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

Effects of CRISPR/Cas9 targeting of the myo-inositol biosynthesis pathway on hyper-osmotic tolerance of tilapia cells

Myo-inositol is an important compatible osmolyte in vertebrates. This osmolyte is produced by the myo-inositol biosynthesis (MIB) pathway composed of myo-inositol phosphate synthase and inositol monophosphatase. These enzymes are among the highest upregulated proteins in tissues and cell cultures from teleost fish exposed to hyperosmotic conditions indicating high importance of this pathway for tolerating this type of stress. CRISPR/Cas9 gene editing of tilapia cells produced knockout lines of MIB enzymes and control genes. Metabolic activity decreased significantly for MIB KO lines in hyperosmotic media. Trends of faster growth of the MIB knockout lines in isosmotic media and faster decline of MIB knockout lines in hyperosmotic media were also observed. These results indicate a decline in metabolic fitness but only moderate effects on cell survival when tilapia cells with disrupted MIB genes are exposed to hyperosmolality. Therefore MIB genes are required for full osmotolerance of tilapia cells.

Metabolic cost of osmoregulation by the gastro-intestinal tract in marine teleost fish

Introduction: Although dozens of studies have attempted to determine the metabolic cost of osmoregulation, mainly by comparing standard metabolic rates (SMR) in fish acclimated to different salinities, consensus is still lacking. Methods: In the present study, using the Gulf toadfish, Opsanus beta, we aimed to determine the metabolic cost of esophageal and intestinal osmoregulatory processes by estimating ATP consumption from known ion transport rates and pathways and comparing these estimates with measurements on isolated tissues. Further, we performed whole animal respirometry on fish acclimated to 9, 34 and 60 ppt. Results and Discussion: Our theoretical estimates of esophageal and intestinal osmoregulatory costs were in close agreement with direct measurements on isolated tissues and suggest that osmoregulation by these tissues amounts to ∼2.5% of SMR. This value agrees well with an earlier attempt to estimate osmoregulation cost from ion transport rates and combined with published measurements of gill osmoregulatory costs suggests that whole animal costs of osmoregulation in marine teleosts is ∼7.5% of SMR. As in many previous studies, our whole animal measurements were variable between fish and did not seem suited to determine osmoregulatory costs. While the esophagus showed constant metabolic rate regardless of acclimation salinity, the intestine of fish acclimated to higher salinities showed elevated metabolic rates. The esophagus and the intestine had 2.1 and 3.2-fold higher metabolic rates than corresponding whole animal mass specific rates, respectively. The intestinal tissue displays at least four different Cl- uptake pathways of which the Na+:Cl-:2 K+ (NKCC) pathway accounts for 95% of the Cl- uptake and is the most energy efficient. The remaining pathways are via apical anion exchange and seem to primarily serve luminal alkalinization and the formation of intestinal CaCO3 which is essential for water absorption.

Invasive round goby shows higher sensitivity to salinization than native European perch

Salinity is an influential abiotic environmental factor in aquatic species, specifically in freshwater, where salinization causes ecosystem degradation. Secondary salinization, that is increases in salinity due to anthropogenic activities, can affect both osmoregulation and behaviour in freshwater fishes. It is generally believed that invasive species handle climatic change and environmental degradation better than native species, which is one reason for their invasion success. However, how invasive and native species cope with salinity changes remains little understood. Therefore, we investigated how low (500 µS/cm) and high salinity (2000 µS/cm) conditions affected oxygen consumption and behaviour in the invasive round goby (Neogobius melanostomus) and the native European perch (Perca fluviatilis). Our results showed that in round goby oxygen consumption increased and swimming and non-swimming movements changed in response to salinity increments, whereas European perch was not affected by salinity. Thus, it seems as if the invasive round goby is more sensitive to changes in salinity than the native European perch. Our results fit with the minority of studies indicating invasive species being less tolerant than some native species to environmental changes. This finding could be explained by the adaptation of round goby to low salinity due to its long establishment in River Rhine. Further, our results are also confirming that the effect of salinity is species-specific. In addition, European perch and round goby show diametrically different behavioural response to disturbance which could be an effect of holding different ecological niches as well as their anatomical differences.

Physiological and behavioural strategies of aquatic animals living in fluctuating environments

Shallow or near-shore environments, such as ponds, estuaries and intertidal zones, are among the most physiologically challenging of all aquatic settings. Animals inhabiting these environments experience conditions that fluctuate markedly over relatively short temporal and spatial scales. Living in these habitats requires the ability to tolerate the physiological disturbances incurred by these environmental fluctuations. This tolerance is achieved through a suite of physiological and behavioural responses that allow animals to maintain homeostasis, including the ability to dynamically modulate their physiology through reversible phenotypic plasticity. However, maintaining the plasticity to adjust to some stresses in a dynamic environment may trade off with the capacity to deal with other stressors. This paper will explore studies on select fishes and invertebrates exposed to fluctuations in dissolved oxygen, salinity and pH. We assess the physiological mechanisms these species employ to achieve homeostasis, with a focus on the plasticity of their responses, and consider the resulting physiological trade-offs in function. Finally, we discuss additional factors that may influence organismal responses to fluctuating environments, such as the presence of multiple stressors, including parasites. We echo recent calls from experimental biologists to consider physiological responses to life in naturally fluctuating environments, not only because they are interesting in their own right but also because they can reveal mechanisms that may be crucial for living with increasing environmental instability as a consequence of climate change.

Shuttle-box systems for studying preferred environmental ranges by aquatic animals

Animals' selection of environments within a preferred range is key to understanding their habitat selection, tolerance to stressors and responses to environmental change. For aquatic animals, preferred environmental ranges can be studied in so-called shuttle-boxes, where an animal can choose its ambient environment by shuttling between separate choice chambers with differences in an environmental variable. Over time, researchers have refined the shuttle-box technology and applied them in many different research contexts, and we here review the use of shuttle-boxes as a research tool with aquatic animals over the past 50 years. Most studies on the methodology have been published in the latest decade, probably due to an increasing research interest in the effects of environmental change, which underlines the current popularity of the system. The shuttle-box has been applied to a wide range of research topics with regards to preferred ranges of temperature, CO 2 , salinity and O 2  in a vast diversity of species, showing broad applicability for the system. We have synthesized the current state-of-the-art of the methodology and provided best practice guidelines with regards to setup, data analyses, experimental design and study reporting. We have also identified a series of knowledge gaps, which can and should be addressed in future studies. We conclude with highlighting directions for research using shuttle-boxes within evolutionary biology and behavioural and physiological ecology.

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

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

Inter-population differences in salinity tolerance of adult wild Sacramento splittail: osmoregulatory and metabolic responses to salinity

The Sacramento splittail (Pogonichthys macrolepidotus) is composed of two genetically distinct populations endemic to the San Francisco Estuary (SFE). The allopatric upstream spawning habitat of the Central Valley (CV) population connects with the sympatric rearing grounds via relatively low salinity waters, whereas the San Pablo (SP) population must pass through the relatively high-salinity Upper SFE to reach its allopatric downstream spawning habitat. We hypothesize that if migration through SFE salinities to SP spawning grounds is more challenging for adult CV than SP splittail, then salinity tolerance, osmoregulatory capacity, and metabolic responses to salinity will differ between populations. Osmoregulatory disturbances, assessed by measuring plasma osmolality and ions, muscle moisture and Na⁺-K⁺-ATPase activity after 168 to 336 h at 11‰ salinity, showed evidence for a more robust osmoregulatory capacity in adult SP relative to CV splittail. While both resting and maximum metabolic rates were elevated in SP splittail in response to increased salinity, CV splittail metabolic rates were unaffected by salinity. Further, the calculated difference between resting and maximum metabolic values, aerobic scope, did not differ significantly between populations. Therefore, improved osmoregulation came at a metabolic cost for SP splittail but was not associated with negative impacts on scope for aerobic metabolism. These results suggest that SP splittail may be physiologically adjusted to allow for migration through higher-salinity waters. The trends in interpopulation variation in osmoregulatory and metabolic responses to salinity exposures support our hypothesis of greater salinity-related challenges to adult CV than SP splittail migration and are consistent with our previous findings for juvenile splittail populations, further supporting our recommendation of population-specific management.

Chasing away accurate results: exhaustive chase protocols underestimate maximum metabolic rate estimates in European perch Perca fluviatilis

Metabolic rates are one of many measures that are used to explain species' response to environmental change. Static respirometry is used to calculate the standard metabolic rate (SMR) of fish, and when combined with exhaustive chase protocols it can be used to measure maximum metabolic rate (MMR) and aerobic scope (AS) as well. While these methods have been tested in comparison to swim tunnels and chambers with circular currents, they have not been tested in comparison with a no-chase control. We used a repeated-measures design to compare estimates of SMR, MMR and AS in European perch Perca fluviatilis following three protocols: (a) a no-chase control; (b) a 3-min exhaustive chase; and (c) a 3-min exhaustive chase followed by 1-min air exposure. We found that, contrary to expectations, exhaustive chase protocols underestimate MMR and AS at 18°C, compared to the no-chase control. This suggests that metabolic rates of other species with similar locomotorty modes or lifestyles could be similarly underestimated using chase protocols. These underestimates have implications for studies examining metabolic performance and responses to climate change scenarios. To prevent underestimates, future experiments measuring metabolic rates should include a pilot with a no-chase control or, when appropriate, an adjusted methodology in which trials end with the exhaustive chase instead of beginning with it.

Extreme warm acclimation temperature alters oxygen consumption, micronucleus formation in erythrocytes, and gill morphology of rohu (Labeo rohita) fingerlings

Experiencing the seasonal variation and rapid global warming in the tropical climate is a common phenomenon which challenged the aquatic organisms to adapt the physiology and behavior. To investigate the effect of high-temperature acclimation, we selected Indian major carp, rohu (Labeo rohita), a commercially important freshwater aquaculture species. Oxygen consumptions, micronucleus formation in erythrocytes, and gill histopathology were observed in L. rohita fingerlings acclimated at three temperatures (30, 33, and 36 °C) for 30 days. Results showed that the highest acclimated temperature (36 °C) induced higher oxygen consumption and increased frequency of micronucleus formation in erythrocytes. Severity of different histological alterations (hyperplasia, epithelial necrosis, telangiectasis, epithelial lifting, and hypertrophy of chloride cells) in the gills was found to be increased in the highest acclimated temperature (36 °C). These findings indicate the temperature induced adaptive responses and climate vulnerability in a changing environment.

Respirometry and cutaneous oxygen flux measurements reveal a negligible aerobic cost of ion regulation in larval zebrafish (Danio rerio)

Fishes living in fresh water counter the passive loss of salts by actively absorbing ions through specialized cells termed ionocytes. Ionocytes contain ATP-dependent transporters and are enriched with mitochondria; therefore ionic regulation is an energy-consuming process. The purpose of this study was to assess the aerobic costs of ion transport in larval zebrafish (Danio rerio). We hypothesized that changes in rates of Na+ uptake evoked by acidic or low Na+ rearing conditions would result in corresponding changes in whole-body oxygen consumption (ṀO2 ) and/or cutaneous oxygen flux (JO2 ), measured at the ionocyte-expressing yolk sac epithelium using the scanning micro-optrode technique (SMOT). Larvae at 4 days post-fertilization (dpf) that were reared under low pH (pH 4) conditions exhibited a higher rate of Na+ uptake compared with fish reared under control conditions (pH 7.6), yet they displayed a lower ṀO2  and no difference in cutaneous JO2 Despite a higher Na+ uptake capacity in larvae reared under low Na+ conditions, there were no differences in ṀO2  and JO2  at 4 dpf. Furthermore, although Na+ uptake was nearly abolished in 2 dpf larvae lacking ionocytes after morpholino knockdown of the ionocyte proliferation regulating transcription factor foxi3a, ṀO2 and JO2  were unaffected. Finally, laser ablation of ionocytes did not affect cutaneous JO2 Thus, we conclude that the aerobic costs of ion uptake by ionocytes in larval zebrafish, at least in the case of Na+, are below detection using whole-body respirometry or cutaneous SMOT scans, providing evidence that ion regulation in zebrafish larvae incurs a low aerobic cost.

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

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

Salinity modulates biochemical and histopathological changes caused by silver nanoparticles in juvenile Persian sturgeon (Acipenser persicus)

The objective of this study was to evaluate the effect of salinity on the acute and sub-chronic toxicity of silver nanoparticles (AgNPs) in Persian sturgeon. This was evaluated by exposing Persian sturgeon to AgNPs in three salinities: freshwater (F: 0.4 ppt), brackish water 1 (B₁: 6 ± 0.2 ppt), and brackish water 2 (B₂: 12 ± 0.3 ppt) for 14 days, which was followed by analysis of alterations in plasma chemistry and histopathology of the gills, liver, and intestine. Values of 96-h median lethal concentration (LC₅₀) were calculated as 0.89 mg/L in F, 2.07 mg/L in B₁, and 1.59 mg/L in B₂. After sub-chronic exposures, plasma cortisol, glucose, potassium, and sodium levels illustrated no significant changes within each salinity level. In F, 0.2 mg/L AgNP caused the highest levels of alkaline phosphatase and osmolality levels. In B₁, 0.6 mg/L AgNP induced the highest level of alkaline phosphatase and elevated plasma osmolality was recorded in all AgNP-exposed treatments in comparison with the controls. The B₂ treatment combined with 0.6 mg/L AgNP significantly reduced plasma chloride level. The results showed elevating salinity significantly increased osmolality, chloride, sodium, and potassium levels of plasma in the fish exposed to AgNPs. The abundance of the tissue lesions was AgNP concentration-dependent, where the highest number of damages was observed in the gills, followed by liver and intestine, respectively. The histopathological study also confirmed alterations such as degeneration of lamella, lifting of lamellar epithelium, hepatic vacuolation, pyknotic nuclei, and cellular infiltration of the lamina propria elicited by AgNPs in the gills, liver, and intestine of Persian sturgeon. In conclusion, the stability of AgNPs in aquatic environments can be regulated by changing the salinity, noting that AgNPs are more stable in low salinity waters.

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.

FishResp: R package and GUI application for analysis of aquatic respirometry data

Intermittent-flow respirometry is widely used to measure oxygen uptake rates and subsequently estimate aerobic metabolic rates of aquatic animals. However, the lack of a standard quality-control software to detect technical problems represents a potential impediment to comparisons across studies in the field of evolutionary and conservation physiology. Here, we introduce 'FishResp', a versatile R package and its graphical implementation for quality-control and filtering of raw respirometry data. Our goal is to provide a straightforward, cross-platform and free software to help improve the quality and comparability of metabolic rate estimates for reducing methodological fragmentation in the field of aquatic respirometry. FishResp accepts data from various respirometry systems, allows users to detect potential mechanical problems which can occur during oxygen uptake measurements (e.g. chamber leaking, poor water circulation), and offers six options to correct raw data for microbial oxygen consumption. The software performs filtering of raw data based on user criteria, and produces accurate and unbiased estimates of absolute and mass-specific metabolic rates. Using data from three-spined sticklebacks (Gasterosteus aculeatus) and Trinidadian guppies (Poecilia reticulata), we demonstrate the virtues of FishResp, highlighting the importance of detecting mechanical problems and correcting measurements for background respiration.

Maximum salinity tolerance and osmoregulatory capabilities of European perch Perca fluviatilis populations originating from different salinity habitats

Although considered a stenohaline freshwater species, European perch (Perca fluviatilis) inhabit brackish waters. The present study determined the maximum salinity tolerance and osmoregulatory capability on individuals originating from brackish water and from freshwater populations. The fish were acclimated for 3 weeks to salinities of 0, 10, 12.5, 15, 17.5 and 20 after an initial stepwise increase to the target salinity. The maximum salinity tolerance was determined as the test salinity below which the fish could not acclimate and lost equilibrium. Blood plasma osmolality was measured if the fish had not lost equilibrium after the acclimation period. The maximum salinity tolerance was 17.5 for brackish water European perch and 10 for fresh water European perch. The high salinity tolerance of the brackish water European perch was caused by their ability to both hyper- and hypo-osmoregulate, whereas the freshwater originating fish could only hyper-osmoregulate. The results showed that maximum salinity tolerances and osmoregulatory capabilities depends on the origin habitat salinity. Due to genetic differentiation between European perch populations in brackish and fresh water, the possibility of brackish water European perch being a subspecies of European perch is discussed, yet vital knowledge concerning heritability of salinity tolerance traits is still missing. Regardless of species status, within-species plasticity in the ability to cope with varying salinities have substantial ecological and conservation implications and underlines the need for managing brackish water and freshwater European perch stocks separately.

Behavioural salinity preference of juvenile yellow perch Perca flavescens

The present study determined the behavioural salinity preference of a freshwater stock of juvenile yellow perch Perca flavescens acclimated to salinities of 0 and 10. The preferred salinities ranged between 7·3 and 13·0 (mean ± s.d. = 10·4 ± 1·7; n = 13) with no significant effect of acclimation salinity. The results showed that juvenile P. flavescens prefers near isoosmotic salinities, which could be due to a lowered energetic cost of osmoregulation.