly at 6, 12, and 12 h post-transfer, respectively. In the SW-transfer group, relative mRNA and protein abundance of gill NKA alpha1-subunit did not change while NKA activity declined before dying in 5 h. Upon SW transfer, dramatic increases (nearly 2-fold) of plasma osmolality, [Na+], and [Cl(-)] were found prior to death. For the BW-transfer group, plasma osmolality was eventually controlled by 96 h post-transfer by enhancement of NKA expression and subtype III MR cell. The success or failure of NKA activation from gene to functional protein as well as the development of specific SW subtype in gills were crucial for the survival of euryhaline tilapia to various hyperosmotic shocks.

The effect of elevated salinity on 'California' Mozambique tilapia (Oreochromis mossambicus x O. urolepis hornorum) metabolism

California Mozambique tilapia (Oreochromis mossambicus x O. urolepis hornorum) are extremely saline tolerant and have been previously shown to reduce whole-animal oxygen consumption rate (MO(2)) upon exposures to salinities greater than that of seawater (SW). In this study tilapia were acclimated to 15, 30, 45, 60 and 75 g/L salinity for 1, 5, 14, or 28 days. There was little change in plasma osmolality or muscle water content in salinities below 60 g/L, and branchial Na(+), K(+)-ATPase (NKA) activity was low in 15 and 30 g/L relative to 60 and 75 g/L. When tilapia were exposed to 75 g/L, plasma osmolality and NKA activity were significantly increased within 5 days of exposure relative to those in 15 and 30 g/L, and remained elevated over the entire 28 days acclimation, indicating that short term salinity challenges (i.e., 5 days) are predictive of longer exposure durations in this species. MO(2) following transfer to 15 and 30 g/L was elevated, reflecting the high energy demand required for switching from a hyper- to a hypo-osmoregulatory strategy. The MO(2) of 60 g/L-exposed fish was significantly reduced at 1, 5, and 14 days, relative to 30 g/L-exposed fish; however by 28 days there were no significant differences. We investigated the potential for a metabolic basis for the salinity-induced MO(2) reduction, using forward stepwise linear regression to correlate enzyme activities of brain, liver, and kidney with MO(2). Brain NKA was correlated with MO(2) after 5 days (p<0.01, r(2)=0.944) and both brain NKA and hepatic total ATPase were correlated with the reduced MO(2) at 14 days (p=0.027, r(2)=0.980 and p=0.025, r(2)=0.780, respectively). These results may indicate a tissue-level metabolic suppression, which has not been previously described as a response to hypersaline exposure in fishes.

Some insights into energy metabolism for osmoregulation in fish

A sufficient and timely energy supply is a prerequisite for the operation of iono- and osmoregulatory mechanisms in fish. Measurements of whole-fish or isolated-gill (or other organs) oxygen consumption have demonstrated regulation of the energy supply during acclimation to different osmotic environments, and such regulation is dependent on species, the situation of acclimation or acclimatization, and life habits. Carbohydrate metabolism appears to play a major role in the energy supply for iono- and osmoregulation, and the liver is the major source supplying carbohydrate metabolites to osmoregulatory organs. Compared with carbohydrates, the roles of lipids and proteins remain largely unclear. Energy metabolite translocation was recently found to occur between fish gill ionocytes and neighboring glycogen-rich (GR) cells, indicating the physiological significance of a local energy supply for gill ion regulatory mechanisms. Spatial and temporal relationships between the liver and other osmoregulatory and non-osmoregulatory organs in partitioning the energy supply for ion regulatory mechanisms during salinity challenges were also proposed. A novel glucose transporter was found to specifically be expressed and function in gill ionocytes, providing the first cue for investigating energy translocation among gill cells. Advanced molecular physiological approaches can be used to examine energy metabolism relevant to a particular cell type (e.g., gill ionocytes), and functional genomics may also provide another powerful approach to explore new metabolic pathways related to fish ion regulation.

Behavioural and gill histopathological effects of acute exposure to sodium chloride in moneda (Metynnis orinocensis)

To evaluate the toxicity of sodium chloride (NaCl), juveniles and adult Metynnis orinocensis were exposed for 96h to 0, 5, 10, 15, 20 or 40gL(-1) of salt. Food intake, behaviour, opercular frequency (OF), mortality, body weight and gill microscopic alterations were evaluated. Behavioural changes were observed in fish exposed to concentrations higher than 10gL(-1). Juveniles and adults showed a progressive decrease in the OF and body weight. Food intake decreased in concentrations below 15gL(-1). Juveniles and adults exposed to 15, 20 or 40gL(-1) had 100% mortality. Lamellar congestion, hyperplasia and fusion were the common microscopic alterations at higher concentrations. The gill congestion severity increased with salt concentration. The LC(50) for juveniles and adults were 10.5gL(-1) and 10.8gL(-1), respectively. These results suggest that salt concentrations lower than 5gL(-1) are safe for preventive and therapeutic practices in Metynnis orinocensis; whereas prolonged exposure higher than 10gL(-1) is deleterious in this species.

Osmoregulation and expression of ion transport proteins and putative claudins in the gill of southern flounder (Paralichthys lethostigma)

The southern flounder is a euryhaline teleost that inhabits ocean, estuarine, and riverine environments. We investigated the osmoregulatory strategy of juvenile flounder by examining the time-course of homeostatic responses, hormone levels, and gill Na(+),K(+)-ATPase and Na(+),K(+),2Cl(-) cotransporter protein expression after salinity challenge. Transfer of freshwater (FW)-acclimated flounder to sea water (SW) induced an increase in plasma osmolality and cortisol and a decrease in muscle water content, plasma insulin-like growth factor I (IGF-I) and hepatic IGF-I mRNA, all returning to control levels after 4 days. Gill Na(+),K(+)-ATPase and Na(+),K(+),2Cl(-) cotransporter protein levels were elevated in response to SW after 4 days. Transfer of SW-acclimated flounder to FW reduced gill Na(+),K(+)-ATPase and Na(+),K(+),2Cl(-) cotransporter protein, increased plasma IGF-I, but did not alter hepatic IGF-I mRNA or plasma cortisol levels. Gill claudin-3 and claudin-4 immunoreactive proteins were elevated in FW versus SW acclimated flounder. The study demonstrates that successful acclimation of southern flounder to SW or FW occurs after an initial crisis period and that the salinity adaptation process is associated with changes in branchial expression of ion transport and putative tight junction claudin proteins known to regulate epithelial permeability in mammalian vertebrates.

Salinity regulates claudin mRNA and protein expression in the teleost gill

The teleost gill carries out NaCl uptake in freshwater (FW) and NaCl excretion in seawater (SW). This transformation with salinity requires close regulation of ion transporter capacity and epithelial permeability. This study investigates the regulation of tight-junctional claudins during salinity acclimation in fish. We identified claudin 3- and claudin 4-like immunoreactive proteins and examined their expression and that of select ion transporters by performing Western blot in tilapia (Oreochromis mossambicus) gill during FW and SW acclimation. Transfer of FW tilapia to SW increased plasma osmolality, which was corrected after 4 days, coinciding with increased gill Na+-K+-ATPase and Na+-K+-2Cl(-) cotransporter expression. Gill claudin 3- and claudin 4-like proteins were reduced with exposure to SW. Transfer to FW increased both claudin-like proteins. Immunohistochemistry shows that claudin 3-like protein was localized deep in the FW gill filament, whereas staining was found apically in SW gill. Claudin 4-like proteins are localized predominantly in the filament outer epithelial layer, and staining appears more intense in the gill of FW versus SW fish. In addition, tilapia claudin 28a and 30 genes were characterized, and mRNA expression was found to increase during FW acclimation. These studies are the first to detect putative claudin proteins in teleosts and show their localization and regulation with salinity in gill epithelium. The data indicate that claudins may be important in permeability changes associated with salinity acclimation and possibly the formation of deeper tight junctions in FW gill. This may reduce ion permeability, which is a critical facet of FW osmoregulation.

Regulation of glycogen metabolism in gills and liver of the euryhaline tilapia (Oreochromis mossambicus) during acclimation to seawater

Glucose, which plays a central role in providing energy for metabolism, is primarily stored as glycogen. The synthesis and degradation of glycogen are mainly initialized by glycogen synthase (GS) and glycogen phosphorylase (GP), respectively. The present study aimed to examine the glycogen metabolism in fish liver and gills during acute exposure to seawater. In tilapia (Oreochromis mossambicus) gill, GP, GS and glycogen were immunocytochemically colocalized in a specific group of glycogen-rich (GR) cells, which are adjacent to the gill's main ionocytes, mitochondrion-rich (MR) cells. Na+/K+-ATPase activity in the gills, protein expression and/or activity of GP and GS and the glycogen content of the gills and liver were examined in tilapia after their acute transfer from freshwater (FW) to 25 per thousand seawater (SW). Gill Na+/K+-ATPase activity rapidly increased immediately after SW transfer. Glycogen content in both the gills and liver were significantly depleted after SW transfer, but the depletion occurred earlier in gills than in the liver. Gill GP activity and protein expression were upregulated 1-3 h post-transfer and eventually recovered to the normal level as determined in the control group. At the same time, GS protein expression was downregulated. Similar changes in liver GP and GS protein expression were also observed but they occurred later at 6-12 h post-transfer. In conclusion, GR cells are initially stimulated to provide prompt energy for neighboring MR cells that trigger ion-secretion mechanisms. Several hours later, the liver begins to degrade its glycogen stores for the subsequent energy supply.

New insights into fish ion regulation and mitochondrion-rich cells

Compared to terrestrial animals, fish have to cope with more-challenging osmotic and ionic gradients from aquatic environments with diverse salinities, ion compositions, and pH values. Gills, a unique and highly studied organ in research on fish osmoregulation and ionoregulation, provide an excellent model to study the regulatory mechanisms of ion transport. The present review introduces and discusses some recent advances in relevant issues of teleost gill ion transport and functions of gill ionocytes. Based on accumulating evidence, a conclusive model of NaCl secretion in gills of euryhaline teleosts has been established. Interpretations of results of studies on freshwater fish gill Na+/Cl- uptake mechanisms are still being debated compared with those for NaCl secretion. Current models for Na+/Cl- uptake are proposed based on studies in traditionally used model species. Many reported inconsistencies are claimed to be due to differences among species, various experimental designs, or acclimation conditions. Having the benefit of advanced techniques in molecular/cellular biology, functional genomics, and model animals, several new notions have recently been raised concerning relevant issues of Na+/Cl- uptake pathways. Several new windows have been opened particularly in terms of molecular mechanisms of ionocyte differentiation and energy metabolite transport between gill cells during environmental challenge.