Leap of faith: voluntary emersion behaviour and physiological adaptations to aerial exposure in a non-aestivating freshwater fish in response to aquatic hypoxia

HIF-3α/PPAR-γ Regulates Hypoxia Tolerance by Altering Glycolysis and Lipid Synthesis in Blunt Snout Bream (Megalobrama amblycephala)

Hypoxic stress causes cell damage and serious diseases in organisms, especially in aquatic animals. It is important to elucidate the changes in metabolic function caused by hypoxia and the mechanisms underlying these changes. This study focuses on the low oxygen tolerance feature of a new blunt snout bream strain (GBSBF1). Our data show that GBSBF1 has a different lipid and carbohydrate metabolism pattern than wild-type bream, with altering glycolysis and lipid synthesis. In GBSBF1, the expression levels of phd2 and vhl genes are significantly decreased, while the activation of HIF-3α protein is observed to have risen significantly. The results indicate that enhanced HIF-3α can positively regulate gpd1ab and gpam through PPAR-γ, which increases glucose metabolism and reduces lipolysis of GBSBF1. This research is beneficial for creating new aquaculture strains with low oxygen tolerance traits.

Cardiac Hypoxia Tolerance in Fish: From Functional Responses to Cell Signals

Aquatic animals are increasingly challenged by O₂ fluctuations as a result of global warming, as well as eutrophication processes. Teleost fish show important species-specific adaptability to O₂ deprivation, moving from intolerance to a full tolerance of hypoxia and even anoxia. An example is provided by members of Cyprinidae which includes species that are amongst the most tolerant hypoxia/anoxia teleosts. Living at low water O₂ requires the mandatory preservation of the cardiac function to support the metabolic and hemodynamic requirements of organ and tissues which sustain whole organism performance. A number of orchestrated events, from metabolism to behavior, converge to shape the heart response to the restricted availability of the gas, also limiting the potential damages for cells and tissues. In cyprinids, the heart is extraordinarily able to activate peculiar strategies of functional preservation. Accordingly, by using these teleosts as models of tolerance to low O₂, we will synthesize and discuss literature data to describe the functional changes, and the major molecular events that allow the heart of these fish to sustain adaptability to O₂ deprivation. By crossing the boundaries of basic research and environmental physiology, this information may be of interest also in a translational perspective, and in the context of conservative physiology, in which the output of the research is applicable to environmental management and decision making.

Individual metabolism and behaviour as complementary endpoints to better understand mangrove crab community variations linked to wastewater inputs

Mangrove forests are impacted by a large range of anthropogenic activities that challenge their functioning. For example, domestic wastewater (WW) discharges are known to increase vegetation growth but recent studies indicate that they have negative effects on benthic macrofauna, especially on mangrove crabs, these ecosystem engineers playing a key role on the functioning of the mangrove. In experimental areas regularly receiving WW at low tide (Mayotte Island, Indian Ocean), a drastic decrease in burrowing crab density has been reported. In this context, the individual behavioural and physiological responses of the fiddler crab Paraleptuca chlorophthalmus exposed to short-term (6 h) pulse of WW and ammonia-N (as a potential proxy of WW) were investigated. This species is one of the most sensitive to WW within the mangrove crab community. For the behavioural experiment, crabs could choose between the aquatic and aerial environment. Individual metabolic rate (O₂ consumption) was monitored after 6 h of exposure in WW or ammonia-N. Aerobic and anaerobic metabolic markers (citrate synthase and lactate dehydrogenase activities, respectively) were also evaluated. Results indicate that crabs exposed to WW are more active and mobile than controls after 3 h. Crabs actively emersed from WW and reduced their activity and mobility after 6 h. A higher metabolic rate in WW occurred immediately (t = 0 h), 3 and 6 h after WW exposure, with also, a burst in aerobic bacterial consumption in WW, but no effect of ammonia-N. No effect of WW or ammonia-N was observed on enzymatic aerobic and anaerobic metabolic markers. Therefore, short-term pulses with domestic polluted wastewater trigger quick behavioural and metabolic responses that could be deleterious if prolonged. These results could contribute to the understanding of the community-scale changes observed in benthic macrofauna after several years of regular domestic pollution pulses.

Influence of environmental factors on biology and catch composition of Barbonymus schwanenfeldii in a tropical lake, northern Malaysia: implications for conservation planning

Very little work has determined the relative importance of uncontrolled environmental factors for affecting fish biology, and how these might influence gillnet catches. This study addresses this deficit for an important Southeast Asian cyprinid (Barbonymus schwanenfeldii). Fish were caught monthly for 12 months using gillnets of three different mesh sizes, each of which was deployed in duplicate at the surface of one of three randomly selected sites in Lake Kenyir, Malaysia, concurrent with determining various environmental parameters and the abundance of phytoplankton (chlorophyll-a). Results indicated that growth co-efficient of B. schwanenfeldii was positively influenced by dissolved oxygen and negatively influenced by total inorganic nitrogen, whereas an opposite result was observed in case of the hepatosomatic index of fish. Water turbidity was a limiting factor only for small fish (mean total length: 15.74±1.10 cm). B. schwanenfeldii could best be caught during the period of high phytoplankton abundance or at the location of high phytoplankton density in the water. Water temperature negatively influenced the gillnet catches of the fish. The remaining environmental factors such as water depth, pH, and phosphate had a weak and insignificant influence (P >0.05) on the biology and gillnet catches of fish. The observed results can be very useful for the ecological monitoring and conservation plans for this species in relation to climate change. Furthermore, the utility of the similar data for other species would be useful not only for regional but also for international fishery by optimizing catches considering environmental conditions.

The utility and determination of P crit in fishes

The critical O₂ tension (P _crit) is the lowest P _O2  at which an animal can maintain some benchmark rate of O₂ uptake (Ṁ _O2 ). This P _O2  has long served as a comparator of hypoxia tolerance in fishes and aquatic invertebrates, but its usefulness in this role, particularly when applied to fishes, has recently been questioned. We believe that P _crit remains a useful comparator of hypoxia tolerance provided it is determined using the proper methods and hypoxia tolerance is clearly defined. Here, we review the available methods for each of the three steps of P _crit determination: (1) measuring the most appropriate benchmark Ṁ _O2  state for P _crit determination (Ṁ _O2,std, the Ṁ _O2  required to support standard metabolic rate); (2) reducing water P _O2 ; and (3) calculating P _crit from the Ṁ _O2  versus P _O2  curve. We make suggestions on best practices for each step and for how to report P _crit results to maximize their comparative value. We also discuss the concept of hypoxia tolerance and how P _crit relates to a fish's overall hypoxia tolerance. When appropriate methods are used, P _crit provides useful comparative physiological and ecological information about the aerobic contributions to a fish's hypoxic survival. When paired with other hypoxia-related physiological measurements (e.g. lactate accumulation, calorimetry-based measurements of metabolic depression, loss-of-equilibrium experiments), P _crit contributes to a comprehensive understanding of how a fish combines aerobic metabolism, anaerobic metabolism and metabolic depression in an overall strategy for hypoxia tolerance.

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

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

Hydrogen sulphide toxicity and the importance of amphibious behaviour in a mangrove fish inhabiting sulphide-rich habitats

We investigated amphibious behaviour, hydrogen sulphide (H₂S) tolerance, and the mechanism of H₂S toxicity in the amphibious mangrove rivulus (Kryptolebias marmoratus). We found that fish emersed (left water) in response to acutely elevated [H₂S] (~ 130-200 µmol l^-1). The emersion response to H₂S may be influenced by prior acclimation history due to acclimation-induced alterations in gill morphology and/or the density and size of neuroepithelial cells (NECs) on the gills and skin. Thus, we acclimated fish to water (control), H₂S-rich water, or air and tested the hypotheses that acclimation history influences H₂S sensitivity due to acclimation-induced changes in (i) gill surface area and/or (ii) NEC density and/or size. Air-acclimated fish emersed at significantly lower [H₂S] relative to fish acclimated to control or H₂S-rich water, but exhibited no change in gill surface area or in NEC density or size in the gills or skin. Despite possessing exceptional H₂S tolerance, all fish lost equilibrium when unable to emerse from environments containing extremely elevated [H₂S] (2272 ± 46 µmol l^-1). Consequently, we tested the hypothesis that impaired blood oxygen transport (i.e., sulphemoglobin formation) causes H₂S toxicity in amphibious fishes. In vitro exposure of red blood cells to physiologically relevant [H₂S] did not cause a substantial increase in sulphemoglobin formation. We found evidence, however, for an alternative hypothesis that H₂S toxicity is caused by impaired oxidative phosphorylation (i.e., cytochrome c oxidase inhibition). Collectively, our results show that amphibious behaviour is critical for the survival of K. marmoratus in H₂S-rich environments as fish experience impaired oxidative phosphorylation when unable to emerse.

Are all bony fishes oxygen regulators? Evidence for oxygen regulation in a putative oxygen conformer, the swamp eel Synbranchus marmoratus

This study investigated the oxygen consumption of the putative oxygen conformer marbled swamp eel Synbranchus marmoratus during progressive hypoxia. Earlier studies have not reached an agreement on whether S. marmoratus is a conformer or a regulator. Our results support the view that S. marmoratus is an oxygen regulator, like most bony fishes.

Emersion behaviour underlies variation in gill morphology and aquatic respiratory function in the amphibious fish Kryptolebias marmoratus

Fishes acclimated to hypoxic environments often increase gill surface area to improve O₂ uptake. In some species, surface area is increased via reduction of an interlamellar cell mass (ILCM) that fills water channels between gill lamellae. Amphibious fishes, however, may not increase gill surface area in hypoxic water because these species can, instead, leave water and breathe air. To differentiate between these possibilities, we compared wild amphibious mangrove rivulus Kryptolebias marmoratus from two habitats that varied in O₂ availability - a hypoxic freshwater pool versus nearly anoxic crab burrows. Fish captured from crab burrows had less gill surface area (as ILCMs were enlarged by ∼32%), increased rates of normoxic O₂ consumption and increased critical O₂ tension compared with fish from the freshwater pool. Thus, wild mangrove rivulus do not respond to near-anoxic water by decreasing metabolism or increasing O₂ extraction. Instead, fish from the crab burrow habitat spent three times longer out of water, which probably caused the observed changes in gill morphology and respiratory phenotype. We also tested whether critical O₂ tension is influenced by genetic heterozygosity, as K. marmoratus is one of only two hermaphroditic vertebrate species that can produce both self-fertilized (inbred) or out-crossed (more heterozygous) offspring. We found no evidence for inbreeding depression, suggesting that self-fertilization does not impair respiratory function. Overall, our results demonstrate that amphibious fishes that inhabit hypoxic aquatic habitats can use a fundamentally different strategy from that used by fully aquatic water-breathing fishes, relying on escape behaviour rather than metabolic depression or increased O₂ extraction ability.

Metabolism drives distribution and abundance in extremophile fish

Differences in population density between species of varying size are frequently attributed to metabolic rates which are assumed to scale with body size with a slope of 0.75. This assumption is often criticised on the grounds that 0.75 scaling of metabolic rate with body size is not universal and can vary significantly depending on species and life-history. However, few studies have investigated how interspecific variation in metabolic scaling relationships affects population density in different sized species. Here we predict inter-specific differences in metabolism from niche requirements, thereby allowing metabolic predictions of species distribution and abundance at fine spatial scales. Due to the differences in energetic efficiency required along harsh-benign gradients, an extremophile fish (brown mudfish, Neochanna apoda) living in harsh environments had slower metabolism, and thus higher population densities, compared to a fish species (banded kōkopu, Galaxias fasciatus) in physiologically more benign habitats. Interspecific differences in the intercepts for the relationship between body and density disappeared when species mass-specific metabolic rates, rather than body sizes, were used to predict density, implying population energy use was equivalent between mudfish and kōkopu. Nevertheless, despite significant interspecific differences in the slope of the metabolic scaling relationships, mudfish and kōkopu had a common slope for the relationship between body size and population density. These results support underlying logic of energetic equivalence between different size species implicit in metabolic theory. However, the precise slope of metabolic scaling relationships, which is the subject of much debate, may not be a reliable indicator of population density as expected under metabolic theory.

Amphibious fishes: evolution and phenotypic plasticity

Amphibious fishes spend part of their life in terrestrial habitats. The ability to tolerate life on land has evolved independently many times, with more than 200 extant species of amphibious fishes spanning 17 orders now reported. Many adaptations for life out of water have been described in the literature, and adaptive phenotypic plasticity may play an equally important role in promoting favourable matches between the terrestrial habitat and behavioural, physiological, biochemical and morphological characteristics. Amphibious fishes living at the interface of two very different environments must respond to issues relating to buoyancy/gravity, hydration/desiccation, low/high O2 availability, low/high CO2 accumulation and high/low NH3 solubility each time they traverse the air-water interface. Here, we review the literature for examples of plastic traits associated with the response to each of these challenges. Because there is evidence that phenotypic plasticity can facilitate the evolution of fixed traits in general, we summarize the types of investigations needed to more fully determine whether plasticity in extant amphibious fishes can provide indications of the strategies used during the evolution of terrestriality in tetrapods.

Effects of Acute Hypoxia and Reoxygenation on Physiological and Immune Responses and Redox Balance of Wuchang Bream (Megalobrama amblycephala Yih, 1955)

To study Megalobrama amblycephala adaption to water hypoxia, the changes in physiological levels, innate immune responses, redox balance of M.amblycephala during hypoxia were investigated in the present study. When M. amblycephala were exposed to different dissolved oxygen (DO) including control (DO: 5.5 mg/L) and acute hypoxia (DO: 3.5 and 1.0 mg/L, respectively), hemoglobin (Hb), methemoglobin (MetHb), glucose, Na⁺, succinatedehydrogenase (SDH), lactate, interferon alpha (IFNα), and lysozyme (LYZ), except hepatic glycogen and albumin gradually increased with the decrease of DO level. When M. amblycephala were exposed to different hypoxia time including 0.5 and 6 h (DO: 3.5 mg/L), and then reoxygenation for 24 h after 6 h hypoxia, Hb, MetHb, glucose, lactate, and IFNα, except Na⁺, SDH, hepatic glycogen, albumin, and LYZ increased with the extension of hypoxia time, while the above investigated indexes (except albumin, IFNα, and LYZ) decreased after reoxygenation. On the other hand, the liver SOD, CAT, hydrogen peroxide (H₂O₂), and total ROS were all remained at lower levels under hypoxia stress. Finally, Hif-1α protein in the liver, spleen, and gill were increased with the decrease of oxygen concentration and prolongation of hypoxia time. Interestingly, one Hsp70 isoforms mediated by internal ribozyme entry site (IRES) named junior Hsp70 was only detected in liver, spleen and gill. Taken together, these results suggest that hypoxia affects M. amblycephala physiology and reduces liver oxidative stress. Hypoxia-reoxygenation stimulates M. amblycephala immune parameter expressions, while Hsp70 response to hypoxia is tissue-specific.

Fisheries, low oxygen and climate change: how much do we really know?

As a result of long-term climate change, regions of the ocean with low oxygen concentrations are predicted to occur more frequently and persist for longer periods of time in the future. When low levels of oxygen are present, this places additional pressure on marine organisms to meet their metabolic requirements, with implications for growth, feeding and reproduction. Extensive research has been carried out on the effects of acute hypoxia, but far less on long-term chronic effects of low oxygen zones, especially with regard to commercially important fishes and shellfishes. To provide further understanding on how commercial species could be affected, the results of relevant experiments must support population and ecosystem models. This is not easy because individual effects are wide-ranging; for example, studies to date have shown that low oxygen zones can affect predator-prey relationships as some species are able to tolerate low oxygen more than others. Some fishes may move away from areas until oxygen levels return to acceptable levels, while others take advantage of a reduced start response in prey fishes and remain in the area to feed. Sessile or less mobile species such as shellfishes are unable to move out of depleted oxygen zones. Some species can tolerate low oxygen levels for only short periods of time, while others are able to acclimatize. To advance the knowledge-base further, a number of promising technological and modelling-based developments and the role of physiological data within these, are proposed. These include advances in remote telemetry (tagging) and sensor technologies, trait-based analyses to provide insight into how whole assemblages might respond in the future, research into long-term adaptability of species, population and ecosystem modelling techniques and quantification of economic effects. In addition, more detailed oxygen monitoring and projections are required to better understand the likely temporal and local-scale changes in oxygen.

Temporal variation on environmental variables and pollution indicators in marine sediments under sea Salmon farming cages in protected and exposed zones in the Chilean inland Southern Sea

The impacts of any activity on marine ecosystems will depend on the characteristics of the receptor medium and its resilience to external pressures. Salmon farming industry develops along a constant gradient of hydrodynamic conditions in the south of Chile. However, the influence of the hydrodynamic characteristics (weak or strong) on the impacts of intensive salmon farming is still poorly understood. This one year study evaluates the impacts of salmon farming on the marine sediments of both protected and exposed marine zones differing in their hydrodynamic characteristics. Six physico-chemical, five biological variables and seven indexes of marine sediments status were evaluated under the salmon farming cages and control sites. Our results identified a few key variables and indexes necessary to accurately evaluate the salmon farming impacts on both protected and exposed zones. Interestingly, the ranking of importance of the variables and the temporality of the observed changes, varied depending on the hydrodynamic characteristics. Biological variables (nematodes abundance) and environmental indexes (Simpson's dominance, Shannon's diversity and Pielou evenness) are the first to reflect detrimental impacts under the salmon farming cages. Then the physico-chemical variables such as redox, sulphurs and phosphorus in both zones also show detrimental impacts. Based on the present results we propose that the hydrodynamic regime is an important driver of the magnitude and temporality of the effects of salmon farming on marine sediments. The variables and indexes that best reflect the effects of salmon farming, in both protected and exposed zones, are also described.

A new analysis of hypoxia tolerance in fishes using a database of critical oxygen level (P crit)

Hypoxia is a common occurrence in aquatic habitats, and it is becoming an increasingly frequent and widespread environmental perturbation, primarily as the result of anthropogenic nutrient enrichment and climate change. An in-depth understanding of the hypoxia tolerance of fishes, and how this varies among individuals and species, is required to make accurate predictions of future ecological impacts and to provide better information for conservation and fisheries management. The critical oxygen level (P crit) has been widely used as a quantifiable trait of hypoxia tolerance. It is defined as the oxygen level below which the animal can no longer maintain a stable rate of oxygen uptake (oxyregulate) and uptake becomes dependent on ambient oxygen availability (the animal transitions to oxyconforming). A comprehensive database of P crit values, comprising 331 measurements from 96 published studies, covering 151 fish species from 58 families, provides the most extensive and up-to-date analysis of hypoxia tolerance in teleosts. Methodologies for determining P crit are critically examined to evaluate its usefulness as an indicator of hypoxia tolerance in fishes. Various abiotic and biotic factors that interact with hypoxia are analysed for their effect on P crit, including temperature, CO2, acidification, toxic metals and feeding. Salinity, temperature, body mass and routine metabolic rate were strongly correlated with P crit; 20% of variation in the P crit data set was explained by these four variables. An important methodological issue not previously considered is the inconsistent increase in partial pressure of CO2 within a closed respirometer during the measurement of P crit. Modelling suggests that the final partial pressure of CO2 reached can vary from 650 to 3500 µatm depending on the ambient pH and salinity, with potentially major effects on blood acid-base balance and P crit itself. This database will form part of a widely accessible repository of physiological trait data that will serve as a resource to facilitate future studies of fish ecology, conservation and management.

Phenotypic plasticity associated to environmental hypoxia in the neotropical serrasalmid Piaractus mesopotamicus (Holmberg, 1887) (Characiformes: Serrasalmidae)

ABSTRACT Many South American characid fishes develop reversible dermal protuberances in the jaws to optimize aquatic surface respiration (ASR) during hypoxia. To date, basic aspects of this adaptation remain unknown, mainly due to the scarcity of experimental studies. In laboratory experiments, we determined time necessary for the complete formation and reversion of these structures in Piaractus mesopotamicus , and studied comparatively behavioral, morphological, and respiratory responses along gradients of dissolved oxygen (DO) concentration. Morphological changes during hypoxia consisted in dermal protuberances of lower lip, anterior border of maxillary and distal border of opercular valve, increasing the known number of structures modified. These structures developed completely in less than 6 hours and reversed in less than 3 hours. Most of observed traits showed a logistic response curve with threshold DO values between 0.90 and 2.70 mgL-1. Respiratory frequency and opercular valve development showed similar threshold values above the level of tolerance of DO, whereas ASR and dermal protuberances of the jaws showed threshold values below this level. This observation supports the functional link between these groups of behavioral and morphological traits. This study demonstrates that this species is able to modify reversibly portions of the respiratory system to optimize responses to hypoxia.

Variations in temperature acclimation effects on glycogen storage, hypoxia tolerance and swimming performance with seasonal acclimatization in juvenile Chinese crucian carp

The aim of this study was to test whether temperature acclimation (10 vs 20 °C) effects on tissue glycogen content, hypoxia tolerance, and swimming performance of Chinese crucian carp (Carassius auratus) varied with seasonal acclimatization (winter vs spring) and potential combined interactions. Both the routine metabolic rate (MO(2rout)) and critical oxygen tension (P(crit)) of the MO(2rout) increased significantly with temperature, whereas the seasonal acclimatization showed no significant effect. Only the high temperature group that acclimatized in spring showed a significantly higher aquatic surface respiration (ASR(crit)) value compared with the other three groups. Fish in spring tended to show ASR behavior at higher oxygen tension compared with those in winter, which might have been caused by a more active lifestyle. Time to show LOE prolonged by 25-34% under low temperature. Spring fish showed 20% shorter LOE duration at 10 °C, whereas the difference tended to vanish at 20 °C. Glycogen contents in both liver and muscle were higher in winter than spring. The liver and muscle glycogen content decreased by 5-42% after exposure to anoxic conditions, whereas the magnitude was much smaller in spring. When fish swam in normoxic conditions, fish in higher temperatures showed higher critical swimming speed (Ucrit) than low temperature (5.49 vs 3.74 BL s(-1) in winter and 4.27 vs 3.21 BL s(-1) in spring), whereas fish in winter also showed higher U(crit) than fish in spring for each temperature. However, when fish swam in hypoxic waters, fish in higher temperatures showed a more profound decrease (52-61%) in U(crit) compared to those in lower temperature (25-27%). Fish in lower temperatures that had acclimatized in winter showed the highest U(crit), which might have been caused by higher glycogen storage. The present study suggested that both glycogen storage and alterations in lifestyle had profound effects on hypoxia tolerance and swimming performance, which resulted in a profound difference between seasons and acclimation temperatures.

Effect of temperature on hypoxia tolerance and its underlying biochemical mechanism in two juvenile cyprinids exhibiting distinct hypoxia sensitivities

It is increasingly important to investigate the effect of temperature on hypoxia tolerance in fish species, as worldwide hypoxia worsens with increases in global warming. We selected the hypoxia-tolerant crucian carp (Carassius carassius) and the hypoxia-sensitive Chinese bream (Parabramis pekinensis) as model fish and investigated their hypoxia tolerance based on the critical oxygen tension of the routine metabolic rate (M˙O2rout) (Pcrit), aquatic surface respiration (ASRcrit) and loss of equilibrium (LOEcrit) after two weeks of acclimation at either 10, 20 or 30 °C. We also measured the tissue substrate (glycogen and glucose of muscle and liver) and lactate levels of both normoxia- and hypoxia-treated fish (post-LOE). Crucian carp exhibited significantly lower Pcrit and LOEcrit but not ASRcrit. Crucian carp possessed higher hypoxia tolerance, partially due to a higher tissue glycogen reserve, which provides cellular fuel under severe hypoxia, as well as higher lactate tolerance and clearance ability than Chinese bream. The hypoxia tolerance was maintained in crucian carp but was decreased in Chinese bream as the temperature increased. The difference between the two species is based on the greater recruitment of tissue glycogen, resulting in an increased level of cellular fuel during hypoxia in crucian carp than in Chinese bream. In addition, crucian carp possessed the greater liver lactate clearance capacity, and the smaller increase in the M˙O2rout at higher temperatures compared to Chinese bream. Furthermore, substrate shortage and decreased lactate tolerance at high temperatures in Chinese bream might also contribute to the difference in hypoxia tolerance between the two species.

Constraints upon the response of fish and crayfish to environmental flow releases in a regulated headwater stream network

In dry climate zones, headwater streams are often regulated for water extraction causing intermittency in perennial streams and prolonged drying in intermittent streams. Regulation thereby reduces aquatic habitat downstream of weirs that also form barriers to migration by stream fauna. Environmental flow releases may restore streamflow in rivers, but are rarely applied to headwaters. We sampled fish and crayfish in four regulated headwater streams before and after the release of summer-autumn environmental flows, and in four nearby unregulated streams, to determine whether their abundances increased in response to flow releases. Historical data of fish and crayfish occurrence spanning a 30 year period was compared with contemporary data (electrofishing surveys, Victoria Range, Australia; summer 2008 to summer 2010) to assess the longer-term effects of regulation and drought. Although fish were recorded in regulated streams before 1996, they were not recorded in the present study upstream or downstream of weirs despite recent flow releases. Crayfish (Geocharax sp. nov. 1) remained in the regulated streams throughout the study, but did not become more abundant in response to flow releases. In contrast, native fish (Gadopsis marmoratus, Galaxias oliros, Galaxias maculatus) and crayfish remained present in unregulated streams, despite prolonged drought conditions during 2006-2010, and the assemblages of each of these streams remained essentially unchanged over the 30 year period. Flow release volumes may have been too small or have operated for an insufficient time to allow fish to recolonise regulated streams. Barriers to dispersal may also be preventing recolonisation. Indefinite continuation of annual flow releases, that prevent the unnatural cessation of flow caused by weirs, may eventually facilitate upstream movement of fish and crayfish in regulated channels; but other human-made dispersal barriers downstream need to be identified and ameliorated, to allow native fish to fulfil their life cycles in these headwater streams.

The importance of cutaneous gas exchange during aerial and aquatic respiration in galaxiids

The Canterbury mudfish Neochanna burrowsius was found to be a pseudo-aestivating galaxiid with a low metabolic rate and significant cutaneous oxygen uptake (c. 43%) in both air and water. Another galaxiid, inanga Galaxias maculatus, had a higher metabolic rate in both media but the proportion of oxygen uptake met by cutaneous respiration rose significantly from 38 to 63% when the fish were exposed to air. Besides its important role in oxygen uptake, the skin of both species also contributed significantly to excretion of carbon dioxide in air, indicating the critical role of the integument as a respiratory tissue. In air, G. maculatus may increase cutaneous gas exchange to meet metabolic demands owing to the reduced utility of the gills, but as emersed G. maculatus were only able to maintain metabolic rates at c. 67% of that measured in water, this strategy probably only permits short-term survival. By contrast, the low and unchanging metabolic rate in water and air in N. burrowsius is a feature that may facilitate tolerance of long periods of emersion in the desiccating environments they inhabit.

The impacts of stress on sodium metabolism and copper accumulation in a freshwater fish

In freshwater fish, stress can often result in significant modifications to Na(+) metabolism and may be an important aspect to consider in conservation efforts; as maintaining ion balance is critical to survival and ion transport is also a key determinant of metal toxicity. In order to better quantify the response of stress, Na(+) influx, Na(+) efflux, and copper accumulation were measured as a result of handling stress in inanga (Galaxias maculatus). This species is a culturally and economically important fish in New Zealand as one of the major species in the local 'whitebait' fishery. Na(+) influx rates in inanga were found to be 2-3 times greater after handling than in 'recovered' fish, and Na(+) efflux rates increased in the range of 5-6 times. Both influx and efflux rates quickly returned to resting levels within 24h. Increases in Na(+) efflux were strongly correlated with opercular beat frequency. This suggests an increas in ventilation, and subsequent enhanced diffusive loss of Na(+), as the mechanism of increased Na(+) efflux. Total body copper levels were also measured under similar treatments. Fish had significantly higher levels of copper directly after handling than following a 24h recovery; likely due to a shared Na(+)/copper uptake pathway. As accumulation is linked to toxicity, fish exposed to elevated copper levels in stressful environments will consequently be more at risk to metal toxicity. In a natural environment, stress can come from many different sources; among which, anthropogenic disturbances can often be a cause. Given that inanga must migrate through metal-contaminated coastal regions to reach breeding habitats, they will be exposed to toxicants under conditions where perfusion and ventilation of the gill is increased. As such, ion loss would be exacerbated, leading to an enhanced compensatory ion uptake and an increase in accumulation of ion-mimicking toxicants such as copper, exacerbating toxicity. This is a concern as conservation efforts in more disruptive environments may not be adequately protected.

Relationship between fish size and metabolic rate in the oxyconforming inanga Galaxias maculatus reveals size-dependent strategies to withstand hypoxia

The relationship between metabolic rate and body size in animals is unlikely to be a constant but is instead shaped by a variety of intrinsic (i.e., physiological) and extrinsic (i.e., environmental) factors. This study examined the effect of environmental oxygen tension on oxygen consumption as a function of body mass in the galaxiid fish, inanga (Galaxias maculatus). As an oxyconformer, this fish lacks overt intrinsic regulation of oxygen consumption, eliminating this as a factor affecting the scaling relationship at different oxygen tensions. The relationship between oxygen consumption rate and body size was best described by a power function, with an exponent of 0.82, higher than the theoretical values of 0.66 or 0.75. The value of this exponent was significantly altered by environmental P(O2), first increasing as P(O2) decreased and then declining at the lowest P(O2) tested. These data suggest that the scaling exponent is species specific and regulated by extrinsic factors. Furthermore, the external P(O2) at which fish lost equilibrium was related to fish size, an effect explained by the scaling of anaerobic capacity with fish mass. Therefore, although bigger fish were forced to depress aerobic metabolism more rapidly than small fish when exposed to progressive hypoxia, they were better able to enact anaerobic metabolism, potentially extending their survival in hypoxia.

Genome analysis and signature discovery for diving and sensory properties of the endangered Chinese alligator

Crocodilians are diving reptiles that can hold their breath under water for long periods of time and are crepuscular animals with excellent sensory abilities. They comprise a sister lineage of birds and have no sex chromosome. Here we report the genome sequence of the endangered Chinese alligator (Alligator sinensis) and describe its unique features. The next-generation sequencing generated 314 Gb of raw sequence, yielding a genome size of 2.3 Gb. A total of 22 200 genes were predicted in Alligator sinensis using a de novo, homology- and RNA-based combined model. The genetic basis of long-diving behavior includes duplication of the bicarbonate-binding hemoglobin gene, co-functioning of routine phosphate-binding and special bicarbonate-binding oxygen transport, and positively selected energy metabolism, ammonium bicarbonate excretion and cardiac muscle contraction. Further, we elucidated the robust Alligator sinensis sensory system, including a significantly expanded olfactory receptor repertoire, rapidly evolving nerve-related cellular components and visual perception, and positive selection of the night vision-related opsin and sound detection-associated otopetrin. We also discovered a well-developed immune system with a considerable number of lineage-specific antigen-presentation genes for adaptive immunity as well as expansion of the tripartite motif-containing C-type lectin and butyrophilin genes for innate immunity and expression of antibacterial peptides. Multifluorescence in situ hybridization showed that alligator chromosome 3, which encodes DMRT1, exhibits significant synteny with chicken chromosome Z. Finally, population history analysis indicated population admixture 0.60-1.05 million years ago, when the Qinghai-Tibetan Plateau was uplifted.

Gill remodelling during terrestrial acclimation reduces aquatic respiratory function of the amphibious fish Kryptolebias marmoratus

The skin-breathing amphibious fish Kryptolebias marmoratus experiences rapid environmental changes when moving between water- and air-breathing, but remodelling of respiratory morphology is slower (~1 week). We tested the hypotheses that (1) there is a trade-off in respiratory function of gills displaying aquatic versus terrestrial morphologies and (2) rapidly increased gill ventilation is a mechanism to compensate for reduced aquatic respiratory function. Gill surface area, which varied inversely to the height of the interlamellar cell mass, was increased by acclimating fish for 1 week to air or low ion water, or decreased by acclimating fish for 1 week to hypoxia (~20% dissolved oxygen saturation). Fish were subsequently challenged with acute hypoxia, and gill ventilation or oxygen uptake was measured. Fish with reduced gill surface area increased ventilation at higher dissolved oxygen levels, showed an increased critical partial pressure of oxygen and suffered impaired recovery compared with brackish water control fish. These results indicate that hyperventilation, a rapid compensatory mechanism, was only able to maintain oxygen uptake during moderate hypoxia in fish that had remodelled their gills for land. Thus, fish moving between aquatic and terrestrial habitats may benefit from cutaneously breathing oxygen-rich air, but upon return to water must compensate for a less efficient branchial morphology (mild hypoxia) or suffer impaired respiratory function (severe hypoxia).

Should I stay or should I go?: Physiological, metabolic and biochemical consequences of voluntary emersion upon aquatic hypoxia in the scaleless fish Galaxias maculatus

Hypoxia represents a significant challenge to most fish, forcing the development of behavioural, physiological and biochemical adaptations to survive. It has been previously shown that inanga (Galaxias maculatus) display a complex behavioural repertoire to escape aquatic hypoxia, finishing with the fish voluntarily emerging from the water and aerially respiring. In the present study we evaluated the physiological, metabolic and biochemical consequences of both aquatic hypoxia and emersion in inanga. Inanga successfully tolerated up to 6 h of aquatic hypoxia or emersion. Initially, this involved enhancing blood oxygen-carrying capacity, followed by the induction of anaerobic metabolism. Only minor changes were noted between emersed fish and those maintained in aquatic hypoxia, with the latter group displaying a higher mean cell haemoglobin content and a reduced haematocrit after 6 h. Calculations suggest that inanga exposed to both aquatic hypoxia and air reduced oxygen uptake and also increased anaerobic contribution to meet energy demands, but the extent of these changes was small compared with hypoxia-tolerant fish species. Overall, these findings add to previous studies suggesting that inanga are relatively poorly adapted to survive aquatic hypoxia.

A novel oxyconforming response in the freshwater fish Galaxias maculatus

How fish oxygen consumption is modulated by external PO(2) has long been a matter of interest, yet is an experimentally complicated question to answer. In this study closed and semi-closed respirometry were used to evaluate the oxygen consumption rate of the scaleless galaxiid fish, inanga (Galaxias maculatus) as a function of decreasing external PO(2). Both respirometry techniques showed that as environmental oxygen levels declined, oxygen consumption rates also decreased. At no point did inanga regulate oxygen consumption. This is strong evidence that inanga is an oxyconformer. Partitioned respirometry experiments showed that skin plays an important role in oxygen uptake in this fish species, and cutaneous oxygen uptake may have an important role in shaping the oxygen consumption response to hypoxia.