Routine Metabolic Rate Research Articles

The ability of blue crab (Callinectes sapidus, Rathbun 1886) to sustain aerobic metabolism during hypoxia

To assess the ability of adult blue crab (Callinectes sapidus) to function under the hypoxic conditions becoming increasingly common in their inshore habitats, critical oxygen levels (i.e., the minimum oxygen levels at which aerobic metabolism can be maintained) were determined over a range of metabolic rates using automated intermittent-flow respirometry. Different metabolic rates were induced by conducting experiments at three temperatures (17°, 23°, and 28°C), testing recently fed crabs, and those infected with the parasitic dinoflagellate Hematodinium perezi. The effects of hypoxia on the metabolic rates and recovery times of individuals following enforced exhaustive activity, and metabolic rates following feeding, were also measured to determine the levels of hypoxia likely to impact feeding, digestion, and overall energetics. Contrary to previously published results, blue crab were found not to be partial oxygen conformers (i.e., where metabolic rate falls in concert with reductions in ambient oxygen), but rather to be oxygen regulators (i.e., to have the ability to maintain a constant aerobic metabolic rate until the critical oxygen level was reached). By this measure, at routine metabolic rates blue crab are as hypoxia-tolerant as other decapod crustaceans with a median critical oxygen level of ~20% air saturation (at 17° and 23°C). Critical oxygen levels increased in concert with the increases in metabolic rate occurring at 28°C, in individuals infected with H. perezii, and those recently fed. At the highest metabolic rates (measured in recently fed individuals at 28°C) median critical oxygen level was ~45% air saturation. Consistent with this latter observation, metabolic rates after feeding or exercise were not compromised until below 50% air saturation, although maximum metabolic rates were lower at this level of hypoxia. The results presented are consistent with the oxygen levels shown to influence blue crab behaviors (~2 to 4mgl−1) in both field and laboratory settings.

Testing for relationships between individual crab behavior and metabolic rate across ecological contexts

Two classes of explanations for covariation between activity level and metabolic rate among conspecifics have been proposed. First, individual-level variation in activity exhibited during the measurement of metabolic rate should covary with routine metabolic rate because movement increases respiration (a methodological relationship). Second, energetic-based hypotheses posit relationships between individual activity measured under more natural conditions and baseline measures of metabolic rate, among other behavioral, physiological, and life-history traits (functional relationships). Here, we examined these potential relationships between individual behavior and metabolic rate in the mud crab (Panopeus herbstii). Specifically, we tested for (1) an effect of crab activity in metabolic chambers (activitychamber) on routine metabolic rate (RMR), and (2) an effect of crab activity in mesocosms that mimicked field conditions (activitymesocosm) on standard metabolic rate (SMR). To test for context dependence, we assessed both activity-metabolic rate relationships in the absence and presence of predation threat from toadfish (Opsanus tau) in the form of waterborne chemical cues. Individual variation in activitymesocosm and RMR was repeatable over time. In support of a methodological relationship, individual differences in RMR were partially explained by crab activitychamber. After accounting for this methodological relationship, individual SMR was inversely related to activitymesocosm, supporting an allocation model that predicts behavior and baseline metabolic rate compete for finite energy reserves. We found no evidence of context dependence in either activity-metabolic rate relationship. Thus, our study emphasizes the importance of considering methodological artifacts in elucidating functional relationships between individual behavior and energetics.

Steelhead trout Oncorhynchus mykiss metabolic rate is affected by dietary Aloe vera inclusion but not by mounting an immune response against formalin-killed Aeromonas salmonicida.

The oxygen consumption (MO2) of two groups of 10° C acclimated steelhead trout Oncorhynchus mykiss was measured for 72 h after they were given a 100 µl kg(-1) intraperitoneal injection of formalin-killed Aeromonas salmonicida (ASAL) or phosphate-buffered saline (PBS). In addition, plasma cortisol levels were measured in fish from both groups prior to, and 1 and 3 h after, they were given a 30 s net stress. The first group was fed an unaltered commercial diet for 4 weeks, whereas the second group was fed the same diet but with 0·5% (5 g kg(-1) ) Aloe vera powder added; A. vera has potential as an immunostimulant for use in aquaculture, but its effects on basal and acute phase response (APR)-related metabolic expenditures and stress physiology, are unknown. Injection of ASAL v. PBS had no measurable effect on the MO2 of O. mykiss indicating that the APR in this species is not associated with any net increase in energy expenditure. In contrast, incorporating 0·5% A. vera powder into the feed decreased routine metabolic rate by c. 8% in both injection groups and standard metabolic rate in the ASAL-injected group (by c. 4 mg O2 kg(-1) h(-1) ; 5%). Aloe vera fed fish had resting cortisol levels that were approximately half of those in fish on the commercial diet (c. 2·5 v. 5·0 ng ml(-1) ), but neither this difference nor those post-stress reached statistical significance (P > 0·05).

Branchial NH 4 + ‐dependent Acid‐base Transport Mechanisms and Energy Metabolism of Squid ( Sepioteuthis lessoniana ) Affected by Seawater Acidification

CO2 induced pH fluctuation in the extracellular compartments are a unifying physiological phenomenon that all animals are confronted with. Cephalopods have evolved acid-base regulatory abilities to accommodate temporal pH fluctuations via highly pH sensitive hemocyanins. Interestingly, it has been suggested that metabolic energetic limitations may be a critical factor when exposed to acidified seawater. Here we show the effects of short-term (few hours) to medium-term (up to 168 h) seawater acidification on pelagic squids, Sepioteuthis lessoniana. Routine metabolic rates, NH4+ secretion and extracellular acid-base balance were monitored during exposure to control (pH 8.1) and acidified conditions of pH 7.7 and 7.3 along a period of 168h. Metabolic rates were significantly depressed by 40% after 168 hours in pH 7.3 conditions exposure. The extracellular pH is fully restored within 20 h and is maintained along the period of 168 h, and the accompanied acid-base transporter transcripts were up-regulated including V-type H+-ATPase (VHA), Rhesus protein (RhP), Na/HCO3- cotransporter (NBC) and cystolic carbonic anhydrase (CAc). Immunohistochemical analyses demonstrated co-localization of Na+/K+-ATPase (NKA), VHA in basolateral and Na+/H+-exchanger 3 (NHE3), RhP in apical membranes of the branchial epithelium. Protein levels of branchial VHA and RhP were increased in response to acidified conditions. The present work demonstrated the NH4+ based proton secretion via RhP may contribute to a well-developed acid-base regulatory machinery in branchial epithelia of cephalopods.

Reduction of Energetic Demands through Modification of Body Size and Routine Metabolic Rates in Extremophile Fish.

Variation in energy availability or maintenance costs in extreme environments can exert selection for efficient energy use, and reductions in organismal energy demand can be achieved in two ways: reducing body mass or metabolic suppression. Whether long-term exposure to extreme environmental conditions drives adaptive shifts in body mass or metabolic rates remains an open question. We studied body size variation and variation in routine metabolic rates in locally adapted populations of extremophile fish (Poecilia mexicana) living in toxic, hydrogen sulfide-rich springs and caves. We quantified size distributions and routine metabolic rates in wild-caught individuals from four habitat types. Compared with ancestral populations in nonsulfidic surface habitats, extremophile populations were characterized by significant reductions in body size. Despite elevated metabolic rates in cave fish, the body size reduction precipitated in significantly reduced energy demands in all extremophile populations. Laboratory experiments on common garden-raised fish indicated that elevated routine metabolic rates in cave fish likely have a genetic basis. The results of this study indicate that adaptation to extreme environments directly impacts energy metabolism, with fish living in cave and sulfide spring environments expending less energy overall during routine metabolism.

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 (M·O2rout) and critical oxygen tension (Pcrit) of the M·O2rout 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 (ASRcrit) 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 3.74BLs−1 in winter and 4.27 vs 3.21BLs−1 in spring), whereas fish in winter also showed higher Ucrit 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 Ucrit compared to those in lower temperature (25–27%). Fish in lower temperatures that had acclimatized in winter showed the highest Ucrit, 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.

Shape shifting predicts ontogenetic changes in metabolic scaling in diverse aquatic invertebrates.

Metabolism fuels all biological activities, and thus understanding its variation is fundamentally important. Much of this variation is related to body size, which is commonly believed to follow a 3/4-power scaling law. However, during ontogeny, many kinds of animals and plants show marked shifts in metabolic scaling that deviate from 3/4-power scaling predicted by general models. Here, we show that in diverse aquatic invertebrates, ontogenetic shifts in the scaling of routine metabolic rate from near isometry (bR = scaling exponent approx. 1) to negative allometry (bR < 1), or the reverse, are associated with significant changes in body shape (indexed by bL = the scaling exponent of the relationship between body mass and body length). The observed inverse correlations between bR and bL are predicted by metabolic scaling theory that emphasizes resource/waste fluxes across external body surfaces, but contradict theory that emphasizes resource transport through internal networks. Geometric estimates of the scaling of surface area (SA) with body mass (bA) further show that ontogenetic shifts in bR and bA are positively correlated. These results support new metabolic scaling theory based on SA influences that may be applied to ontogenetic shifts in bR shown by many kinds of animals and plants.

No evidence for a bioenergetic advantage from forced swimming in rainbow trout under a restrictive feeding regime

Sustained swimming at moderate speeds is considered beneficial in terms of the productive performance of salmonids, but the causative mechanisms have yet to be unequivocally established. In the present study, the effects of moderate exercise on the bioenergetics of rainbow trout were assessed during a 15 week growth experiment, in which fish were reared at three different current speeds: 1 BL s−1, 0.5 BL s−1 and still water (≈ 0 BL s−1). Randomly selected groups of 100 fish were distributed among twelve 600 L tanks and maintained on a restricted diet regime. Specific growth rate (SGR) and feed conversion ratio (FCR) were calculated from weight and length measurements every 3 weeks. Routine metabolic rate (RMR) was measured every hour as rate of oxygen consumption in the tanks, and was positively correlated with swimming speed. Total ammonia nitrogen (TAN) excretion rates showed a tendency to decrease with increasing swimming speeds, yet neither they nor the resulting nitrogen quotients (NQ) indicated that swimming significantly reduced the fraction of dietary protein used to fuel metabolism. Energetic budgets revealed a positive correlation between energy expenditure and the current speed at which fish were reared, fish that were forced to swim and were fed restrictively consequentially had poorer growth and feed utilization. The results show that for rainbow trout, water current can negatively affect growth despite promoting minor positive changes in substrate utilization. We hypothesize that this may be the result of either a limited dietary energy supply from diet restriction being insufficient for both covering the extra costs of swimming and supporting enhanced growth.

Interactive effects of ocean acidification and rising sea temperatures alter predation rate and predator selectivity in reef fish communities.

Ocean warming and acidification are serious threats to marine life. While each stressor alone has been studied in detail, their combined effects on the outcome of ecological interactions are poorly understood. We measured predation rates and predator selectivity of two closely related species of damselfish exposed to a predatory dottyback. We found temperature and CO2 interacted synergistically on overall predation rate, but antagonistically on predator selectivity. Notably, elevated CO2 or temperature alone reversed predator selectivity, but the interaction between the two stressors cancelled selectivity. Routine metabolic rates of the two prey showed strong species differences in tolerance to CO2 and not temperature, but these differences did not correlate with recorded mortality. This highlights the difficulty of linking species-level physiological tolerance to resulting ecological outcomes. This study is the first to document both synergistic and antagonistic effects of elevated CO2 and temperature on a crucial ecological process like predator-prey dynamics.

Environmental heterogeneity generates opposite gene-by-environment interactions for two fitness-related traits within a population.

Theory predicts that environmental heterogeneity offers a potential solution to the maintenance of genetic variation within populations, but empirical evidence remains sparse. The live-bearing fish Xiphophorus variatus exhibits polymorphism at a single locus, with different alleles resulting in up to five distinct melanistic "tailspot" patterns within populations. We investigated the effects of heterogeneity in two ubiquitous environmental variables (temperature and food availability) on two fitness-related traits (upper thermal limits and body condition) in two different tailspot types (wild-type and upper cut crescent). We found gene-by-environment (G × E) interactions between tailspot type and food level affecting upper thermal limits (UTL), as well as between tailspot type and thermal environment affecting body condition. Exploring mechanistic bases underlying these G × E patterns, we found no differences between tailspot types in hsp70 gene expression despite significant overall increases in expression under both thermal and food stress. Similarly, there was no difference in routine metabolic rates between the tailspot types. The reversal of relative performance of the two tailspot types under different environmental conditions revealed a mechanism by which environmental heterogeneity can balance polymorphism within populations through selection on different fitness-related traits.

Metabolic depression in cunner (Tautogolabrus adspersus) is influenced by ontogeny, and enhances thermal tolerance.

To examine the effect of ontogeny on metabolic depression in the cunner (Tautogolabrus adspersus), and to understand how ontogeny and the ability to metabolically depress influence this species' upper thermal tolerance: 1) the metabolic rate of 9°C-acclimated cunner of three size classes [0.2–0.5 g, young of the year (YOY); 3–6 g, small; and 80–120 g, large (adult)] was measured during a 2°C per day decrease in temperature; and 2) the metabolic response of the same three size classes of cunner to an acute thermal challenge [2°C h−1 from 10°C until Critical Thermal Maximum, CTMax] was examined, and compared to that of the Atlantic cod (Gadus morhua). The onset-temperature for metabolic depression in cunner increased with body size, i.e. from 5°C in YOY cunner to 7°C in adults. In contrast, the extent of metabolic depression was ∼80% (Q10 = ∼15) for YOY fish, ∼65% (Q10 = ∼8) for small fish and ∼55% (Q10 = ∼5) for adults, and this resulted in the metabolic scaling exponent (b) gradually increasing from 0.84 to 0.92 between 9°C to 1°C. All size classes of cunner had significantly (approximately 60%) lower routine metabolic rates at 10°C than Atlantic cod. However, there was no species' difference in the temperature-induced maximum metabolic rate, and this resulted in factorial metabolic scope values that were more than two-fold greater for cunner, and CTMax values that were 6–9°C higher (∼21 vs. 28°C). These results: 1) show that ontogeny influences the temperature of initiation and the extent of metabolic depression in cunner, but not O2 consumption when in a hypometabolic state; and 2) suggest that the evolution of cold-induced metabolic depression in this northern wrasse species has not resulted in a trade-off with upper thermal tolerance, but instead, an enhancement of this species' metabolic plasticity.

Early-life exposure to climate change impairs tropical shark survival.

Sharks are one of the most threatened groups of marine animals worldwide, mostly owing to overfishing and habitat degradation/loss. Although these cartilaginous fish have evolved to fill many ecological niches across a wide range of habitats, they have limited capability to rapidly adapt to human-induced changes in their environments. Contrary to global warming, ocean acidification was not considered as a direct climate-related threat to sharks. Here we show, for the first time, that an early ontogenetic acclimation process of a tropical shark (Chiloscyllium punctatum) to the projected scenarios of ocean acidification (ΔpH = 0.5) and warming (+4°C; 30°C) for 2100 elicited significant impairments on juvenile shark condition and survival. The mortality of shark embryos at the present-day thermal scenarios was 0% both at normocapnic and hypercapnic conditions. Yet routine metabolic rates (RMRs) were significantly affected by temperature, pH and embryonic stage. Immediately after hatching, the Fulton condition of juvenile bamboo sharks was significantly different in individuals that experienced future warming and hypercapnia; 30 days after hatching, survival rapidly declined in individuals experiencing both ocean warming and acidification (up to 44%). The RMR of juvenile sharks was also significantly affected by temperature and pH. The impact of low pH on ventilation rates was significant only under the higher thermal scenario. This study highlights the need of experimental-based risk assessments of sharks to climate change. In other words, it is critical to directly assess risk and vulnerability of sharks to ocean acidification and warming, and such effort can ultimately help managers and policy-makers to take proactive measures targeting most endangered species.

The use of spontaneous behavior, swimming performances and metabolic rate to evaluate toxicity of PFOS on topmouth gudgeon Pseudorasbora parva

Perfluorooctane sulfonate (PFOS) is a ubiquitous environmental contaminant that has been found to pose various risks to fish health and the safety of aquatic ecosystem. Swimming performance is an integrated index of fitness in fish. However, little research has sought on the effects of PFOS on swimming performances of fish. Experiments were carried out to clarify the impacts of acute exposure to PFOS on behavior, swimming ability and metabolic rate in topmouth gudgeon (Pseudorasbora parva), to understand the underlying ecotoxicological effects of waterborne PFOS exposure on fish physiology and behavior. Fish were exposed to PFOS (0, 0.5, 2, 8 or 32 mg/L) for 96 h. Afterwards, the routine metabolic rate (RMR), spontaneous swimming behavior (SSB), fast-start swimming performance (FSP) and critical swimming speed (Ucrit) of the topmouth gudgeon were examined. The results show reduced behavioral performance and increased physiological stress with increasing PFOS concentration. Both RMR, SSB and Ucrit were significantly affected by PFOS exposure (p < 0.05). The lowest observed effect concentration (LOEC) is 2 mg/L for SSB. PFOS treatment resulted in increased RMR (p = 0.001) and decreased Ucrit (p = 0.005), whereas FSP was not influenced by PFOS (p > 0.05). The results indicate that the anaerobic swimming capacity was conserved, but the metabolic level, SSB and aerobic swimming performance in topmouth gudgeon were susceptible to PFOS contamination, and hence might be useful as considerable potential biomarkers of pollution.

Physiological mechanisms underlying individual variation in tolerance of food deprivation in juvenile European sea bass, Dicentrarchus labrax.

Although food deprivation is a major ecological pressure in fishes, there is wide individual variation in tolerance of fasting, whose mechanistic bases are poorly understood. Two thousand individually tagged juvenile European sea bass were submitted to two 'fasting/feeding' cycles each comprising 3 weeks of food deprivation followed by 3 weeks of ad libitum feeding at 25°C. Rates of mass loss during the two fasting periods were averaged for each individual to calculate a population mean. Extreme fasting tolerant (FT) and sensitive (FS) phenotypes were identified that were at least one and a half standard deviations, on opposing sides, from this mean. Respirometry was used to investigate two main hypotheses: (1) tolerance of food deprivation reflects lower mass-corrected routine metabolic rate (RMR) in FT phenotypes when fasting, and (2) tolerance reflects differences in substrate utilisation; FT phenotypes use relatively less proteins as metabolic fuels during fasting, measured as their ammonia quotient (AQ), the simultaneous ratio of ammonia excretion to RMR. There was no difference in mean RMR between FT and FS over 7 days fasting, being 6.70±0.24 mmol h(-1) fish(-1) (mean ± s.e.m., N=18) versus 6.76±0.22 mmol h(-1) fish(-1) (N=17), respectively, when corrected to a body mass of 130 g. For any given RMR, however, the FT lost mass at a significantly lower rate than FS, overall 7-day average being 0.72±0.05 versus 0.90±0.05 g day(-1) fish(-1), respectively (P<0.01, t-test). At 20 h after receiving a ration equivalent to 2% body mass as food pellets, ammonia excretion and simultaneous RMR were elevated and similar in FT and FS, with AQs of 0.105±0.009 and 0.089±0.007, respectively. At the end of the period of fasting, ammonia excretion and RMR had fallen in both phenotypes, but AQ was significantly lower in FT than FS, being 0.038±0.004 versus 0.061±0.005, respectively (P<0.001, t-test). There was a direct linear relationship between individual fasted AQ and rate of mass loss, with FT and FS individuals distributed at opposing lower and upper extremities, respectively. Thus the difference between the phenotypes in their tolerance of food deprivation did not depend upon their routine energy use when fasting. Rather, it depended upon their relative use of tissue proteins as metabolic fuels when fasting, which was significantly lower in FT phenotypes.

Bioenergetics of captive Pacific bluefin tuna (Thunnus orientalis)

Tuna bioenergetics can be described by the following relationship: the energy available for growth is equal to the food energy minus all metabolic costs. These costs include routine metabolic rate, specific dynamic action, increased activity level, eliminated waste, and gonadal development. Captive populations of Pacific bluefin tuna ( Thunnus orientalis ) were held at ~ 20 °C in fiberglass tanks and fed on a regular schedule with a diet formulated to achieve an energetic content of 176 ± 36 kJ · kg − 1 of biomass · day − 1 (mean ± s.d.). To conduct a bioenergetic study, growth rates during the captive period and tissue energy values post-mortem were empirically determined. Daily growth rates were obtained from a von Bertalanffy growth function based on curved fork length (CFL) measurements of live fish and post-mortem morphometrics. The parameters obtained for the captive bluefin growth function were 225.13 cm straight fork length (SFL), 0.173, and − 0.497 years for L ∞ , k , and t o , respectively. The growth equation, SFL = 225.13 · (1 − e (− 0.173(t-(− 0.497))) ) in conjunction with the length-mass regression (where body mass M = 4.98 × 10 − 6 × SFL 3.3186 ) gave a daily growth increase of 32.60 ± 2.40 g · day − 1 for Pacific bluefin tuna of 2.2 years of age and 11.4 ± 1.0 kg (the average age and mass of a fish in the study). The average tissue energy value from four sampled tuna was 7.66 ± 0.40 kJ · g − 1 , and applying the daily growth increase estimate provides a daily energy gain of 249.7 kJ, which is 12.4% of an ingested meal's total energy content. A food conversion ratio of 17.8:1 is estimated for a meal consisting solely of sardines and 22.6:1 for a mixed diet consisting of sardines, squid, and a gelatin-vitamin mixture at the stated feeding regimen. This paper presents the first data on actual food conversion ratios and bioenergetic utilization for Pacific bluefin tuna. • This paper presents the first data on actual food conversion ratios and bioenergetic utilization for Pacific bluefin tuna. • This work gives an important insight into the energy requirements of an endothermic fish species. • New information on tuna growth rates may be useful in understanding Pacific bluefin tuna aquaculture operations.

Brain functioning under acute hypothermic stress supported by dynamic monocarboxylate utilization and transport in ectothermic fish

BackgroundThe vertebrate brain is a highly energy consuming organ that requires continuous energy provision. Energy metabolism of ectothermic organisms is directly affected by environmental temperature changes and has been demonstrated to affect brain energy balance in fish. Fish were hypothesized to metabolize lactate as an additional energy substrate during acute exposure to energy demanding environmental abiotic fluctuations to support brain functionality. However, to date the pathways of lactate mobilization and transport in the fish brain are not well understood, and may represent a critical physiological feature in ectotherms during acclimation to low temperature.ResultsWe found depressed routine metabolic rates in zebrafish during acute exposure to hypothermic (18°C) conditions accompanied by decreased lactate concentrations in brain tissues. No changes in brain glucose content were observed. Acute cold stress increased protein concentrations of lactate dehydrogenase 1 (LDH1) and citrate synthase (CS) in brain by 1.8- and- 2.5-fold, paralleled by an increased pyruvate to acetyl-CoA transformation. To test the involvement of monocarboxylate transporters (MCTs) under acute cold stress in zebrafish, we cloned and sequenced seven MCT1-4 homologues in zebrafish. All drMCT1-4 are expressed in brain tissues and in response to cold stress drmct2a and drmct4a transcripts were up-regulated 5- and 3-fold, respectively. On the contrary, mRNA levels of drmct1a, -1b and -4b in zebrafish brain responded with a down regulation in response to cold stress. By expressing drMCTs in Xenopus oocytes we could provide functional evidence that hypothermic stress leads to a 2-fold increase in lactate transport in drMCT4b expressing oocytes. Lactate transport of other paralogues expressed in oocytes was unaffected, or even decreased during cold stress.ConclusionThe present work provides evidence that lactate utilization and transport pathways represent an important energy homeostatic feature to maintain vital functions of brain cells during acute cold stress in ectotherms.

Branchial NH4+-dependent acid–base transport mechanisms and energy metabolism of squid (Sepioteuthis lessoniana) affected by seawater acidification

Cephalopods have evolved strong acid–base regulatory abilities to cope with CO2 induced pH fluctuations in their extracellular compartments to protect gas transport via highly pH sensitive hemocyanins. To date, the mechanistic basis of branchial acid–base regulation in cephalopods is still poorly understood, and associated energetic limitations may represent a critical factor in high power squids during prolonged exposure to seawater acidification. The present work used adult squid Sepioteuthis lessoniana to investigate the effects of short-term (few hours) to medium-term (up to 168 h) seawater acidification on pelagic squids. Routine metabolic rates, NH4+ excretion, extracellular acid–base balance were monitored during exposure to control (pH 8.1) and acidified conditions of pH 7.7 and 7.3 along a period of 168 h. Metabolic rates were significantly depressed by 40% after exposure to pH 7.3 conditions for 168 h. Animals fully restored extracellular pH accompanied by an increase in blood HCO3− levels within 20 hours. This compensation reaction was accompanied by increased transcript abundance of branchial acid–base transporters including V-type H+-ATPase (VHA), Rhesus protein (RhP), Na+/HCO3− cotransporter (NBC) and cytosolic carbonic anhydrase (CAc). Immunocytochemistry demonstrated the sub-cellular localization of Na+/K+-ATPase (NKA), VHA in basolateral and Na+/H+-exchanger 3 (NHE3) and RhP in apical membranes of the ion-transporting branchial epithelium. Branchial VHA and RhP responded with increased mRNA and protein levels in response to acidified conditions indicating the importance of active NH4+ transport to mediate acid–base balance in cephalopods. The present work demonstrated that cephalopods have a well developed branchial acid–base regulatory machinery. However, pelagic squids that evolved a lifestyle at the edge of energetic limits are probably more sensitive to prolonged exposure to acidified conditions compared to their more sluggish relatives including cuttlefish and octopods.

The response of striped surfperch Embiotoca lateralis to progressive hypoxia: Swimming activity, shoal structure, and estimated metabolic expenditure

When exposed to moderately low O2 conditions (75–30% O2 saturation), shoaling striped surfperch Embiotoca lateralis show no changes in swimming activity, uniform levels of metabolic expenditure and an unchanging shoal structure. As O2 levels become more hypoxic surfperch reduce their locomotor activity, initially with a decrease in turning rate at 20% O2 saturation, followed by a reduction in swimming speed at 15% O2 saturation. Reductions in activity occur in combination with individuals adopting more uniform turning behaviors, which translate into an increase in the distance traveled per unit time (i.e. net displacement). This increase in net displacement may enhance the ability of surfperch to distance themselves from discrete zones of hypoxia. Measures of shoal structure, including nearest neighbor distances and shoal polarity, were constant throughout low O2 exposure despite changes in swimming activity. Estimation of the energetic costs of these behaviors reveals that metabolic expenditure (routine metabolic rate) remained steady at ~120mgO2kg−1h−1 between 100 and 30% O2 saturation, dropping to ~108mgO2kg−1h−1 at 20 and 15% O2 saturation. Given that surfperch possess a critical oxygen saturation (Scrit) of 15.2%, reductions in swimming activity and metabolic energy expenditure clearly occur as individuals reach their aerobic metabolic limits. These results identify that surfperch demonstrate a multi-faceted coping strategy when exposed to low oxygen conditions, which may prove advantageous when the species experience hypoxic episodes in their natural habitat.

Ice age fish in a warming world: minimal variation in thermal acclimation capacity among lake trout (Salvelinus namaycush) populations.

In the face of climate change, the persistence of cold-adapted species will depend on their adaptive capacity for physiological traits within and among populations. The lake trout (Salvelinus namaycush) is a cold-adapted salmonid and a relict from the last ice age that is well suited as a model species for studying the predicted effects of climate change on coldwater fishes. We investigated the thermal acclimation capacity of upper temperature resistance and metabolism of lake trout from four populations across four acclimation temperatures. Individuals were reared from egg fertilization onward in a common environment and, at 2 years of age, were acclimated to 8, 11, 15 or 19°C. Although one population had a slightly higher maximal metabolic rate (MMR), higher metabolic scope for activity and faster metabolic recovery across all temperatures, there was no interpopulation variation for critical thermal maximum (CTM) or routine metabolic rate (RMR) or for the thermal acclimation capacity of CTM, RMR, MMR or metabolic scope. Across the four acclimation temperatures, there was a 3°C maximal increase in CTM and 3-fold increase in RMR for all populations. Above 15°C, a decline in MMR and increase in RMR resulted in sharply reduced metabolic scope for all populations acclimated at 19°C. Together, these data suggest there is limited variation among lake trout populations in thermal physiology or capacity for thermal acclimatization, and that climate change may impact lake trout populations in a similar manner across a wide geographical range. Understanding the effect of elevated temperatures on the thermal physiology of this economically and ecologically important cold-adapted species will help inform management and conservation strategies for the long-term sustainability of lake trout populations.

Diurnal and water salinity-dependent metabolic activity of juvenile white steenbras Lithognathus lithognathus

The euryhaline white steenbras Lithognathus lithognathus is an important estuarine-dependent fish species in the South African marine and estuarine environment. Its tolerance to water quality fluctuations, in particular its metabolic rate responses to salinity changes, has not been studied. In order to contribute to ecophysiogical data on this species, oxygen consumption by white steenbras was determined to quantify metabolic activity. White steenbras were most active during daylight hours when oxygen consumption was 28% higher than during the scotophase. Standard, routine and active metabolic rate estimates were 0.04, 0.09 and 0.12 mg O2 g−1 h−1, respectively. In juveniles (body mass [W] = 19.8–85 g fish−1), the metabolic rate (MR; mg O2 g−1 h−1) was estimated as MR = 0.62W−0.526. The MR was reduced in fish exposed to a fast drop in salinity within 10 min from 35 to 5, but not in fish exposed to a fast drop to 25 or a slow drop within 3 or 9 h to 25 or 5, respectively. At a salinity of 5, oxygen consumption rate was 51% lower than at salinities of 25 and 35.