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The mechanism(s) of sodium, chloride and pH regulation in teleost fishes has been the subject of intense interest for researchers over the past 100 years. The primary organ responsible for ionoregulatory homeostasis is the gill, and more specifically, gill ionocytes. Building on the theoretical and experimental research of the past, recent advances in molecular and cellular techniques in the past two decades have allowed for substantial advances in our understanding of mechanisms involved. With an increased diversity of teleost species and environmental conditions being investigated, it has become apparent that there are multiple strategies and mechanisms employed to achieve ion and acid-base homeostasis. This review will cover the historical developments in our understanding of the teleost fish gill, highlight some of the recent advances and conflicting information in our understanding of ionocyte function, and serve to identify areas that require further investigation to improve our understanding of complex cellular and molecular machineries involved in iono- and acid-base regulation., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

The fish gill serves many physiological functions, among which is the excretion of ammonia, the primary nitrogenous waste in most fishes. Although it is the end-product of nitrogen metabolism, ammonia serves many physiological functions including acting as an acid equivalent and as a counter-ion in mechanisms of ion regulation. Our current understanding of the mechanisms of ammonia excretion have been influenced by classic experimental work, clever mechanistic approaches, and modern molecular and genetic techniques. In this review, I will overview the history of the study of ammonia excretion by the gills of fishes, highlighting the important advancements that have shaped this field with a nearly 100-year history. The developmental and evolutionary implications of an ammonia and gill-dominated nitrogen regulation strategy in most fishes will also be discussed. Throughout the review, I point to areas in which more work is needed to push forward this field of research that continues to produce novel insights and discoveries that will undoubtedly shape our overall understanding of fish physiology., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Amazonia is home to 15% (>2700, in 18 orders) of all the freshwater fish species of the world, many endemic to the region, has 65 million years of evolutionary history and accounts for 20% of all freshwater discharge to the oceans. These characteristics make Amazonia a unique region in the world. We review the geological history of the environment, its current biogeochemistry and the evolutionary forces that led to the present endemic fish species that are distributed amongst three very different water types: black waters [acidic, ion-poor, rich in dissolved organic carbon (DOC)], white waters (circumneutral, particle-rich) and clear waters (circumneutral, ion-poor, DOC-poor). The annual flood pulse is the major ecological driver for fish, providing feeding, breeding and migration opportunities, and profoundly affecting O2, CO2 and DOC regimes. Owing to climate change and other anthropogenic pressures such as deforestation, pollution and governmental mismanagement, Amazonia is now in crisis. The environment is becoming hotter and drier, and more intense and frequent flood pulses are now occurring, with greater variation between high and low water levels. Current projections are that Amazon waters of the near future will be even hotter, more acidic, darker (i.e. more DOC, more suspended particles), higher in ions, higher in CO2 and lower in O2, with many synergistic effects. We review current physiological information on Amazon fish, focusing on temperature tolerance and ionoregulatory strategies for dealing with acidic and ion-poor environments. We also discuss the influences of DOC and particles on gill function, the effects of high dissolved CO2 and low dissolved O2, with emphasis on water- versus air-breathing mechanisms, and strategies for pH compensation. We conclude that future elevations in water temperature will be the most critical factor, eliminating many species. Climate change will likely favour predominantly water-breathing species with low routine metabolic rates, low temperature sensitivity of routine metabolic rates, high anaerobic capacity, high hypoxia tolerance and high thermal tolerance., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2022. Published by The Company of Biologists Ltd.)

August Krogh made fundamental discoveries about both respiratory gas exchange and osmo/iono-regulation in fish gills. Dave Randall and co-workers identified a tradeoff between these two functions such that high functional surface area and low diffusion distance would favour O 2 uptake (e.g. exercise, hypoxia), whereas low functional surface area and high diffusion distance would favour osmo/iono-regulation (rest, normoxia). Today we call this concept the "osmorespiratory compromise" and realize that it is much more complex than originally envisaged. There are at least 6 mechanisms by which fish can change functional branchial area and diffusion distance. Three involve reorganizing blood flow pathways: (i) flow redistribution within the secondary (respiratory) lamellae; (ii) flow shunting between "respiratory" and "ionoregulatory" pathways in the filament; (iii) opening up more distal lamellae on the filament and closing non-respiratory pathways. Three more involve "reversible gill remodeling": (iv) proliferation of the interlamellar gill cell mass (ILCM); (v) proliferation of ionocytes up the sides of the lamellae; (vi) covering over the apical exposure of ionocytes by extension of pavement cells. In ways that remain incompletely understood, these mechanisms allow dynamic regulation of the osmorespiratory compromise, such that ion and water fluxes can be decoupled from O 2 uptake during continuous exercise. Furthermore, hypoxia-tolerant species can reduce branchial ion and water fluxes below normoxic levels despite hyperventilating during hypoxia. In marine fish, the osmorespiratory conflict is intensified by the greater ionic and osmotic gradients from seawater to blood, but underlying mechanisms remain poorly understood., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Acidic freshwater habitats disrupt ion-homeostasis in fishes, yet the often acidic waters of the Mekong host the second highest diversity of freshwater fish in the world. To investigate how five Mekong fish species tolerate water acidity, we measured: time to loss of equilibrium (LOE) at sustained (4 days) low pH (3.5) and net ion flux in acute low pH (3.5 and 3) in Chitala ornata, Pangasianodon hypophthalmus, Osphronemus goramy, Trichogaster pectoralis, and Monopterus albus. Our sustained low pH exposures revealed that C. ornata was least tolerant, P. hypophthalmus and M. albus were moderately tolerant, and O. goramy and T. pectoralis were highly tolerant to low pH. In general, net ion loss in acute low pH exposures was greatest in species with the shortest time to LOE in the sustained low pH exposure. We also explored how low water [Ca 2+ ] (relative to current Mekong levels) affected ion flux at low water pH in the least tolerant C. ornata and highly tolerant T. pectoralis. In C. ornata, low water Ca 2+ (56 ± 1 μmol L -1 ) increased net ion loss relative to high Ca 2+ (342 ± 3 μmol L -1 ) water while no effect was observed in T. pectoralis. Finally, we find that T. pectoralis is among the most acid-tolerant fish species examined to date., (Crown Copyright © 2019. Published by Elsevier Inc. All rights reserved.)

Amphibious fishes have evolved a variety of physiological modifications allowing them to survive in water and air. In air, the amphibious mangrove rivulus, Kryptolebias marmoratus, uses its skin as a site of ionoregulation. Skin ionocytes actively transport ions into/out of the body; however, it is unclear if there are specific morphological or functional changes occurring in skin ionocytes during air exposure. We used two microscopy techniques to describe skin ionocyte morphology and to investigate their plasticity after salinity challenges and air exposure. Immunohistochemical staining in air-exposed fish revealed ionocytes with Na + /K + ATPase (NKA), Na + /H + exchanger (NHE3b) and cystic fibrosis transmembrane conductance regulator (CFTR) immunoreactivity, whereas ionocytes from aquatic fish had only NKA (freshwater) or NKA and CFTR (brackish and hypersaline water). Following salinity challenges, we noted increases in the number and area of ionocyte apical surfaces, indicating that skin ionocyte activity increased in high salinity environments compared with control conditions. Furthermore, we show increased ionocyte area during air exposure suggesting increased ionocyte activity in all salinity conditions. Using energy dispersive X-ray spectroscopy to analyze the skin surface, we report decreases in magnesium, phosphorous, and sulfur after 7 days in air compared with fish in water, suggesting ionic movement in the skin surface during air exposure. Our study highlights morphological and functional features of skin ionocytes that are involved in ionoregulation in an air-exposed amphibious fish., (© 2018 Wiley Periodicals, Inc.)

Freshwater fish actively take up ions via specific transporters to counter diffusive losses to their hypotonic environment. While much is known about the specific mechanisms employed by teleosts, almost nothing is known about the basal fishes, such as white sturgeon (Acipenser transmontanus) which may offer insight into the evolution of osmo- and ionoregulation in fishes. We investigated Na + uptake in juvenile white sturgeon in the presence and absence of transporter inhibitors. We found that sturgeon acclimated to 100μmoll -1 Na + have Na + uptake kinetics typical of teleosts and that a Na + /H + exchanger (NHE) is the predominant transporter for Na + uptake. White sturgeon are tolerant to hypercarbia-induced respiratory acidoses and recover blood pH (pH e ) at 1.5kPa PCO 2 but not at higher PCO 2 (6kPa PCO 2 ) where they preferentially regulate intracellular pH (pH i ). It was hypothesized that during exposure to hypercarbia Na + uptake would increase at CO 2 tensions at which fish were capable of pH e regulation but decrease at higher tensions when they were preferentially regulating pH i . We found that Na + uptake did not increase at 1.5kPa PCO 2 , but at 6kPa PCO 2 Na + uptake was reduced by 95% while low water pH equivalent to 6kPa PCO 2 reduced Na + uptake by 71%. Lastly, we measured net acid flux during hypercarbia, which indicates that net acid flux is not associated with Na + uptake. These findings indicate Na + uptake in sturgeon is not different from freshwater teleosts but is sensitive to hypercarbia and is not associated with pH e compensation during hypercarbia., (Copyright © 2016 Elsevier Inc. All rights reserved.)

Milkfish, a species within the primitive teleost lineage Otocephala, can survive in water conditions ranging from hypo- to hyper-saline. This study explored the effects of environmental salinity on apical morphologies of ionocytes and the expression of villin homologs in the gills of milkfish acclimated to either seawater (SW) or fresh water (FW). Scanning electron microscopy revealed that the ionocytes in the gill filaments of SW and FW milkfish, respectively, cellular apical morphologies were hole-type and squint-type. The flat-type ionocytes were observed in the gill lamellae of FW milkfish. Furthermore, apical surfaces of some lamellar ionocytes exhibited microvilli. Villin 1 is a microvilli marker expressed in the epithelial cells of various vertebrates. In the phylogenetic tree of villin 1 homologs, primitive teleosts exhibit villin 1-like (VILL) and villin 1 proteins. Two mRNA sequences, villin 1 and VILL, were identified from the milkfish transcriptome by next generation sequencing. Low but constant expression of villin 1 (gene and protein) was observed in the gills for both SW and FW fish. VILL gene and protein expression levels in the gills were higher in FW fish, compared to SW fish. Double immunofluorescence staining demonstrated that VILL protein was present in some lamellar ionocytes of FW milkfish, but not in the filament ionocytes of either FW or SW milkfish. Taken together, these findings indicated that the VILL expression of ionocytes is hypoosmotic-dependent. The VILL might be involved in the formation of microvilli in the lamellar ionocytes for hyperosmoregulation of the milkfish., (Copyright © 2016 Elsevier Inc. All rights reserved.)

The evolution of air breathing during the Devonian provided early fishes with bimodal respiration with a stable O2 supply from air. This was, however, probably associated with challenges and trade-offs in terms of acid-base balance and ionoregulation due to reduced gill:water interaction and changes in gill morphology associated with air breathing. While many aspects of acid-base and ionoregulation in air-breathing fishes are similar to water breathers, the specific cellular and molecular mechanisms involved remain largely unstudied. In general, reduced ionic permeability appears to be an important adaptation in the few bimodal fishes investigated but it is not known if this is a general characteristic. The kidney appears to play an important role in minimizing ion loss to the freshwater environment in the few species investigated, and while ion uptake across the gut is probably important, it has been largely unexplored. In general, air breathing in facultative air-breathing fishes is associated with an acid-base disturbance, resulting in an increased partial pressure of arterial CO2 and a reduction in extracellular pH (pHE ); however, several fishes appear to be capable of tightly regulating tissue intracellular pH (pHI ), despite a large sustained reduction in pHE , a trait termed preferential pHI regulation. Further studies are needed to determine whether preferential pHI regulation is a general trait among bimodal fishes and if this confers reduced sensitivity to acid-base disturbances, including those induced by hypercarbia, exhaustive exercise and hypoxia or anoxia. Additionally, elucidating the cellular and molecular mechanisms may yield insight into whether preferential pHI regulation is a trait ultimately associated with the early evolution of air breathing in vertebrates., (© 2014 The Fisheries Society of the British Isles.)

The objective of this study was to verify net ion fluxes and ammonia excretion in silver catfish transported in plastic bags at three different loading densities: 221, 286 and 365g L-1 for 5h. A water sample was collected at the beginning and at the end of the transport for analysis of water parameters. There was a significant positive relationship between net ion effluxes and negative relationship between ammonia excretion and loading density, demonstrated by the following equations: Na+: y-24.5-0.27x, r²=0.99, Cl-: y=40.2-0.61x, r²=0.98, K+: y=8.0-27.6x, r²=0.94; ammonia excretion: y=-11.43+0.017x, r²=0.95, where y: net ion flux (mmol kg-1 h-1) or ammonia excretion (mg kg-1 h-1) and x: loading density (g). Therefore, the increase of loading density increases net ion loss, but reduces ammonia excretion during the transport of silver catfish, indicating the possibility of ammonia accumulation. [ABSTRACT FROM AUTHOR]

Red drum, Sciaenops ocellatus , is an estuarine-dependent fish species commonly found in the Gulf of Mexico and along the coast of the southeastern United States. This economically important species has demonstrated freshwater tolerance; however, the physiological mechanisms and costs related to freshwater exposure remain poorly understood. The current study therefore investigated the physiological response of red drum using an acute freshwater transfer protocol. Plasma osmolality, Cl − , Mg 2 + and Ca 2 + were all significantly reduced by 24 h post-transfer; Cl − and Mg 2 + recovered to control levels by 7 days post-transfer. No effect of transfer was observed on muscle water content; however, muscle Cl − was significantly reduced. Interestingly, plasma and muscle Na + content was unaffected by freshwater transfer. Intestinal fluid was absent by 24 h post-transfer indicating cessation of drinking. Branchial gene expression analysis showed that both CFTR and NKCC1 exhibited significant down-regulation at 8 and 24 h post-transfer, respectively, although transfer had no impact on NHE2, NHE3 or Na + , K + ATPase (NKA) activity. These general findings are supported by immunohistochemical analysis, which revealed no apparent NKCC containing cells in the gills at 7 days post transfer while NKA cells localization was unaffected. The results of the current study suggest that red drum can effectively regulate Na + balance upon freshwater exposure using already present Na + uptake pathways while also down-regulating ion excretion mechanisms. [ABSTRACT FROM AUTHOR]

Oscars are often subjected to a combination of low levels of oxygen and fasting during nest-guarding on Amazonian floodplains. We questioned whether this anorexia would aggravate the osmo-respiratory compromise. We compared fed and fasted oscars (10-14 days) in both normoxia and hypoxia (10-20 Torr, 4 h). Routine oxygen consumption rates (M02) were increased by 75% in fasted fish, reflecting behavioural differences, whereas fasting improved hypoxia resistance and critical oxygen tensions (Pcrit) lowered from 54 Torr in fed fish to 34 Torr when fasting. In fed fish, hypoxia reduced liver lipid stores by approximately 50% and total liver energy content by 30%. Fasted fish had a 50% lower hepatosomatic index, resulting in lower total liver protein, glycogen and lipid energy stores under normoxia. Compared with hypoxic fed fish, hypoxic fasted fish only showed reduced liver protein levels and even gained glycogen (+50%) on a per gram basis. This confirms the hypothesis that hypoxia-tolerant fish protect their glycogen stores as much as possible as a safeguard for more prolonged hypoxic events. In general, fasted fish showed lower hydroxyacylCoA dehydrogenase activities compared with fed fish, although this effect was only significant in hypoxic fasted fish. Energy stores and activities of enzymes related to energy metabolism in muscle or gills were not affected. Branchial Na+ uptake rates were more than two times lower in fed fish, whereas Na+ efflux was similar. Fed and fasted fish quickly reduced Na+ uptake and efflux during hypoxia, with fasting fish responding more rapidly. Ammonia excretion and K+ efflux were reduced under hypoxia, indicating decreased transcellular permeability. Fasted fish had more mitochondria-rich cells (MRC), with larger crypts, indicating the increased importance of the branchial uptake route when feeding is limited. Gill MRC density and surface area were greatly reduced under hypoxia, possibly to reduce ion uptake and efflux rates. Density of mucous cells of normoxic fasted fish was approximately fourfold of that in fed fish. Overall, a 10-14 day fasting period had no negative effects on hypoxia tolerance in oscars, as fasted fish were able to respond more quickly to lower oxygen levels, and reduced branchial permeability effectively. [ABSTRACT FROM AUTHOR]

Abstract: Recently mercury pollution has been increased considerably in aquatic resources throughout the world and it is a growing global concern. In this study, the 96h LC50 value of waterborne mercuric chloride for Cirrhinus mrigala was found to be 0.34mg/L (with 95% confidence limits). Fingerlings of C. mrigala were exposed to 0.068 and 0.034mg/L of mercuric chloride for 96h to assess the Na+/K+-ATPase activity and ionoregulation (Na+, K+ and Cl−) in gill and brain. Results showed that Na+/K+-ATPase activity and ionic levels (Na+, K+ and Cl−) in gill and brain of fish exposed to different concentrations of mercuric chloride were found to be significantly (p <0.05) decreased throughout the study period. Mercury inactivates many enzymes by attaching to sulfur atoms in which the enzyme Na+/K+-ATPase is highly sensitive to mercury. The inhibition of gill and brain Na+/K+-ATPase activity might have resulted from the physicochemical alteration of the membrane due to mercury toxicity. Moreover, inhibition of Na+/K+-ATPase may affect the ion transport and osmoregulatory function by blocking the transport of substances across the membrane by active transport. The present study indicates that the alterations in these parameters can be used in environmental biomonitoring of mercury contamination in aquatic ecosystem. [Copyright &y& Elsevier]

Abstract: In freshwater teleosts, the primary mechanism of acute silver toxicity is inhibition of Na+/K+ ATPase and carbonic anhydrase at the gill, leading to net Na+ and Cl− loss due to the continued diffusion of these ions into the hypoosmotic external environment. External Cl− has been shown to protect rainbow trout (Oncorhychus mykiss) against silver toxicity presumably by complexation to form AgCl. However, Cl− does not appear to greatly influence silver toxicity to at least two other species, the European eel (Anguilla Anguilla) and the fathead minnow (Pimephales promelas). We hypothesized that differences in protective effects of Cl− at the gill were due to differing requirements or mechanisms for Cl− uptake among fish species. To test this hypothesis, we exposed Fundulus heteroclitus, which does not take up Cl− across the gills, and Danio rerio and P. promelas, which do rely on Cl− uptake across the gills, to Ag+ in waters of varying Cl− concentration. The 96-h LC50s of F. heteroclitus exposed to Ag+ in soft water with 10μM Cl−, 1mM KCl, and 0.5mM MgCl2 were 3.88, 1.20, and 3.20μg/L, respectively, and not significantly different. The 96-h LC50s for D. rerio exposed to Ag+ in soft water with 10μM Cl− and 1mM KCl were 10.3 and 11.3μg/L, respectively and P. promelas exposed under the same conditions were 2.32 and 2.67μg/L, respectively. Based on these results, increasing external Cl− concentration by as much as 1mM (35.5mg/L) did not offer protection against Ag+ toxicity to any fish species tested. Although previous results in our laboratory have demonstrated that P. promelas do take up Cl− at the gill, a mechanism of uptake has not been identified. Additional experiments, investigating the mechanisms of Na+ and Cl− influx at the gill of P. promelas and the influence of silver, demonstrated that Cl− uptake in P. promelas acclimated to soft water occurs through both a Na+:K+:2Cl− co-transporter and a Cl−/HCO3 − exchanger, but is not dependent on carbonic anhydrase. Further, acclimation water chemistry was found to greatly influence subsequent branchial silver accumulation, but Cl− uptake was not sensitive to 10μg/L Ag+. [Copyright &y& Elsevier]

The main objective of this study was to compare the development of seawater adaptation and changes in body composition of smolt produced using three different strategies, and to investigate growth during the first month in seawater. Smolt released at different times of the year experience different environmental conditions. In northern Norway there are large annual variations in seawater temperature and photoperiod, and both are known to influence growth rates of salmonids. Three groups of smolts were raised and released 15 April (Group 1, large 1+ smolt), 15 June (Group 2, medium sized 1+ smolt) and 15 September (Group 3, medium sized 0+ smolt). The 0+ out-of-season smolt (group 3) transferred to seawater in September had the best hypo-osmoregulatory ability of the three groups after one month in seawater. These fish also had the highest growth rate (assessed using TGC), and deposited body fat during their first month in seawater. By contrast large 1+ salmon released in April had poorer hypo-osmoregulatory ability, failed to grow during their first month in the sea, and appeared to mobilise body fat stores to support metabolism during this critical phase. The medium-sized 1+ smolt, transferred to seawater in June, occupied an intermediate position. Although they appeared to experience some problems with hypo-osmotic regulation, the fish grew during their first month in seawater, and they were able to maintain their concentration of body fat. Our findings conclude that the 0+ out-of-season smolt were not disadvantaged during the early phase of seawater rearing compared to the smolt that were produced by a ‘traditional’ strategy and transferred to seawater in spring. Indications were that the 0+ smolt fared better following transfer to seawater than did the fish making up both the large 1+ (group 1) and the medium-sized 1+ (group 2) groups. [Copyright &y& Elsevier]

Two silver-contaminated diets were prepared by exposing juvenile rainbow trout for 8 d to waterborne silver thiosulfate as Ag at either 0.1 µg/L (low-Ag diet) or 80 µg/L (high-Ag diet). The level of total Ag accumulated in whole low-Ag fish was below the detection limit of analysis. Whole high-Ag fish accumulated Ag at 21.3 nmol/g. The livers of the low- and high-Ag fish accumulated Ag at 0.43 nmol/g and 1.01 µmol/g, respectively. The Ag-contaminated fish were then fed whole to adult crayfish in an 80-d dietary study to determine the effects of long-term trophic accumulation of Ag. In a second experiment, the livers of the high-Ag trout were fed to juvenile crayfish for either one or five weeks. Accumulation of Ag was demonstrated in both adult and juvenile crayfish. Silver accumulation in juvenile crayfish peaked at approximately 650 nmol/g at three weeks, after which Ag depuration occurred. In adult crayfish that consumed the high-Ag diet, the hepatopancreas accumulated more than 90% of assimilated Ag, rising 1,000-fold over control animals to approximately 740 nmol/g at 80 d. Crayfish that consumed the low-Ag diet had small, statistically insignificant elevations of Ag in some tissues. Dietary Ag had no effect on juvenile crayfish growth or adult mortality. Disturbances in osmoregulation, which are normally associated with acute waterborne Ag exposure, were not detected. Dietary Ag also had no effect on hemolymph concentrations of Na-, Cl-, Ca2+, Mg2+, or Cu; did not affect the concentration kinetics of Na+ or Cl- influx; and had no effect on the activity of gill Na+/K+-dependent adenosine triphosphatase. Hemolymph concentrations of glucose and lactate were similarly unaffected, indicating an absence of stress-related metabolic disturbance. However, a disproportionately low number of ecdysis events occurred among crayfish that consumed the high-Ag diet. [ABSTRACT FROM AUTHOR]

Freshwater salmonids exposed to low environmental pH typically suffer a net loss of ions, primarily Na+ and Cl-, across the gills, resulting in reduced plasma and tissue ion concentrations. However, in recent experiments in our laboratory, juvenile rainbow trout, Oncorhynchus mykiss, fed a ration of 1% body weight d-1 or greater showed no ionoregulatory disturbance during chronic, sublethal acidification. This raised the possibility that these fish had acclimated to low pH in that they would be better able to withstand further, more severe acidification than fish that had no prior experience of acid conditions: previous studies had concluded that such acclimation does not occur. This hypothesis was tested by measuring unidirectional ion fluxes during a 24 h acute acid challenge (pH 4.2) in juvenile rainbow trout that had previously been exposed to either ambient pH 6.2 (naive fish) or sublethal low pH 5.2 (acid pre-exposed fish) for 90 days, and fed a ration of either 1.0 or 0.25% d-1 (wet basis). No mortalities were observed during the acute acid challenge in the fish fed the higher ration and no differences between the two groups in the response of Na+ fluxes were observed. Sodium influx in both groups was significantly inhibited throughout the challenge and Na+ net flux was significantly stimulated over the first 6 h. Prior to the acute acid challenge, the fish fed the lower ration that had previously been exposed to pH 5.2 had significantly lower plasma ion concentrations than those fish previously exposed to pH 6.2. Both groups suffered mortalities; those of the naive fish (22% by 24 h) being markedly lower than those of the acid pre-exposed fish (68% by 24 h). However, there were no significant differences in either Na+ or Cl- fluxes between the two groups of fish during the acid challenge: both showed significant inhibition of ion influxes and significantly greater net ion losses, resulting in reduced plasma ion concentrations. These results indicate that rainbow trout are unable to acclimate to environmental acidification irrespective of the availability of dietary salts. [ABSTRACT FROM AUTHOR]