Specific dynamic action: the energy cost of digestion or growth?

The physiological processes underlying the post-prandial rise in metabolic rate, most commonly known as the 'specific dynamic action' (SDA), remain debated and controversial. This Commentary examines the SDA response from two opposing hypotheses: (i) the classic interpretation, where the SDA represents the energy cost of digestion, versus (ii) the alternative view that much of the SDA represents the energy cost of growth. The traditional viewpoint implies that individuals with a reduced SDA should grow faster given the same caloric intake, but experimental evidence for this effect remains scarce and inconclusive. Alternatively, we suggest that the SDA reflects an organism's efficacy in allocating the ingested food to growth, emphasising the role of post-absorptive processes, particularly protein synthesis. Although both viewpoints recognise the trade-offs in energy allocation and the dynamic nature of energy distribution among physiological processes, we argue that equating the SDA with 'the energy cost of digestion' oversimplifies the complexities of energy use in relation to the SDA and growth. In many instances, a reduced SDA may reflect diminished nutrient absorption (e.g. due to lower digestive efficiency) rather than increased 'free' energy available for somatic growth. Considering these perspectives, we summarise evidence both for and against the opposing hypotheses with a focus on ectothermic vertebrates. We conclude by presenting a number of future directions for experiments that may clarify what the SDA is, and what it is not.

The biotic ligand model as a promising tool to predict Cu toxicity in amazon blackwaters

The Rio Negro basin of Amazonia (Brazil) is a hotspot of fish biodiversity that is under threat from copper (Cu) pollution. The very ion-poor blackwaters have a high dissolved organic carbon (DOC) concentration. We investigated the Cu sensitivity of nine Amazonian fish species in their natural blackwaters (Rio Negro). The acute lethal concentration of Cu (96 h LC50) was determined at different dilutions of Rio Negro water (RNW) in ion-poor well water (IPW), ranging from 0 to 100%. The IPW was similar to RNW in pH and ionic composition but deficient in DOC, allowing this parameter to vary 20-fold from 0.4 to 8.3 mg/L in tests. The Biotic Ligand Model (BLM; Windward version 3.41.2.45) was used to model Cu speciation and toxicity over the range of tested water compositions, and to estimate lethal Cu accumulations on the gills (LA50). The modeling predicted a high relative abundance of Cu complexes with DOC in test waters. As these complexes became more abundant with increasing RNW content, a concomitant decrease in free Cu2+ was observed. In agreement with this modeling, acute Cu toxicity decreased (i.e. 96 h LC50 values increase) with increasing RNW content. The three most sensitive species (Hemigrammus rhodostomus, Carnegiella strigatta and Hyphessobrycon socolofi) were Characiformes, whereas Corydoras schwartzi (Siluriformes) and Apistogramma agassizii (Cichliformes) were the most tolerant. These sensitivity differences were reflected in the BLM-predicted lethal gill copper accumulation (LA50), which were generally lower in Characiformes than in Cichliformes. Using these newly estimated LA50 values in the BLM allowed for accurate prediction of acute Cu toxicity in the nine Amazonian fish. Our data emphasize that the BLM approach is a promising tool for assessing Cu risk to Amazonian fish species in blackwater conditions characterized by very low concentrations of major ions but high concentrations of DOC.

The first direct measurements of ventilatory flow and oxygen utilization after exhaustive exercise and voluntary feeding in a teleost fish, Oncorhynchus mykiss

A new "less invasive" device incorporating an ultrasonic flow probe and a divided chamber, but no stitching of membranes to the fish, was employed to make the first direct measurements of ventilatory flow rate (V̇w) and % O2 utilization (%U) in juvenile rainbow trout (37 g, 8ºC) after exhaustive exercise (10-min chasing) and voluntary feeding (2.72% body mass ration). Under resting conditions, the allometrically scaled V̇w (300 ml kg-1 min-1 for a 37-g trout = 147 ml kg-1 min-1 for a 236-g trout exhibiting the same mass-specific O2 consumption rate, ṀO2) and the convection requirement for O2 (CR = 4.13 L mmol-1) were considerably lower, and the %U (67%) was considerably higher than in previous studies using surgically attached masks or the Fick principle. After exhaustive exercise, V̇w and ṀO2 approximately doubled whereas frequency (fr) and %U barely changed, so increased ventilatory stroke volume (Vsv) was the most important contributor to increased ṀO2. CR declined slightly. Values gradually returned to control conditions after 2-3 h. After voluntary feeding, short-term increases in V̇w, Vsv and ṀO2 were comparable to those after exercise, and fr again did not change. However, %U increased so CR declined even more. The initial peaks in V̇w, Vsv and ṀO2, similar to those after exercise, were likely influenced by the excitement and exercise component of voluntary feeding. However, in contrast to post-exercise fish, post-prandial fish exhibited second peaks in these same parameters at 1-3 h after feeding, and %U increased further, surpassing 85%, reflecting the true "specific dynamic action" response. We conclude that respiration in trout is much more efficient than previously believed.

The physiological consequences of a very large natural meal in a voracious marine fish, the staghorn sculpin (Leptocottus armatus)

Little information exists on physiological consequences when wild fish eat natural food. Staghorn sculpins at 10-13°C voluntarily consumed 15.8% of their body mass in anchovies. Gastric clearance was slow with >60% of the meal retained in the stomach at 48 h, and was not complete until 84 h. At 14-24 h post-feeding, pH was depressed by 3 units and Cl- concentration was elevated 2-fold in gastric chyme, reflecting HCl secretion, while in all sections of the intestine, pH declined by 1 pH unit but Cl- concentration remained unchanged. PCO2 and total ammonia concentration were greatly elevated throughout the tract, whereas PNH3 and HCO3- concentration were depressed. Intestinal HCO3- secretion rates, measured in gut sacs in vitro, were also lower in fed fish. Whole-animal O2 consumption rate was elevated approximately 2-fold for 72 h post-feeding, reflecting 'specific dynamic action', whereas ammonia and urea-N excretion rates were elevated about 5-fold. Arterial blood exhibited a modest 'alkaline tide' for about 48 h, but there was negligible excretion of metabolic base to the external seawater. PaCO2 and PaO2 remained unchanged. Plasma total amino acid concentration and total lipid concentration were elevated about 1.5-fold for at least 48 h, whereas small increases in plasma total ammonia concentration, PNH3 and urea-N concentration were quickly attenuated. Plasma glucose concentration remained unchanged. We conclude that despite the very large meal, slow processing with high efficiency minimizes internal physiological disturbances. This differs greatly from the picture provided by previous studies on aquacultured species using synthetic diets and/or force-feeding. Questions remain about the role of the gastro-intestinal microbiome in nitrogen and acid-base metabolism.

Osmo-respiratory compromise in the mosshead sculpin (Clinocottus globiceps): effects of temperature, hypoxia, and re-oxygenation on rates of diffusive water flux and oxygen uptake

In nature, mosshead sculpins (Clinocottus globiceps) are challenged by fluctuations in temperature and oxygen levels in their environment. However, it is unclear how mosshead sculpins modulate the permeability of their branchial epithelia to water and O2 in response to temperature or hypoxia stress. Acute decrease in temperature from 13 to 6 oC reduced diffusive water flux rate by 22% and ṀO2 by 51%, whereas acute increase in temperature from 13 to 25 oC increased diffusive water flux rate by 217% and ṀO2 by 140%, yielding overall Q10 values of 2.08 and 2.47 respectively. Acute reductions in oxygen tension from >95% to 20% or 10% air saturation did not impact diffusive water flux rates, however, ṀO2 was reduced significantly by 36% and 65% respectively. During 1-h or 3-h recovery periods diffusive water flux rates were depressed while ṀO2 exhibited overshoots beyond the normoxic control level. Many responses differed from those seen in our parallel earlier study on the tidepool sculpin, a cottid with similar hypoxia tolerance but much smaller gill area that occupies a similar environment. Overall, our data suggest that during temperature stress, diffusive water flux rates and ṀO2 follow the traditional osmo-respiratory compromise pattern, but during hypoxia and re-oxygenation stress, diffusive water flux rates are decoupled from ṀO2.

Role of the kidneys in acid-base regulation and ammonia excretion in freshwater and seawater fish: implications for nephrocalcinosis

Maintaining normal pH levels in the body fluids is essential for homeostasis and represents one of the most tightly regulated physiological processes among vertebrates. Fish are generally ammoniotelic and inhabit diverse aquatic environments that present many respiratory, acidifying, alkalinizing, ionic and osmotic stressors to which they are able to adapt. They have evolved flexible strategies for the regulation of acid-base equivalents (H⁺, NH₄ ⁺, OH^- and HCO₃ ^-), ammonia and phosphate to cope with these stressors. The gills are the main regulatory organ, while the kidneys play an important, often overlooked accessory role in acid-base regulation. Here we outline the kidneys role in regulation of acid-base equivalents and two of the key 'urinary buffers', ammonia and phosphate, by integrating known aspects of renal physiology with recent advances in the molecular and cellular physiology of membrane transport systems in the teleost kidneys. The renal transporters (NHE3, NBC1, AE1, SLC26A6) and enzymes (V-type H⁺ATPase, CAc, CA IV, ammoniagenic enzymes) involved in H⁺ secretion, bicarbonate reabsorption, and the net excretion of acidic and basic equivalents, ammonia, and inorganic phosphate are addressed. The role of sodium-phosphate cotransporter (Slc34a2b) and rhesus (Rh) glycoproteins (ammonia channels) in conjunction with apical V-type H⁺ ATPase and NHE3 exchangers in these processes are also explored. Nephrocalcinosis is an inflammation-like disorder due to the precipitation of calcareous material in the kidneys, and is listed as one of the most prevalent pathologies in land-based production of salmonids in recirculating aquaculture systems. The causative links underlying the pathogenesis and etiology of nephrocalcinosis in teleosts is speculative at best, but acid-base perturbation is probably a central pathophysiological cause. Relevant risk factors associated with nephrocalcinosis are hypercapnia and hyperoxia in the culture water. These raise internal CO₂ levels in the fish, triggering complex branchial and renal acid-base compensations which may promote formation of kidney stones. However, increased salt loads through the rearing water and the feed may increase the prevalence of nephrocalcinosis. An increased understanding of the kidneys role in acid-base and ion regulation and how this relates to renal diseases such as nephrocalcinosis will have applied relevance for the biologist and aquaculturist alike.

Emersion and recovery alter oxygen consumption, ammonia and urea excretion, and oxidative stress parameters, but not diffusive water exchange or transepithelial potential in the green crab (Carcinus maenas)

The green crab (C. maenas) is an "inshore" species affected by intertidal zonation patterns, facing periods of emersion during low tide, and submersion during high tide. During these periods of air and subsequent water exposure, these species can face physiological challenges. We examined changes in O2 consumption rate (MO2), ammonia (JAmmnet) and urea (JUreanet) excretion rates over sequential 14 h periods in seawater (32 ppt, control), in air, and during recovery in seawater after air exposure (13°C throughout). At the end of each exposure, the anterior (5th) and posterior (8th) gills, and the hepatopancreas were removed for measurements of oxidative stress parameters (TBARs and catalase in the gills and hepatopancreas, and protein carbonyls in the gills). MO2 remained unchanged during air exposure, but increased greatly (3.4-fold above control levels) during the recovery period. Ammonia and urea net fluxes (JAmmnet and JUreanet) were reduced by 98% during air exposure, but rebounded during recovery to >2-fold the control rates. Exchangeable water pools (VH2O), rate constants of diffusive water exchange (k), and unidirectional diffusive water flux rates (JH2O) (using tritiated water) and transepithelial potential (TEP) were also measured during control and recovery treatments, but exhibited no significant changes. Damage to proteins was not observed in either gill. However, lipid damage occurred in the anterior (respiratory) gill after the air exposure but not in the posterior (ionoregulatory) gill or hepatopancreas. Catalase activity also decreased significantly in recovery relative to levels during air exposure in both the anterior gill and hepatopancreas, but not in the posterior gill. The crabs did not modify water metabolism or permeability. We conclude that MO2 can be maintained during air exposure, it does not meet increased metabolic demand, and ammonia and urea-N excretion are impaired. As a result, all these parameters increase greatly during re-immersion recovery, and oxidative stress also occurs. Clearly, emersion is not without physiological costs.

Exercise and emersion in air and recovery in seawater in the green crab (Carcinus maenas): Effects on nitrogenous wastes and branchial chamber fluid chemistry

At low tide, the green crab, which is capable of breathing air, may leave the water and walk on the foreshore, carrying branchial chamber fluid (BCF). N-waste metabolism was examined in crabs at rest in seawater (32 ppt, 13°C), and during 18-h recovery in seawater after 1 h of exhaustive exercise (0.25 BL s^-1 ) on a treadmill in air (20°C-23°C), or 1 h of quiet emersion in air. Measurements were made in parallel to O₂ consumption (ṀO₂ ), acid-base, cardio-respiratory, and ion data reported previously. At rest, the ammonia-N excretion rate (Ṁ_Amm  = 44 µmol-N kg^-1  h^-1 ) and ammonia quotient (AQ; Ṁ_Amm /ṀO₂  = 0.088) were low for a carnivore. Immediately after exercise and return to seawater, Ṁ_Amm increased by 65-fold above control rates. After emersion alone and return to seawater, Ṁ_Amm increased by 17-fold. These ammonia-N bursts were greater, but transient relative to longer-lasting elevations in ṀO₂ , resulting in temporal disturbances of AQ. Intermittent excretion of urea-N and urate-N at rest and during recovery indicated the metabolic importance of these N-wastes. Hemolymph glutamate, glutamine, and PNH₃ did not change. Hemolymph ammonia-N, urea-N, and urate-N concentrations increased after exercise and more moderately after emersion, with urate-N exhibiting the largest absolute increments, and urea-N the longest-lasting elevations. All three N-wastes were present in the BCF, with ammonia-N and PNH₃ far above hemolymph levels even at rest. BCF volume declined by 34% postemersion and 77% postexercise, with little change in osmolality but large increases in ammonia-N concentrations. Neither rapid flushing of stored BCF nor clearance of hemolymph ammonia-N could explain the surges in Ṁ_Amm after return to seawater.

Ammonia transport is independent of PNH3 gradients across the gastrointestinal epithelia of the rainbow trout: A role for the stomach

Although the gastrointestinal tract (GIT) is an important site for nitrogen metabolism in teleosts, the mechanisms of ammonia absorption and transport remain to be elucidated. Both protein catabolism in the lumen and the metabolism of the GIT tissues produce ammonia which, in part, enters the portal blood through the anterior region of the GIT. The present study examined the possible roles of different GIT sections of rainbow trout (Oncorhynchus mykiss) in transporting ammonia in its unionized gas form-NH₃ -by changing the PNH₃ gradient across GIT epithelia using in vitro gut sac preparations. We also surveyed messenger RNA expression patterns of three of the identified Rh proteins (Rhbg, Rhcg1, and  Rhcg2) as potential NH₃ transporters and NKCC as a potential ammonium ion (NH₄ ⁺ ) transporter along the GIT of rainbow trout. We found that ammonia absorption is not dependent on the PNH₃ gradient despite expression of Rhbg and Rhcg2 in the intestinal tissues, and Rhcg2 in the stomach. We detected no expression of Rhbg in the stomach and no expression of Rhcg1 in any GIT tissues. There was also a lack of correlation between ammonia transport and [NH₄ ⁺ ] gradient despite NKCC expression in all GIT tissues. Regardless of PNH₃ gradients, the stomach showed the greatest absorption and net tissue consumption of ammonia. Overall, our findings suggest nitrogen metabolism zonation of GIT, with stomach serving as an important site for the absorption, handling and transport of ammonia that is independent of the PNH₃ gradient.

Do extreme postprandial levels of oxygen, carbon dioxide, and ammonia in the digestive tract equilibrate with the bloodstream in the freshwater rainbow trout (Oncorhynchus mykiss)?

The gastrointestinal tract (GIT) lumen of teleosts harbors extreme conditions, especially after feeding: high PCO₂ (20-115 Torr), total ammonia (415-3710 μM), PNH₃ (79-1760 μTorr in the intestine), and virtual anoxia (PO₂ < 1 Torr). These levels could be dangerous if they were to equilibrate with the bloodstream. Thus, we investigated the potential equilibration of O₂, CO₂, and ammonia across the GIT epithelia in freshwater rainbow trout by monitoring postprandial arterial and venous blood gases in vivo and in situ. In vivo blood was sampled from the indwelling catheters in the dorsal aorta (DA) and subintestinal vein (SIV) draining the posterior intestine in the fasting state and at 4 to 48 h following catheter-feeding. To investigate possible ammonia absorption in the anterior part of the GIT, blood was sampled from the DA, SIV and hepatic portal vein (HPV) from anaesthetized fish in situ following voluntary feeding. We found minimal equilibration of all three gases between the GIT lumen and the SIV blood, with the latter maintaining pre-feeding levels (PO₂ = 25-49 Torr, PCO₂ = 6-8 Torr, and total ammonia = 117-134 μM and PNH₃ = 13-30 μTorr at 48 h post-feeding). In contrast to the SIV, we found that the HPV total ammonia more than doubled 24 h after feeding (128 to 297 μM), indicative of absorption in the anterior GIT. Overall, the GIT epithelia of trout, although specialized for absorption, prevent dangerous levels of PO₂, PCO₂ and ammonia from equilibrating with the blood circulation.

The roles of calcium and salinity in protecting against physiological symptoms of waterborne zinc toxicity in the euryhaline killifish (Fundulus heteroclitus)

In fresh water, environmental Ca ameliorates Zn toxicity because Ca²⁺ and Zn²⁺ compete for uptake at the gills. Zn toxicity is also lower in sea water, but it is unclear whether this is due to increased Ca²⁺ concentration, and/or to the other ions present at higher salinity. Using the euryhaline killifish, we evaluated the relative roles of Ca²⁺ (as CaNO₃) versus the other ions contributing to salinity in protecting against physiological symptoms of Zn²⁺ toxicity. Killifish were exposed to a sublethal level of Zn (500 μg/L, as ZnSO₄) for 96 h in either fresh water (0 % salinity) at low (1 mmol/L) and High Ca (10 mmol/L) or 35 ppt sea water (100 % salinity) at low (1 mmol/L) and High Ca (10 mmol/L). At 0 % salinity, High Ca partly or completely protected against the following effects of Zn seen at Low Ca: elevated plasma Zn, hypocalcaemia, inhibited unidirectional Ca²⁺ influx, inhibited branchial Na⁺/K⁺ATPase and Ca²⁺ATPase activities, and oxidative stress in gills, liver, intestine, and muscle. At 100 % salinity, in the presence of 1 mmol/L (Low Ca), Zn caused no disturbances in most of these same parameters, showing that the "non-Ca" components of sea water alone provided complete protection. However, for a few endpoints (inhibited intestinal Ca²⁺ATPase activity, oxidative stress in gill and liver), High Ca (10 mmol/L) was needed to provide full protection against Zn in 100 % salinity. There was no instance where the combination of 100 % salinity and High Ca failed to provide complete protection against Zn-induced disturbances in sea water.

Investigating the mechanisms of dissolved organic matter protection against copper toxicity in fish of Amazon's black waters

We investigated how natural dissolved organic matter (DOM) of the Rio Negro (Amazon) affects acute copper (Cu) toxicity to local fish: the cardinal tetra (Paracheirodon axelrodi) and the dwarf cichlid (Apistogramma agassizii). It is established that Cu²⁺ complexation with DOM decreases Cu bioavailability (and thus toxicity) to aquatic organisms, as conceptualized by the Biotic Ligand Model (BLM). However, we also know that Rio Negro's DOM can interact with fish gills and have a beneficial effect on Na⁺ homeostasis, the main target of acute Cu toxicity in freshwater animals. We aimed to tease apart these potential protective effects of DOM against Cu-induced Na⁺ imbalances in fish. In the laboratory, we acclimated fish to Rio Negro water (10 mg L^-1 DOC) and to a low-DOM water (1.4 mg L^-1 DOC) with similar ion composition and pH (5.9). We measured 3-h Cu uptake in gills and unidirectional and net Na⁺ physiological fluxes across a range of Cu concentrations in both waters. Various DOM pre-acclimation times (0, 1 and 5 days) were evaluated in experiments with P. axelrodi. Copper exposure led to similar levels of net Na⁺ loss in the two fish, but with distinct effects on Na⁺ influx and efflux rates reflecting their different ionoregulation strategies. Rio Negro DOM protected against Cu uptake and toxicity in the two fish species. Both Cu uptake in fish gills and Na⁺ regulation disturbances were relatively well predicted by the modelled aqueous free Cu²⁺ ion concentration. These findings suggest that protection by DOM occurs mainly from Cu complexation under the tested conditions. The prevalence of this geochemical-type protection over a physiological-type protection agrees with the BLM conceptual framework, supporting the use of the BLM to assess the risk of Cu in these Amazonian waters.

How the green crab Carcinus maenas copes physiologically with a range of salinities

To evaluate the physiological ability to adjust to environmental variations of salinity, Carcinus maenas were maintained in 10, 20, 32 (control), 40, and 50 ppt (13.8 ± 0.6 °C) for 7 days. Closed respirometry systems were used to evaluate oxygen consumption ([Formula: see text]), ammonia excretion (Jamm), urea-N excretion (Jurea-N) and diffusive water fluxes (with ³H₂O). Ions, osmolality, metabolites, and acid-base status were determined in the hemolymph and seawater, and transepithelial potential (TEP) was measured. At the lowest salinity, there were marked increases in [Formula: see text] and Jamm, greater reliance on N-containing fuels to support aerobic metabolism, and a state of internal metabolic alkalosis (increased [HCO₃^-]) despite lower seawater pH. At higher salinities, an activation of anaerobic metabolism and a state of metabolic acidosis (decreased [HCO₃^-] and increased [lactate]), in combination with respiratory compensation (decreased PCO₂), were detected. TEP became more negative with decreasing salinity. Osmoregulation and osmoconformation occurred at low and high salinities, respectively, with complex patterns in individual ions; hemolymph [Mg²⁺] was particularly well regulated at levels well below the external seawater at all salinities. Diffusive water flux rates increased at higher salinities. Our results show that C. maenas exhibits wide plasticity of physiological responses when acclimated to different salinities and tolerates substantial disturbances of physiological parameters, illustrating that this species is well adapted to invade and survive in diverse habitats.

Exercise and emersion in air, and recovery in seawater in the green crab (Carcinus maenas): metabolic, acid-base, cardio-ventilatory, and ionoregulatory responses

In nature, the green crab exhibits emersion and terrestrial activity at low tide. Treadmill exercise in air (20-23°C) of crabs acclimated to 32ppt seawater (13°C) revealed an inverse relationship between velocity and duration: 2.0 BL sec−1sustainable for several minutes, and 0.25 BL sec−1 for long periods. Fatigue was not due to dehydration. Physiological responses over 18-h recovery in seawater after near-exhaustive exercise (0.25 BL sec−1, 1h) in air were compared with responses after quiet emersion (1h) in air. Exercising crabs exhibited transient scaphognathite slowing and progressive increases in heart rate, whereas emersed crabs exhibited persistent inhibition of ventilation and transient heart slowing. Upon return to seawater, all these rates increased above both control and treatment levels. Post-exercise disturbances were more marked and/or longer lasting (e.g. EPOC, hyperventilation, tachycardia, metabolic acidosis, lactate elevation, ionic disturbances) than those after simple air exposure. However, an increase in net acidic equivalent excretion to the environment occurred after emersion but not after exercise. Instead, post-exercise crabs relied on carapace buffering, signaled by elevated haemolymph Ca2+ and Mg2+. Prolonged lowering of haemolymph PCO2 associated with hyperventilation also played a key role in acid-base recovery. EPOC after exercise was 3-fold greater than after emersion, sufficient to support control M˙O2for&gt;14h. This reflected clearance of a large lactate load, likely by glycogen re-synthesis rather than oxidation. We conclude that the amphibious green crab uses a combination of aquatic and terrestrial strategies to support exercise in air, emersion in air, and recovery in seawater.

Global change and physiological challenges for fish of the Amazon today and in the near future

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.

Transepithelial potential remains indicative of major ion toxicity in rainbow trout (Oncorhynchus mykiss) after 4-day pre-exposure to major salts

The Multi-Ion Toxicity (MIT) Model uses electrochemical theory to predict the transepithelial potential (TEP) across the gills as an index of major ion toxicity in freshwater animals. The goal is to determine environmental criteria that will be protective of aquatic organisms exposed to salt pollution. In recent studies, TEP disturbances above baseline (ΔTEP) during short-term exposures to major ions have been proven as indicative of their toxicity to fish, in accord with the MIT model. However, the acute 1-h exposures used in these previous studies might not be realistic relative to the 24 h or 96 h test periods used for toxicity assessment. To address this temporal inconsistency, the current study investigated both the TEP responses to serial concentrations of 10 major salts (NaCl, Na₂SO₄, NaHCO₃, KCl, K₂SO₄, KHCO₃, CaCl₂, CaSO₄, MgCl₂, MgSO₄) and plasma ion levels in juvenile rainbow trout after they had been pre-exposed to 50% of the 96h-LC50 levels of these same salts for 4 days. The pre-exposures caused no mortalities. In general, plasma ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^-) were well-regulated; however, pre-exposure to sulfate salts resulted in the greatest number of alterations in plasma ion levels. TEP responses remained largely similar to those of naïve trout (without salt pre-exposure). All salts caused hyperbolic concentration-dependent increases in TEP that were well-described by the Michaelis-Menten equation. In the pre-exposed trout, the variation of ∆TEP at the 96h-LC50 concentrations was only 2.2-fold, compared to nearly 28-fold variation among the molar concentrations of the various salts at the 96h-LC50s, identical to the conclusion for naïve trout. Overall, the results remove the temporal inconsistency of previous tests and remain supportive of the MIT model. In addition, the recorded alterations in certain plasma ions, baseline TEP, and Michaelis-Menten constants improve our knowledge on specific physiological responses after extended major ion exposure.

Breathing versus feeding in the Pacific hagfish

Hagfish represent the oldest extant connection to the ancestral vertebrates, but their physiology is not well understood. Using behavioural (video), physiological (respirometry, flow measurements), classical morphological (dissection, silicone injection) and modern imaging approaches (micro-MRI, DICE micro-CT) we examined the interface between feeding and the unique breathing mechanism (nostril, high frequency velum, low frequency gill pouches (24) and pharyngo-cutaneous duct,PCD) in the Pacific hagfish, Eptatretus stoutii. A video tour via micro-MRI is presented through the breathing and feeding passages. We have reconciled earlier disagreement as to the position of the velum chamber, which powers inhalation through the nostril, placing it downstream of the merging point of food and water passages, such that the oronasal septum terminates at the anterior end of the velum chamber. When feeding occurs by engulfment of large chunks by the dental plates, food movement through the chamber may transiently interfere with breathing. Swallowing is accelerated by peristaltic body undulation involving the ventral musculature, and is complete within 5 sec. After a large meal (anchovy, 20% body mass), hagfish remain motionless, defecating bones and scales at 1.7 days and an intestinal peritrophic membrane at 5 days. O2 consumption rate approximately doubles within 1 h after feeding, remaining elevated through 12-24 h. This is achieved by combinations of elevated O2 utilization and ventilatory flow, the latter caused by varying increases in velar frequency and stroke volume. Additional imaging casts light on the reasons for the trend for greater O2 utilization by more posterior pouches and PCD in fasted hagfish.

A novel K+ -dependent Na+ uptake mechanism during low pH exposure in adult zebrafish (Danio rerio): New tricks for old dogma

AIM

To determine whether Na⁺ uptake in adult zebrafish (Danio rerio) exposed to acidic water adheres to traditional models reliant on Na⁺ /H⁺ Exchangers (NHEs), Na⁺ channels and Na⁺ /Cl^- Cotransporters (NCCs) or if it occurs through a novel mechanism.

METHODS

Zebrafish were exposed to control (pH 8.0) or acidic (pH 4.0) water for 0-12 hours during which ²² Na⁺ uptake ( J Na in ), ammonia excretion, net acidic equivalent flux and net K⁺ flux ( J H net ) were measured. The involvement of NHEs, Na⁺ channels, NCCs, K⁺ -channels and K⁺ -dependent Na⁺ /Ca²⁺ exchangers (NCKXs) was evaluated by exposure to Cl^- -free or elevated [K⁺ ] water, or to pharmacological inhibitors. The presence of NCKXs in gill was examined using RT-PCR.

RESULTS

J Na in was strongly attenuated by acid exposure, but gradually recovered to control rates. The systematic elimination of each of the traditional models led us to consider K⁺ as a counter substrate for Na⁺ uptake during acid exposure. Indeed, elevated environmental [K⁺ ] inhibited J Na in during acid exposure in a concentration-dependent manner, with near-complete inhibition at 10 mM. Moreover, J H net loss increased approximately fourfold at 8-10 hours of acid exposure which correlated with recovered J Na in in 1:1 fashion, and both J Na in and J H net were sensitive to tetraethylammonium (TEA) during acid exposure. Zebrafish gills expressed mRNA coding for six NCKX isoforms.

CONCLUSIONS

During acid exposure, zebrafish engage a novel Na⁺ uptake mechanism that utilizes the outwardly directed K⁺ gradient as a counter-substrate for Na⁺ and is sensitive to TEA. NKCXs are promising candidates to mediate this K⁺ -dependent Na⁺ uptake, opening new research avenues about Na⁺ uptake in zebrafish and other acid-tolerant aquatic species.

Arapaima gigas maintains gas exchange separation in severe aquatic hypoxia but does not suffer branchial oxygen loss

One of the most air-reliant obligate air-breathing fish is the South American Arapaima gigas, with substantially reduced gills impeding gas diffusion, thought to be a result of recurring aquatic hypoxia in its habitat. In normoxic water, A. gigas is reported to satisfy 70-80% of its O2 requirement from the air while excreting 60-90% of its CO2 to the water. If this pattern of gas exchange were to continue in severely hypoxic water, O2 loss at the gills would be expected. We hypothesized therefore that partitioning of CO2 would shift to the air phase in severe aquatic hypoxia eliminating the risk of branchial O2 loss. By adapting a respirometer designed to measure aquatic MO2/MCO2 we were able to run intermittent closed respirometry on both water and air phase for both of these gasses as well as sample water for N-waste measurements (ammonia-N, urea-N) so as to calculate metabolic fuel utilization. In contrast to our prediction, we found that partitioning of CO2 excretion changed little between normoxia and severe hypoxia (83% vs 77% aquatic excretion respectively) and at the same time there was no evidence of branchial O2 loss in hypoxia. This indicates that A. gigas can utilize distinct transfer pathways for O2 and CO2. Routine and standard MO2, N-waste excretion, and metabolic fuel utilization did not change with water oxygenation. Metabolism was fueled mostly by protein oxidation (53%) while carbohydrates and lipids accounted for 27% and 20% respectively.

The effect of marine dissolved organic carbon on nickel accumulation in early life-stages of the sea urchin, Strongylocentrotus purpuratus

Dissolved organic carbon (DOC) is known to ameliorate the toxicity of the trace metal nickel (Ni) to aquatic animals. In theory, this effect is mediated by the capacity of DOC to bind Ni, rendering it less bioavailable, with the resulting reduction in accumulation limiting toxicological effects. However, there is a lack of experimental data examining Ni accumulation in marine settings with natural sources of DOC. In the current study, radiolabelled Ni was used to examine the time- and concentration-dependence of Ni accumulation, using naturally sourced DOC, on developing larvae of the sea urchin Strongylocentrotus purpuratus. Contrary to prediction, the two tested natural DOC samples (collected from the eastern United States, DOC 2 (Seaview park, Rhode Island (SVP)) and DOC 7 (Aubudon Coastal Center, Connecticut)) which had previously been shown to protect against Ni toxicity, did not limit accumulation. The control (artificial seawater with no added DOC), and the DOC 2 sample could mostly be described as having saturable Ni uptake, whereas Ni uptake in the presence of DOC 7 was mostly linear. These data provide evidence that DOC modifies the bioavailability of Ni, through either indirect effects (e.g. membrane permeability) or by the absorption of DOC-Ni complexes. There was some evidence for regulation of Ni accumulation in later-stage embryos (96-h) where the bioconcentration factor for Ni declined with increasing Ni exposure concentration. These data have implications for predictive modelling approaches that rely on known relationships between Ni speciation, bioavailability and bioreactivity, by suggesting that these relationships may not hold for natural marine DOC samples in the developing sea urchin model system.

Post-prandial respiratory gas and acid-base profiles in the gastrointestinal tract and its venous drainage in freshwater rainbow trout (Oncorhynchus mykiss) and seawater English sole (Parophrys vetulus)

The basic respiratory gas and acid-base conditions inside the lumen of the gastrointestinal tract (GIT) and blood draining the tract are largely unestablished in teleost fishes after feeding, though there have been some recent novel discoveries on freshwater rainbow trout (Oncorhynchus mykiss) and seawater English sole (Parophrys vetulus). The present study examined in greater detail the gas (PO₂, PCO₂, PNH₃) and acid-base profiles (pH, [HCO₃^-], total [ammonia]) in the lumen of the stomach, the anterior, mid, and posterior intestine, as well as the venous drainage (subintestinal and/or hepatic portal vein) of the GIT in these two species 20 h post-feeding. Both species had high PCO₂, PNH₃, and total [ammonia], and low PO₂ (virtual anoxia) in the lumens throughout all sections of the GIT, and high [HCO₃^-] in the intestine. Total [ammonia], PNH₃, and [HCO₃^-] increased from anterior to posterior intestine in both species. P. vetulus had higher intestinal total [ammonia] and lower [HCO₃^-] than O. mykiss post feeding, but total [ammonia] was much higher in the stomach of O. mykiss. Despite the extreme conditions in the lumen, both arterial and venous blood showed relatively lower PCO₂, total [ammonia] and higher PO₂, implying limited equilibration between the two compartments. The higher [HCO₃^-] and lower total [ammonia] in the intestinal lumen of the freshwater O. mykiss than the seawater P. vetulus suggest the need for future comparative studies using conspecifics fed identical diets but acclimated to the two different salinities in order to understand the potential role of environmental salinity and associated osmoregulatory processes underlying these differences.

Physicochemical properties of the dissolved organic carbon can lead to different physiological responses of zebrafish (Danio rerio) under neutral and acidic conditions

Previous studies have suggested that the capacity of natural dissolved organic carbon (DOC) molecules to interact with biological membranes is associated with their aromaticity (SAC₃₄₀ ); origin (allochthonous versus autochthonous, FI); molecular weight (Abs_254/365 ); and relative fluorescence of DOC moieties (PARAFAC analysis). These interactions may be especially important when fish are challenged by acidic waters, which are known to inhibit the active uptake of Na⁺ and Cl^- , while stimulating diffusive ion losses in freshwater fishes. Therefore, zebrafish were acclimated (7 days, pH 7.0) to five natural DOC sources (10 mg C/L), two from the Amazon Basin and three from Canada, together with a "no-added DOC" control. After the acclimation, fish were challenged by exposure to acidic water (pH 4.0) for 3 h. Osmoregulatory parameters were measured at pH 7.0 and 4.0. Acclimation to the five DOC sources did not disturb Na⁺ , Cl^- and ammonia net fluxes, but resulted in differential elevations in Na⁺ , K⁺ ATPase and v-type H⁺ ATPase activities in fish at pH 7.0. However, after transfer to pH.4.0, the control fish exhibited rapid increases in both enzymes. In contrast the DOC- acclimated animals exhibited unchanged (Na⁺ , K⁺ ATPase) or differentially increased (v-type H⁺ ATPase) activities. Na⁺ , Cl^- and ammonia net fluxes remained unchanged in the control fish, but were differentially elevated in most of the DOC treatments at pH 4.0, relative to the same DOC treatments at pH 7.0. Correlations between the osmoregulatory data the DOCs properties highlight that the DOC properties drive different effects on gill physiology.

Interplay of oxygen and light in the photo-oxidation of dissolved organic carbon

Light energy and oxygen drive photo-oxidation of dissolved organic carbon (DOC). The longer the wavelength the greater its depth of penetration into water, changing the spectral environment with depth. We asked how oxygen concentration and light spectral composition might affect photo-oxidation processes in DOC. Outdoor experiments compared responses of fluorescence and absorbance indices to photo-oxidation of filtered (0.45 µm) Rio Negro water (Amazon Basin) under near-anoxia, normoxia and hyperoxia exposed to natural sunlight or reduced sunlight (≥340, reduced-UVR). Near-anoxia decreased all absorbance and fluorescence indices. Absorbance changed across the spectrum (≥250 nm) even under reduced-UVR provided that oxygen was present. This phenomenon maintains broader photo-oxidation and the release of CO₂ at depth. Slope_350-400 was responsive to changes in the irradiance field but not to oxygen concentration, while Slope_275-295 responded to both. Thus, larger molecules are broken down near the water's surface and medium to smaller molecules continue to be processed at depth. The production of fulvic acid-like fluorescence required both UVB and oxygen, restricting its production to surface waters. The relatively small increase in R_254/365 compared with the loss of SUVA₂₅₄ under near-anoxia indicated a slower breakdown of larger DOC molecules as oxygen becomes limiting. Breakdown of larger molecules which absorb in the 350-400 nm range, appears to involve two steps - one by radiant energy and another involving oxygen. The study results reflect the dynamic gradients in photo-oxidation with depth.

The effects of dissolved organic carbon on the reflex ventilatory responses of the neotropical teleost (Colossoma macropomum) to hypoxia or hypercapnia

The tambaqui (Colossoma macropomum), migrates annually between whitewater and blackwater rivers of the Amazon. Unlike the whitewater, blackwater is characterized by higher levels of dissolved organic carbon (DOC), including humic acids (HA). Because humic substances impair sensory processes, the current study tested the hypothesis that O₂ and/or CO₂ chemoreception is impeded in blackwater owing to the presence of HA. Thus, the ventilatory responses of tambaqui to hypoxia or hypercapnia were assessed in well water transported from Manaus, local blackwater, and in well water containing HA either extracted from Rio Negro water or obtained commercially (Sigma Aldrich; SA). In well water, tambaqui exhibited typical hyperventilatory responses to hypoxia or hypercapnia. These responses were prevented by simultaneously exposing fish to SA HA (20 mg l^-1). The negative effects of SA HA on ventilation were prevented when natural DOC (30 mg l^-1; extracted from Rio Negro water after first removing the endogenous HA fraction) was added concurrently, indicating a protective effect of this non-humic acid DOC fraction. The hyperventilatory responses were unaffected during acute exposure or after acclimation of fish to Rio Negro water. HA extracted from Rio Negro water did not impair the hyperventilatory responses to hypoxia or hypercapnia. This study, while demonstrating a negative effect of SA HA derived from peat (coal) on the control of breathing in tambaqui, failed to reveal any detrimental consequences of HA (derived from the decomposition of a variety of lignin-rich plants) naturally occurring in the blackwaters of the Rio Negro.

The physiology of fish in acidic waters rich in dissolved organic carbon, with specific reference to the Amazon basin: Ionoregulation, acid-base regulation, ammonia excretion, and metal toxicity

Although blackwaters, named for their rich content of dissolved organic carbon (DOC), are often very poor in ions and very acidic, they support great fish biodiversity. Indeed, about 8% of all freshwater fish species live in the blackwaters of the Rio Negro watershed in the Amazon basin. We review how native fish survive these harsh conditions that would kill most freshwater fish, with a particular focus on the role of DOC, a water quality parameter that has been relatively understudied. DOC, which is functionally defined by its ability to pass through a 0.45-µm filter, comprises a diverse range of compounds formed by the breakdown of organic matter and is quantified by its carbon component that is approximately 50% by mass. Adaptations of fish to acidic blackwaters include minimal acid-base disturbances associated with a unique, largely unknown, high-affinity Na⁺ uptake system that is resistant to inhibition by low pH in members of the Characiformes, and very tight regulation of Na⁺ efflux at low pH in the Cichliformes. Allochthonous (terrigenous) DOC, which predominates in blackwaters, consists of larger, more highly colored, reactive molecules than autochthonous DOC. The dissociation of protons from allochthonous components such as humic and fulvic acids is largely responsible for the acidity of these blackwaters, yet at the same time, these components may help protect organisms against the damaging effects of low water pH. DOC lowers the transepithelial potential (TEP), mitigates the inhibition of Na⁺ uptake and ammonia excretion, and protects against the elevation of diffusive Na⁺ loss in fish exposed to acidic waters. It also reduces the gill binding and toxicity of metals. At least in part, these actions reflect direct biological effects of DOC on the gills that are beneficial to ionoregulation. After chronic exposure to DOC, some of these protective effects persist even in the absence of DOC. Two characteristics of allochthonous DOC, the specific absorbance coefficient at 340 nm (determined optically) and the PBI (determined by titration), are indicative of both the biological effectiveness of DOC and the ability to protect against metal toxicity. Future research needs are highlighted, including a greater mechanistic understanding of the actions of DOCs on gill ionoregulatory function, morphology, TEP, and metal toxicity. These should be investigated in a wider range of native fish Orders that inhabit one of the world's greatest biodiversity hotspots for freshwater fishes.

Trans-epithelial potential (TEP) response as an indicator of major ion toxicity in rainbow trout and goldfish exposed to 10 different salts in ion-poor water

Freshwater ecosystems are facing increasing contamination by major ions. The Multi-Ion Toxicity (MIT) model, a new tool for risk assessment and regulation, predicts major ion toxicity to aquatic organisms by relating it to a critical disturbance of the trans-epithelial potential (TEP) across the gills, as predicted by electrochemical theory. The model is based on unproven assumptions. We tested some of these by directly measuring the acute TEP responses to a geometric series of 10 different single salts (NaCl, Na₂SO₄, KCl, K₂SO₄, CaCl₂, CaSO₄, MgCl₂, MgSO₄, NaHCO₃, KHCO₃) in the euryhaline rainbow trout (Oncorhynchus mykiss) and the stenohaline goldfish (Carassius auratus) acclimated to very soft, ion-poor water (hardness 10 mg CaCO₃/L). Results were compared to 24-h and 96-h LC50 data from the literature, mainly from fathead minnow (Pimephales promelas). All salts caused concentration-dependent increases in TEP to less negative/more positive values, in patterns well-described by the Michaelis-Menten equation, or a modified version incorporating substrate inhibition. The ΔTEP above baseline became close to a maximum at the 96-h LC50, except for the HCO₃^- salts. Furthermore, the range of ΔTEP values at the LC50 within one species was much more consistent (1.6- to 2.1-fold variation) than the molar concentrations of the different salts at the LC50 (19- to 25-fold variation). ΔTEP responses were related to cation rather than anion concentrations. Overall patterns were qualitatively similar between trout and goldfish, with some quantitative differences, and also in general accord with recently published data on three other species in harder water where ΔTEP responses were much smaller. Blood plasma Na⁺ and K⁺ concentrations were minimally affected by the exposures. The results are in accord with most but not all of the assumptions of the MIT model and support its further development as a predictive tool.

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

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

Understanding ventilation and oxygen uptake of Pacific hagfish (Eptatretus stoutii), with particular emphasis on responses to ammonia and interactions with other respiratory gases

The hagfishes are an ancient and evolutionarily important group, with breathing mechanisms and gills very different from those of other fishes. Hagfish inhale through a single nostril via a velum pump, and exhale through multiple separate gill pouches. We assessed respiratory performance in E. stoutii (31 ppt, 12 ºC, 50-120 g) by measuring total ventilatory flow ([Formula: see text]) at the nostril, velar (respiratory) frequency (fr), and inspired (P_IO₂) and expired (P_EO₂) oxygen tensions at all 12 gill pouch exits plus the pharyngo-cutaneous duct (PCD) on the left side, and calculated ventilatory stroke volume (S[Formula: see text]), % O₂ utilization, and oxygen consumption (ṀO₂). At rest under normoxia, spontaneous changes in [Formula: see text] ranged from apnea to > 400 ml kg^-1 min^-1, due to variations in both fr and S[Formula: see text]; "normal" [Formula: see text] averaged 137 ml kg^-1 min^-1, ṀO₂ was 718 µmol kg^-1 h^-1, so the ventilatory convection requirement for O₂ was about 11 L mmol^-1. Relative to anterior gill pouches, lower P_EO₂ values (i.e. higher utilization) occurred in the more posterior pouches and PCD. Overall, O₂ utilization was 34% and did not change during hyperventilation but increased to > 90% during hypoventilation. Environmental hypoxia (P_IO₂ ~ 8% air saturation, 1.67 kPa, 13 Torr) caused hyperventilation, but neither acute hyperoxia (P_IO₂ ~ 275% air saturation, 57.6 kPa, 430 Torr) nor hypercapnia (P_ICO₂ ~ 1% CO₂, 1.0 kPa, 7.5 Torr) significantly altered [Formula: see text]. ṀO₂ decreased in hypoxia and increased in hyperoxia but did not change in hypercapnia. Acute exposure to high environmental ammonia (HEA, 10 mM NH₄HCO₃) caused an acute decrease in [Formula: see text], in contrast to the hyperventilation of long-term HEA exposure described in a previous study. The hypoventilatory response to HEA still occurred during hypoxia and hyperoxia, but was blunted during hypercapnia. Under all treatments, ṀO₂ increased with increases in [Formula: see text]. Overall, there were lower convection requirements for O₂ during hyperoxia, higher requirements during hypoxia and hypercapnia, but unchanged requirements during HEA. We conclude that this "primitive" fish operates a flexible respiratory system with considerable reserve capacity.

Prevalence and spatial distribution of Coxiella burnetii seropositivity in northern Australian beef cattle adjusted for diagnostic test uncertainty

Q fever is a zoonotic disease caused by infection with Coxiella burnetii transmitted from animals including, but not limited to, cattle, sheep and goats. The infection in cattle is typically sub-clinical with some evidence suggesting associated reproductive loss. There is currently limited data on the true prevalence and distribution of coxiellosis in beef cattle across northern Australia. During this study, 2,012 sera samples from beef cattle managed on commercial farms located in Queensland and the Northern Territory were tested using an indirect immunofluorescent assay (IFA) for serological evidence of IgG antibodies against C. burnetii. Bayesian latent class models were used to estimate the true prevalence, adjusted for diagnostic test sensitivity and specificity and incorporating the hierarchical structure of the cattle within farms and regions. In this study, cattle in the Northern Territory had lower estimated true prevalence than cattle within most regions of Queensland with the exception of south-east Queensland. Results from this study have described the geographic distribution and estimated the true prevalence of antibodies to C. burnetii in a sample of extensively managed beef cattle located across the tropical grazing regions of northern Australia.

Brain and gills as internal and external ammonia sensing organs for ventilatory control in rainbow trout, Oncorhynchus mykiss

Ammonia is both a respiratory gas and a toxicant in teleost fish. Hyperventilation is a well-known response to elevations of both external and internal ammonia levels. Branchial neuroepithelial cells (NECs) are thought to serve as internal sensors of plasma ammonia (peripheral chemoreceptors), but little is known about other possible ammonia-sensors. Here, we investigated whether trout possess external sensors and/or internal central chemoreceptors for ammonia. For external sensors, we analyzed the time course of ventilatory changes at the start of exposure to high environmental ammonia (HEA, 1 mM). Hyperventilation developed gradually over 20 min, suggesting that it was a response to internal ammonia elevation. We also directly perfused ammonia solutions (0.01-1 mM) to the external surfaces of the first gill arches. Immediate hypoventilation occurred. For central chemoreceptors, we injected ammonia solutions (0.5-1.0 mM) directly onto the surface of the hindbrain of anesthetized trout. Immediate hyperventilation occurred. This is the first evidence of central chemoreception in teleost fish. We conclude that trout possess both external ammonia sensors, and dual internal ammonia sensors (perhaps for redundancy), but their roles differ. External sensors cause short term hypoventilation, which would help limit toxic waterborne ammonia uptake. When fish cannot avoid HEA, the diffusion of waterborne ammonia into the blood will stimulate both peripheral (NECs) and central (brain) chemoreceptors, resulting in hyperventilation. This hyperventilation will be beneficial in increasing ammonia excretion via the Rh metabolon system in the gills not only after HEA exposure, but also after endogenous ammonia loading from feeding or exercise.

The osmorespiratory compromise in the fish gill

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 O₂ 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 O₂ 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.

An in vitro study of urea and ammonia production and transport by the intestinal tract of fed and fasted rainbow trout: responses to luminal glutamine and ammonia loading

Digestion of dietary protein in teleosts results in high ammonia levels within the intestinal chyme that may reach concentrations that are many-fold greater than blood plasma levels. We used in vitro gut sac preparations of the ammoniotelic rainbow trout (Oncorhynchus mykiss) to investigate the role of the intestine in producing and transporting ammonia and urea, with specific focus on feeding versus fasting, and on responses to loading of the lumen with 2 mmol L^-1 glutamine or 2 mmol L^-1 ammonia. Feeding increased not only ammonia production and both mucosal and serosal fluxes, but also increased urea production and serosal fluxes. Elevated urea production was accompanied by an increase in arginase activity but minimal CPS III activity, suggesting that urea may be produced by direct arginolysis. The ammonia production and serosal fluxes increased in fasted preparations with glutamine loading, indicating an ability of the intestinal tissue to deaminate glutamine and perhaps use it as an energy source. However, there was little evidence of urea production or transport resulting from the presence of glutamine. Furthermore, the intestinal tissues did not appear to convert surplus ammonia to urea as a detoxification mechanism, as urea production and serosal flux rates decreased in fed preparations, with minimal changes in fasted preparations. Nevertheless, there was indirect evidence of detoxification by another pathway, as ammonia production rate decreased with ammonia loading in fed preparations. Overall, our study suggests that intestinal tissues of rainbow trout have the ability to produce urea and detoxify ammonia, likely via arginolysis.

Influence of environmentally relevant concentrations of Zn, Cd and Ni and their binary mixtures on metal uptake, bioaccumulation and development in larvae of the purple sea urchin Strongylocentrotus purpuratus

Metal accumulation, disturbance of Ca²⁺ homeostasis, and occurrence of abnormalities are well-established consequences of single metal exposure during early development stages of sea urchins. However, the effects caused by low concentrations of metals and metal mixtures need to be better understood in marine invertebrates. Therefore, the present study investigated the effects of environmentally relevant concentrations of Zn (9 μg/L), Cd (30 μg/L) and Ni (5 μg/L) in single and binary exposures (Zn + Cd, Cd + Ni and Ni + Zn) to the early life stages of the purple sea urchin Strongylocentrotus purpuratus. Endpoints checked in all treatments after 48-h exposure were unidirectional metal influx rates, bioaccumulation, and Ca²⁺ influx rates. Additionally, the presence of abnormal larvae and developmental delay was evaluated at 24 h, 48 h and 72 h of exposure. Unidirectional influx rates of all three metals were significantly higher than control background rates in all single exposures and binary mixtures, and were generally not different between them. Net accumulation (body burden) of both Zn and Cd increased significantly as a result of their respective single exposures, while Ni accumulation decreased considerably. When Zn or Cd were presented in binary exposures with other metals, the net accumulations of Zn or Cd were reduced relative to single exposures to these metals, whereas this did not occur for Ni accumulation. Thus, bioaccumulation proved to be a better metric than influx rate measurements to analyze metal competition at a whole organism level at these low metal concentrations. Short-term Ca²⁺ influx also did not appear to be a sensitive metric, as the measured rates did not vary among all single and binary exposures, with the exception of a lower rate in Ni + Zn binary exposure. A critical aspect observed was the relationship between bioaccumulation versus influx measurements, which proved positive for Cd, but negative for Zn and Ni, demonstrating possible capacities for both Zn and Ni regulation by sea urchin larvae. Increases in larval abnormalities relative to controls occurred only after binary mixtures, starting at 48 h exposure and maintained until 72 h. However, delay of the sea urchin development by the presence of gastrula stage at 72 h exposure occurred in Zn and Ni single exposures and all metal mixtures, with very high abnormal development when Ni was present.

A Mystery Tale: Nickel Is Fickle When Snails Fail-Investigating the Variability in Ni Toxicity to the Great Pond Snail

Dissolved Ni concentrations inhibiting the growth of juvenile great pond snails (Lymnaea stagnalis) have been documented to vary from about 1 to 200 µg L^-1 Ni. This variability makes L. stagnalis either a moderately sensitive or the most sensitive freshwater species to chronic Ni exposure tested to date. Given the role of sensitive species in environmental risk assessment frameworks, it is particularly important to understand this variability, i.e., to characterize the factors that modulate Ni toxicity and that may confound toxicity test outcomes when uncontrolled. In the present study, we tested if this variability was due to analytical (growth calculation: biomass versus growth rate), environmental (water quality), lab-specific practices, and/or snail population differences among earlier studies. Specifically, we reanalyzed previously published Ni toxicity data and conducted additional measurements of Ni aqueous speciation, short-term Ni uptake, and chronic Ni toxicity with test waters and snail cultures used in previous studies. Corrections for Ni bioavailability and growth calculations explained a large degree of variability in the published literature. However, a residual 16-fold difference remained puzzling between 2 studies: Niyogi et al. (2014) (low ECxs) and Crémazy et al. (2018) (high ECxs). Indeed, differences in metal bioavailability due to water chemistry, lab-specific practices, and snail population sensitivity could not explain the large variation in Ni toxicity in these 2 very similar studies. Other potentially important toxicity-modifying factors were not directly evaluated in the present work: test duration, diet, snail holding conditions, and snail age at onset of testing. The present analysis highlights the need for further studies to elucidate 1) the mechanisms of growth inhibition in Ni-exposed L. stagnalis and 2) the important abiotic and biotic factors affecting this biological response. Until these processes are understood, substantial uncertainties will remain about inclusion of this species in Ni environmental risk assessment. Integr Environ Assess Manag 2020;16:983-997. © 2020 SETAC.

Gills versus kidney for ionoregulation in the obligate air-breathing Arapaima gigas, a fish with a kidney in its air-breathing organ

In Arapaima gigas, an obligate air-breather endemic to ion-poor Amazonian waters, a large complex kidney runs through the air-breathing organ (ABO). Previous indirect evidence suggested that the kidney, relative to the small gills, may be exceptionally important in ionoregulation and nitrogen (N) waste excretion, with support of kidney function by direct O2 supply from the airspace. We tested these ideas by continuous urine collection and gill flux measurements in ∼700 g fish. ATPase activities were many-fold greater in kidney than gills. In normoxia, gill Na+ influx and efflux were in balance, with net losses of Cl- and K+ Urine flow rate (UFR, ∼11 ml kg-1 h-1) and urinary ions (< 0.2 mmol l-1) were exceptional, with [urine]:[plasma] ratios of 0.02-0.002 for K+, Na+, and Cl-, indicating strong reabsorption with negligible urinary ion losses. Urinary [ammonia] was very high (10 mmol l-1, [urine]:[plasma] ∼17) indicating strong secretion. The kidney accounted for 21-24% of N excretion, with ammonia dominating (95%) over urea-N through both routes. High urinary [ammonia] was coupled to high urinary [HCO3-]. Aerial hypoxia (15.3 kPa) and aerial hyperoxia (>40.9 kPa) had no effects on UFR, but both inhibited branchial Na+ influx, revealing novel aspects of the osmorespiratory compromise. Aquatic hypoxia (4.1 kPa), but not aquatic hyperoxia (>40.9 kPa), inhibited gill Na+ influx, UFR and branchial and urinary ammonia excretion. We conclude that the kidney is more important than gills in ionoregulation, and is significant in N excretion. Although not definitive, our results do not indicate direct O2 supply from the ABO for kidney function.

Dichloroacetate reveals the presence of metabolic inertia at the start of exercise in rainbow trout (Oncorhynchus mykiss, Walbaum 1792)

A lag in the increase in oxygen consumption (MO₂ ) occurs at the start of sustainable exercise in trout. Waterborne dichloroacetate (0.58 and 3.49 mmol l^-1 ), a compound which activates pyruvate dehydrogenase (PDH) by inhibiting PDH kinase in muscle, accelerates the increase in MO₂ during the first 10 min of sustainable exercise when velocity is elevated to 75% critical swimming speed in a swim tunnel. There are no effects on MO₂ thereafter or at rest. This indicates that a delay in PDH activation ("metabolic inertia") contributes to the lag phenomenon.

The potential for salt toxicity: Can the trans-epithelial potential (TEP) across the gills serve as a metric for major ion toxicity in fish?

An emerging Multi-Ion Toxicity (MIT) model for assessment of environmental salt pollution is based on the premise that major ion toxicity to aquatic organisms is related to a critical disturbance of the trans-epithelial potential across the gills (ΔTEP), which can be predicted by electrochemical theory. However, the model has never been evaluated physiologically. We directly tested key assumptions by examining the individual effects of eight different salts (NaCl, Na₂SO₄, MgCl₂, MgSO₄, KCl, K₂SO₄, CaCl₂, and CaSO₄) on measured TEP in three different fish species (fathead minnow, Pimephales promelas = FHM; channel catfish, Ictalurus punctatus = CC; bluegill, Lepomis macrochirus = BG). A geometric concentration series based on previously reported 96-h LC50 values for FHM was used. All salts caused concentration-dependent increases in TEP to less negative/more positive values in a pattern well-described by the Michaelis-Menten equation. The ΔTEP responses for different salts were similar to one another within each species when concentrations were expressed as a percentage of the FHM LC50. A plateau was reached at or before 100 % of the LC50 where the ΔTEP values were remarkably consistent, with only 1.4 to 2.2-fold variation. This relative uniformity in the ΔTEP responses contrasts with 28-fold variation in salt concentration (in mmol L^-1), 9.6-fold in total dissolved solids, and 7.9-fold in conductivity at the LC50. The Michaelis-Menten Km values (salt concentrations causing 50 % of the ΔTEP_max) were positively related to the 96-h LC50 values. ΔTEP responses were not a direct effect of osmolarity in all species and were related to specific cation rather than specific anion concentrations in FHM. These responses were stable for up to 24 h in CC. The results provide strong physiological support for the assumptions of the MIT model, are coherent with electrochemical theory, and point to areas for future research.

Effects of natural light and depth on rates of photo-oxidation of dissolved organic carbon in a major black-water river, the Rio Negro, Brazil

Systems rich in terrigenous dissolved organic carbon (DOC), like the Rio Negro, can contribute significant amounts of carbon dioxide back to the atmosphere and support important microbial communities. We investigated photo-oxidation in the Rio Negro: (1) the depth to which light causes complete photo-oxidation to CO₂ and changes in DOC structure, (2) the daily rate of change of absorbance indices, (3) the relationship between sub-surface rates of photo-oxidation to CO₂ and light exposure, (4) the areal rates of photo-oxidation, and (5) the stability of fluorophore signals. Experiments were run in an outdoor pool of Rio Negro water, under natural sunlight during the dry seasons of 2015 and 2018. In 2018, rates of complete photo-oxidation and changes in absorbance indices decayed exponentially, approaching their asymptotes between 9 and 15 cm depth. In 2015, direct absorbance indices ceased changing at 14 cm depth. Fluorescence of humic acid-like moieties continued to decrease, sometimes to 35-43 cm depth. This indicates that partial photo-oxidation of DOC, and thus interaction with the microbial community, occurs to greater depths than previously expected. Areal rates of CO₂ production were 28.8 and 39.3 mg C m^-2 d^-1 (two experiments, October 2018). Sub-surface (1.1 cm) rates were strongly related to light levels, reaching a maximum of 0.68 mg C l^-1 d^-1 in September. Complete photo-oxidation ceased below 29.6 mW cm^-2 d^-1 UV radiation, providing a daily baseline for observable production of CO₂. Absorbance indices changed by 9 to 14% d^-1 at high light levels, except for R_254/365 (4.4% d^-1). Fluorophore emission ranges were stable between 2014 and 2018, indicating that emissions can be compared across time and space. This study contributes to better estimates and understanding of photo-oxidation in tropical, black-water rivers, which will be useful for carbon modelling.

Interactive effects of temperature and hypoxia on diffusive water flux and oxygen uptake rate in the tidepool sculpin, Oligocottus maculosus

The osmorespiratory compromise hypothesis posits that respiratory epithelial characteristics and physiological regulatory mechanisms which promote gas permeability also increase permeability to ions and water. The hypothesis therefore predicts that physiological responses which increase effective gas permeability will result in increased effective ion and water permeabilities. Though analyses of water and gas effective permeabilities using high temperature have generally supported the hypothesis, water permeability responses to hypoxia remain equivocal and the combination of high temperature and hypoxia untested. We measured diffusive water flux (DWF) and oxygen uptake rate (Ṁo₂) in response to acute temperature change, hypoxia, and the combination of high temperature and hypoxia in a hypoxia-tolerant intertidal fish, the tidepool sculpin (Oligocottus maculosus). In support of the osmorespiratory compromise hypothesis, Ṁo₂ and DWF increased with temperature. In contrast, DWF decreased with hypoxia at a constant temperature, a result consistent with previously observed decoupling of water and gas effective permeabilities during hypoxia exposure in some hypoxia tolerant fishes. However, DWF levels during simultaneous high temperature and hypoxia exposure were not different from fish exposed to high temperature in normoxia, possibly suggesting a failure of the mechanism responsible for down-regulating DWF in hypoxia. These results, together with time-course analysis of hypoxia exposure and normoxic recovery, suggest that tidepool sculpins actively downregulate effective water permeability in hypoxia but the mechanism fails with multi-stressor exposure. Future investigations of the mechanistic basis of the regulation of gill permeability will be key to understanding the role of this regulatory ability in the persistence of this species in the dynamic intertidal environment.

A potential role for hyperpolarization-activated cyclic nucleotide-gated sodium/potassium channels (HCNs) in teleost acid-base and ammonia regulation

Increasing evidence suggests the involvement of hyperpolarization-activated cyclic nucleotide-gated sodium/potassium channels (HCNs) not only in cardiac and neural function, but also in more general physiological processes including acid-base and ammonia regulation. We have identified four different HCN paralogs/isoforms in the goldfish Carassius auratus (CaHCN1, CaHCN2b, CaHCN4a and CaHCN4b) as likely candidates to contribute to renal, branchial and intestinal acid-base and ammonia regulation in this teleost. Quantitative real-time PCR showed not only high mRNA abundance of all isoforms in heart and brain, but also detectable levels (particularly of CaHCN2b and CaHCN4b) in non-excitable tissues, including gills and kidneys. In response to an internal or external acid-base and/or ammonia disturbance caused by feeding or high environmental ammonia, respectively, we observed differential and tissue-specific changes in mRNA abundance of all isoforms except CaHCN4b. Furthermore, our data suggest that the functions of specific HCN channels are supplemented by certain Rhesus glycoprotein functions to help in the protection of tissues from elevated ammonia levels, or as potential direct routes for ammonia transport in gills, kidney, and gut. The present results indicate important individual roles for each HCN isoform in response to acid-base and ammonia disturbances.

Is aquaporin-3 involved in water-permeability changes in the killifish during hypoxia and normoxic recovery, in freshwater or seawater?

Aquaporins are the predominant water-transporting proteins in vertebrates, but only a handful of studies have investigated aquaporin function in fish, particularly in mediating water permeability during salinity challenges. Even less is known about aquaporin function in hypoxia (low oxygen), which can profoundly affect gill function. Fish deprived of oxygen typically enlarge gill surface area and shrink the water-to-blood diffusion distance, to facilitate oxygen uptake into the bloodstream. However, these alterations to gill morphology can result in unfavorable water and ion fluxes. Thus, there exists an osmorespiratory compromise, whereby fish must try to balance high branchial gas exchange with low ion and water permeability. Furthermore, the gills of seawater and freshwater teleosts have substantially different functions with respect to osmotic and ion fluxes; consequently, hypoxia can have very different effects according to the salinity of the environment. The purpose of this study was to determine what role aquaporins play in water permeability in the hypoxia-tolerant euryhaline common killifish (Fundulus heteroclitus), in two important osmoregulatory organs-the gills and intestine. Using immunofluorescence, we localized aquaporin-3 (AQP3) protein to the basolateral and apical membranes of ionocytes and enterocytes, respectively. Although hypoxia increased branchial AQP3 messenger-RNA expression in seawater and freshwater, protein abundance did not correlate. Indeed, hypoxia did not alter AQP3 protein abundance in seawater and reduced it in the cell membranes of freshwater gills. Together, these observations suggest killifish AQP3 contributes to reduced diffusive water flux during hypoxia and normoxic recovery in freshwater and facilitates intestinal permeability in seawater and freshwater.

Gills and air-breathing organ in O2 uptake, CO2 excretion, N-waste excretion, and ionoregulation in small and large pirarucu (Arapaima gigas)

In the pirarucu (Arapaima gigas), gill surface area and thus gas exchange capacity of the gills are reduced with proceeding development. It, therefore, is expected that A. gigas, starting as a water breather, progressively turns into an obligate air-breathing fish using an air-breathing organ (ABO) for gas exchange. We assessed the air-breathing activity, O₂ and CO₂ exchange into air and water, ammonia-N and urea-N excretion, ion flux rates, and activities of ion transport ATPases in large versus small pirarucu. We found that even very young A. gigas (4-6 g, 2-3 weeks post-hatch) with extensive gills are air-breathers (18.1 breaths*h^-1) and cover most (63%) of their O₂ requirements from the air whereas 600-700-g animals (about 3-4 months post-hatch), with reduced gills, obtain 75% of their O₂ from the air (10.8 breaths*h^-1). Accordingly, the reduction in gill surface area hardly affected O₂ uptake, but development had a significant effect on aerial CO₂ excretion, which was very low (3%) in small fish and increased to 12% in larger fish, yielding a hyper-allometric scaling coefficient (1.12) in contrast to 0.82-0.84 for aquatic and total CO₂ excretion. Mass-specific ammonia excretion decreased in approximate proportion to mass-specific O₂ consumption as the fish grew, but urea-N excretion dropped from 18% (at 4-6 g) to 8% (at 600-700 g) of total N-excretion; scaling coefficients for all these parameters were 0.70-0.80. Mass-specific sodium influx and efflux rates, as well as potassium net loss rates, departed from this pattern, being greater in larger fish; hyper-allometric scaling coefficients were > 1.0. Gill V-type H⁺ ATPase activities were greater than Na⁺, K⁺-ATPase activities, but levels were generally low and comparable in large and small fish, and similar activities were detected in the ABO. A. gigas is a carnivorous fish throughout its lifecycle, and, despite fasting, protein oxidation accounted for the major portion (61-82%) of aerobic metabolism in both large and small animals. ABO PO₂ and PCO₂ (measured in 600-700-g fish) were quite variable, and aerial hypoxia resulted in lower ABO PO₂ values. Under normoxic conditions, a positive correlation between breath volume and ABP PO₂ was detected, and on average with a single breath more than 50% of the ABO volume was exchanged. ABO PCO₂ values were in the range of 1.95-3.89 kPa, close to previously recorded blood PCO₂ levels. Aerial hypoxia (PO₂ down to 12.65 kPa) did not increase either air-breathing frequency or breath volume.

Understanding the gastrointestinal physiology and responses to feeding in air-breathing Anabantiform fishes

The Mekong Delta is host to a large number of freshwater species, including a unique group of facultative air-breathing Anabantiforms. Of these, the striped snakehead (Channa striata), the climbing perch (Anabas testudineus), the giant gourami (Osphronemus goramy) and the snakeskin gourami (Trichogaster pectoralis) are major contributors to aquaculture production in Vietnam. The gastrointestinal responses to feeding in these four species are detailed here. Relative intestinal length was lowest in the snakehead, indicating carnivory, and 5.5-fold greater in the snakeskin, indicating herbivory; climbing perch and giant gourami were intermediate, indicating omnivory. N-waste excretion (ammonia-N + urea-N) was greatest in the carnivorous snakehead and least in the herbivorous snakeskin, whereas the opposite trend was observed for net K⁺ excretion. Similarly, the more carnivorous species had a greater stomach acidity than the more herbivorous species. Measurements of acid-base flux to water indicated that the greatest postprandial alkaline tide occurred in the snakehead and a potential acidic tide in the snakeskin. Additional findings of interest were high levels of both PCO₂ (up to 40 mmHg) and HCO₃ ^- (up to 33 mM) in the intestinal chyme of all four of these air-breathing species. Using in vitro gut sac preparations of the climbing perch, it was shown that the intestinal net absorption of fluid, Na⁺ and HCO₃ ^- was upregulated by feeding but not net Cl^- uptake, glucose uptake or K⁺ secretion. Upregulated net absorption of HCO₃ ^- suggests that the high chyme (HCO₃ ^- ) does not result from secretion by the intestinal epithelium. The possibility of ventilatory control of PCO₂ to regulate postprandial acid-base balance in these air-breathing fish is discussed.

Effects of copper on a reconstructed freshwater rainbow trout gill epithelium: Paracellular and intracellular aspects

The barrier properties and intracellular responses of a primary cultured trout gill epithelium (containing both mitochondria-rich and pavement cells) were examined over 24 h of copper (Cu) exposure (0, 200 and 1000 μg/L) in apical fresh water. Transepithelial resistance (TER) and mRNA abundance of tight junction proteins zonula occludens-1, occludin, cingulin, claudin-8d and -28b were examined as endpoints of barrier function and the paracellular pathway. Intracellular endpoints analyzed were Cu accumulation, Na⁺ content, carbonic anhydrase activity and mRNA abundance of carbonic anhydrase (ca-II) and Na⁺/K⁺ ATPase (nka α1a and nka α1b isoforms). After a brief initial drop in TER in the 1000 μg Cu/L treatment, Cu at both levels increased TER over the first 6 h of exposure but there were no differences among groups from 12 h onwards. After 24 h of Cu exposure, there were no differences in mRNA abundance of any of the tight junction proteins examined. Cu accumulation occurred at 1000 μg Cu/L (5.5-fold increase), but no depletion of Na⁺ content. Carbonic anhydrase activity decreased significantly (by 76%), however Cu exposure did not alter the transcript abundance of ca-II, nka α1a, or nka α1b. This study provides a first report of carbonic anhydrase sensitivity to Cu exposure in a cultured model gill epithelium. We conclude that Cu impacts the permeability of this model during the early stages of exposure and that the use of carbonic anhydrase inhibition as an endpoint of metal toxicity in this model preparation may be useful for future mechanistic investigations and environmental monitoring.

Reverse translation: effects of acclimation temperature and acute temperature challenges on oxygen consumption, diffusive water flux, net sodium loss rates, Q10 values and mass scaling coefficients in the rainbow trout (Oncorhynchus mykiss)

Our understanding is limited on how fish adjust the effective permeability of their branchial epithelium to ions and water while altering O₂ uptake rate (MO₂) with acute and chronic changes in temperature. We investigated the effects of acclimation temperature (8 °C, 13 °C and 18 °C) and acute temperature challenges [acute rise (acclimated at 8 °C, measured at 13 °C and 18 °C), acute drop (acclimated at 18 °C, measured at 8 °C and 13 °C) and intermediate (acclimated at 13 °C, measured at 8 °C and 18 °C)] on routine MO₂, diffusive water flux, and net sodium loss rates in 24-h fasted rainbow trout (Oncorhynchus mykiss). In the temperature challenge tests, measurements were made during the first hour. In acclimated trout at all temperatures, allometric mass scaling coefficients were much higher for diffusive water flux than for MO₂. Furthermore, the diffusive water flux rate was more responsive (overall Q₁₀ = 2.75) compared to MO₂ (Q₁₀ = 1.80) over the 8-18 °C range, and for both, Q₁₀ values were greater at 8-13 °C than at 13-18 °C. The net Na⁺ flux rates were highly sensitive to acclimation temperature with an overall Q₁₀ of 3.01 for 8-18 °C. In contrast, very different patterns occurred in trout subjected to acute temperature challenges. The net Na⁺ flux rate was temperature-insensitive with a Q₁₀ around 1.0. Both MO₂ and diffusive water flux rates exhibited lower Q₁₀ values than for the acclimated rates in response to either acute increases or decreases in temperature. These results fit Pattern 5 of Precht (undercompensation, reverse effect) and more precisely Pattern IIB of Prosser (reverse translation). These inverse compensatory patterns suggest that trout do not alter their rates very much when undergoing acute thermal challenges (diurnal fluctuations, migration through the thermocline). The greater changes seen with acclimation may be adaptive to long-term seasonal changes in temperature. We discuss the roles of aquaporins, spontaneous activity, and recent feeding in these responses.

Ionoregulatory aspects of the hypoxia-induced osmorespiratory compromise in the euryhaline Atlantic killifish (Fundulus heteroclitus): the effects of salinity

The osmorespiratory compromise is a physiological trade-off between the characteristics of the gill that promote respiratory gas-exchange and those that limit passive fluxes of ions and water with the environment. In hypoxia, changes in gill blood flow patterns and functional surface area that increase gas transfer can promote an exacerbation in ion and water fluxes. Our goal was to determine whether the osmorespiratory compromise is flexible, depending on environmental salinity (fresh, isosmotic and sea water) and oxygen levels (hypoxia) in euryhaline killifish, Fundulus heteroclitus. Plasma ion concentrations were minimally affected by hypoxia, indicating a maintenance of osmoregulatory homeostasis. In FW-killifish, hypoxia exposure reduced branchial Na+/K+-ATPase and NEM-sensitive-ATPase activities, as well as diffusive water flux rates. Unidirectional Na+ influx and Na+ efflux decreased during hypoxia in FW, but net Na+ flux remained unchanged. Net loss rates of Cl−, K+ and ammonia were also attenuated in hypoxia, suggesting both transcellular and paracellular reductions in permeability. These reductions appeared to be regulated phenomena as fluxes were restored immediately in normoxia. Na+ flux rates increased during hypoxia in 11 ppt, but decreased in 35 ppt, the latter suggesting a similar response to hypoxia as in FW. In summary, FW- and SW-killifish experience a reduction in gill permeability, as seen in other hypoxia-tolerant species. Fish acclimated to isosmotic salinity increased Na+ influx and efflux rates, as well as paracellular permeability in hypoxia, responses in accord with the predictions of the classic osmorespiratory compromise.

Metal Bioavailability Models: Current Status, Lessons Learned, Considerations for Regulatory Use, and the Path Forward

Since the early 2000s, biotic ligand models and related constructs have been a dominant paradigm for risk assessment of aqueous metals in the environment. We critically review 1) the evidence for the mechanistic approach underlying metal bioavailability models; 2) considerations for the use and refinement of bioavailability-based toxicity models; 3) considerations for the incorporation of metal bioavailability models into environmental quality standards; and 4) some consensus recommendations for developing or applying metal bioavailability models. We note that models developed to date have been particularly challenged to accurately incorporate pH effects because they are unique with multiple possible mechanisms. As such, we doubt it is ever appropriate to lump algae/plant and animal bioavailability models; however, it is often reasonable to lump bioavailability models for animals, although aquatic insects may be an exception. Other recommendations include that data generated for model development should consider equilibrium conditions in exposure designs, including food items in combined waterborne-dietary matched chronic exposures. Some potentially important toxicity-modifying factors are currently not represented in bioavailability models and have received insufficient attention in toxicity testing. Temperature is probably of foremost importance; phosphate is likely important in plant and algae models. Acclimation may result in predictions that err on the side of protection. Striking a balance between comprehensive, mechanistically sound models and simplified approaches is a challenge. If empirical bioavailability tools such as multiple-linear regression models and look-up tables are employed in criteria, they should always be informed qualitatively and quantitatively by mechanistic models. If bioavailability models are to be used in environmental regulation, ongoing support and availability for use of the models in the public domain are essential. Environ Toxicol Chem 2019;39:60-84. © 2019 SETAC.