Thermal compensation of peripheral oxygen transport in skeletal muscle of seasonally acclimatized trout

Advances in understanding the impacts of global warming on marine fishes farmed offshore: Sparus aurata as a case study

Monitoring variations in proteins involved in metabolic processes, oxidative stress responses, cell signalling and protein homeostasis is a powerful tool for developing hypotheses of how environmental variations affect marine organisms' physiology and biology. According to the oxygen- and capacity-limited thermal tolerance hypothesis, thermal acclimation mechanisms such as adjusting the activities of enzymes of intermediary metabolism and of antioxidant defence mechanisms, inducing heat shock proteins (Hsps) or activating mitogen-activated protein kinases may all shift tolerance windows. Few studies have, however, investigated the molecular, biochemical and organismal responses by fishes to seasonal temperature variations in the field to link these to laboratory findings. Investigation of the impacts of global warming on fishes farmed offsore, in the open sea, can provide a stepping stone towards understanding effects on wild populations because they experience similar environmental fluctuations. Over the last 30 years, farming of the gilthead sea bream Sparus aurata (Linnaeus 1758) has become widespread along the Mediterranean coastline, rendering this species a useful case study. Based on available information, the prevailing seasonal temperature variations expose the species to the upper and lower limits of its thermal range. Evidence for this includes oxygen restriction, reduced feeding, reduced responsiveness to environmental stimuli, plus a range of molecular and biochemical indicators that change across the thermal range. Additionally, close relationships between biochemical pathways and seasonal patterns of metabolism indicate a connection between energy demand and metabolic processes on the one hand, and cellular stress responses such as oxidative stress, inflammation and autophagy on the other. Understanding physiological responses to temperature fluctuations in fishes farmed offshore can provide crucial background information for the conservation and successful management of aquaculture resources in the face of global change.

Climate change and anadromous fish: How does thermal acclimation affect the mechanics of the myotomal muscle of the Atlantic salmon, Salmo salar?

In response to accelerated temperature shifts due to climate change, the survival of many species will require forms of thermal acclimation to their changing environment. We were interested in how climate change will impact a commercially and recreationally important species of fish, Atlantic salmon (Salmo salar). As climate change alters the thermal environment of their natal streams, we asked how their muscle function will be altered by extended exposure to both warm and cold temperatures. We performed a thermal acclimation study of S. salar muscle mechanics of both fast-twitch, or white, and slow-twitch, or red, myotomal muscle bundles to investigate how temperature acclimated Atlantic salmon would respond across a range of different temperatures. Isometric contraction properties, maximum shortening velocity, and oscillatory power output were measured and compared amongst three groups of salmon-warm acclimated (20°C), cold-acclimated (2°C), and those at their rearing temperature (12°C). The Atlantic salmon showed limited thermal acclimation in their contraction kinetics, and some of the shifts in contractile properties that were observed would not be predicted to mitigate the impact of a warming environment. For instance, the maximum shortening velocity at a common test temperature was higher in the warm acclimated group and lower in the cold-acclimated group. In addition, critical swimming speed did not vary with temperature of acclimation when tested at a common temperature (12°C). Our results suggest that Atlantic salmon populations will continue to struggle in response to a warming environment.

Thermal acclimation leads to variable muscle responses in two temperate labrid fishes

Temperature can be a key abiotic factor in fish distribution, as it affects most physiological processes. Specifically, temperature can affect locomotor capabilities, especially as species are exposed to temperatures nearing their thermal limits. In this study, we aimed to understand the effects of temperature on muscle in two labrids that occupy the Northwest Atlantic Ocean. When exposed to cold temperatures in autumn, cunner (Tautogolabrus adspersus) and tautog (Tautoga onitis) go into a state of winter dormancy. Transitions into dormancy vary slightly, where tautog will make short migrations to overwintering habitats while cunner overwinter in year-round habitats. To understand how muscle function changes with temperature, we held fish for 4 weeks at either 5 or 20°C and then ran muscle kinetic and workloop experiments at 5, 10 and 20°C. Following experiments, we used immunohistochemistry staining to identify acclimation effects on myosin isoform expression. Muscle taken from warm-acclimated cunner performed the best, whereas there were relatively few differences among the other three groups. Cunner acclimated at both temperatures downregulated the myosin heavy chain, suggesting a transition in fiber type from slow-oxidative to fast-glycolytic. This change did not amount to a detectable difference in muscle power production and kinetics. However, overall poor performance at cold temperatures could force these fishes into torpor to overwinter. Tautog, alternatively, retained myosin heavy chains, which likely increases locomotor capabilities when making short migrations to overwintering habitats.

The impact of acute thermal stress on the metabolome of the black rockfish (Sebastes schlegelii)

Acute change in water temperature causes heavy economic losses in the aquaculture industry. The present study investigated the metabolic and molecular effects of acute thermal stress on black rockfish (Sebastes schlegelii). Gas chromatography time-of-flight mass spectrometry (GC-TOF-MS)-based metabolomics was used to investigate the global metabolic response of black rockfish at a high water temperature (27°C), low water temperature (5°C) and normal water temperature (16°C). Metabolites involved in energy metabolism and basic amino acids were significantly increased upon acute exposure to 27°C (P < 0.05), and no change in metabolite levels occurred in the low water temperature group. However, certain fatty acid levels were elevated after cold stress (P < 0.05), and this effect was not observed in the 27°C group, suggesting that acute high and low temperature exposures caused different physiological responses. Using quantitative real-time PCR, we analyzed the expression of ubiquitin (ub), hypoxia-inducible factor (hif), lactate dehydrogenase (ldh), and acetyl-CoA carboxylase (acac). Higher expression levels of ub, hif, and ldh (P < 0.05) were observed in the high water temperature group, but no changes in these expression levels occurred in the low water temperature group. Our findings provide a potential metabolic profile for black rockfish when exposed to acute temperature stress and provide some insights into host metabolic and molecular responses to thermal stress.

Seasonal Changes in Metabolism and Cellular Stress Phenomena in the Gilthead Sea Bream (Sparus aurata)

Seasonal temperature changes may take organisms to the upper and lower limit of their thermal range, with respective variations in their biochemical and metabolic profile. To elucidate these traits, we investigated metabolic and antioxidant patterns in tissues of sea bream Sparus aurata during seasonal acclimatization for 1 yr in the field. Metabolic patterns were assessed by determining lactate dehydrogenase, citrate synthase, and β-hydroxyacyl CoA dehydrogenase activities, their kinetic properties and plasma levels of glucose, lactate, and triglycerides and tissue succinate levels. Oxidative stress was assessed by determining antioxidant enzymes superoxide dismutase, catalase, and glutathione reductase activities and levels of thiobarbituric acid reactive substances. Xanthine oxidase (XO) activity was determined as another source of reactive oxygen species (ROS) production. Furthermore, we studied the antiapoptotic protein indicator Bcl-2 and the apoptotic protein indicators Bax, Bad, ubiquitin, and caspase as well as indexes of autophagy (LC3B II/LC3B I and SQSTM1/p62) in the liver and the heart to identify possible relationships between oxidative stress and cell death. The results indicate clear seasonal metabolic patterns involving oxidative stress during summer as well as winter. During cold acclimatization, lipid oxidation is induced, while during increased temperatures, warm-induced metabolic activation and carbohydrate oxidation are observed. Thus, oxidative stress seems to be more prominent during warming because of the increased aerobic metabolism. The seasonal profile of apoptosis and XO as another source of ROS matches the results obtained in the laboratory and are interpreted within the framework of oxygen- and capacity-limited thermal tolerance.

Transcriptome Analysis Reveals Increases in Visceral Lipogenesis and Storage and Activation of the Antigen Processing and Presentation Pathway during the Mouth-Opening Stage in Zebrafish Larvae

The larval phase of the fish life cycle has the highest mortality, particularly during the transition from endogenous to exogenous feeding. However, the transcriptional events underlying these processes have not been fully characterized. To understand the molecular mechanisms underlying mouth-opening acclimation, RNA-seq was used to investigate the transcriptional profiles of the endogenous feeding, mixed feeding and exogenous feeding stages of zebrafish larvae. Differential expression analysis showed 2172 up-regulated and 2313 down-regulated genes during this stage. Genes associated with the assimilation of exogenous nutrients such as the arachidonic acid metabolism, linoleic acid metabolism, fat digestion and absorption, and lipogenesis were activated significantly, whereas dissimilation including the cell cycle, homologous recombination, and fatty acid metabolism were inhibited, indicating a physiological switch for energy storage occurred during the mouth-opening stage. Moreover, the immune recognition involved in the antigen processing and presentation pathway was activated and nutritional supply seemed to be required in this event confirmed by qPCR. These results suggested the energy utilization during the mouth-opening stage is more tended to be reserved or used for some important demands, such as activity regulation, immune defense, and lipid deposition, instead of rapid growth. The findings of this study are important for understanding the physiological switches during the mouth-opening stage.

Osmotic versus adrenergic control of ion transport by ionocytes of Fundulus heteroclitus in the cold

In eurythermic vertebrates, acclimation to the cold may produce changes in physiological control systems. We hypothesize that relatively direct osmosensitive control will operate better than adrenergic receptor mediated control of ion transport in cold vs. warm conditions. Fish were acclimated to full strength seawater (SW) at 21°C and 5°C for four weeks, gill samples and blood were taken and opercular epithelia mounted in Ussing style chambers. Short-circuit current (I_sc) at 21°C and 5°C (measured at acclimation temperature), was significantly inhibited by the α₂-adrenergic agonist clonidine but the ED₅₀ dose was significantly higher in cold conditions (93.8±16.4nM) than in warm epithelia (47.8±8.1nM) and the maximum inhibition was significantly lower in cold (-66.1±2.2%) vs. warm conditions (-85.6±1.3%), indicating lower sensitivity in the cold. β-Adrenergic responses were unchanged. Hypotonic inhibition of I_sc, was higher in warm acclimated (-95%), compared to cold acclimated fish (-75%), while hypertonic stimulations were the same, indicating equal responsiveness to hyperosmotic stimuli. Plasma osmolality was significantly elevated in cold acclimated fish and, by TEM, gill ionocytes from cold acclimated fish had significantly shorter mitochondria. These data are consistent with a shift in these eurythermic animals from complex adrenergic control to relatively simple biomechanical osmotic control of ion secretion in the cold.

Origins of variation in muscle cytochrome c oxidase activity within and between fish species

Mitochondrial content, central to aerobic metabolism, is thought to be controlled by a few transcriptional master regulators, including nuclear respiratory factor 1 (NRF-1), NRF-2 and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α). Though well studied in mammals, the mechanisms by which these factors control mitochondrial content have been less studied in lower vertebrates. We evaluated the role of these transcriptional regulators in seasonal changes in white muscle cytochrome c oxidase (COX) activity in eight local fish species representing five families: Centrarchidae, Umbridae, Esocidae, Gasterosteidae and Cyprinidae. Amongst centrarchids, COX activity was significantly higher in winter for pumpkinseed (2-fold) and black crappie (1.3-fold) but not bluegill or largemouth bass. In esociforms, winter COX activity was significantly higher in central mudminnow (3.5-fold) but not northern pike. COX activity was significantly higher in winter-acclimatized brook stickleback (2-fold) and northern redbelly dace (3-fold). Though mudminnow COX activity increased in winter, lab acclimation to winter temperatures did not alter COX activity, suggesting a role for non-thermal cues. When mRNA was measured for putative master regulators of mitochondria, there was little evidence for a uniform relationship between COX activity and any of NRF-1, NRF-2α or PGC-1α mRNA levels Collectively, these studies argue against a simple temperature-dependent mitochondrial response ubiquitous in fish, and suggest that pathways which control mitochondrial content in fish may differ in important ways from those of the better studied mammals.

Metabolic and molecular stress responses of gilthead seam bream Sparus aurata during exposure to low ambient temperature: an analysis of mechanisms underlying the winter syndrome

The winter syndrome in the gilthead sea bream Sparus aurata indicates that the species is exposed to critically low temperatures in Mediterranean aquaculture in winter. The present study of metabolic patterns and molecular stress responses during cold exposure was carried out to investigate this "disease", in light of the recent concept of oxygen and capacity limited thermal tolerance. The metabolic profile of fuel oxidation was examined by determining the activities of the enzymes hexokinase (HK), aldolase (Ald), pyruvate kinase (PK), L-lactate dehydrogenase (L-LDH), citrate synthase (CS), malate dehydrogenase (MDH) and 3-hydroxyacyl CoA dehydrogenase (HOAD) in heart, red and white muscle after exposure to temperatures of 10, 14 and 18°C. Especially, the increase in LDH activity combined with the accumulation of L-lactate in tissues indicates that temperatures below 14°C are critical for Sparus aurata and stimulate the anaerobic component of metabolism. Increase in the activity of HOAD suggests that oxidation of free fatty acids might contribute to ATP turnover at low temperatures. The expression of Hsp70 and Hsp90 in all tissues examined revealed a cellular stress response during cooling below 18°C. In the light of winter temperatures in S. aurata cultures around 10°C, our data suggest that the fish are exposed to stressful conditions at the low end of their thermal tolerance window. These conditions likely impair the aerobic capacity of the fish, compromise the rates of growth and reproduction and may contribute to elicit pathological conditions.

Allometry of skeletal muscle fine structure allows maintenance of aerobic capacity during ontogenetic growth

Controversy exists over the scaling of oxygen consumption with body mass in vertebrates. A combination of biochemical and structural analyses were used to examine whether individual elements influencing oxygen delivery and demand within locomotory muscle respond similarly during ontogenetic growth of striped bass. Mass-specific metabolic enzyme activity confirmed that glycolytic capacity scaled positively in deep white muscle (regression slope, b=0.1 to 0.8) over a body mass range of ∼20–1500 g, but only creatine phosphokinase showed positive scaling in lateral red muscle(b=0.5). Although oxidative enzymes showed negative allometry in red muscle (b=–0.01 to –0.02), mass-specific myoglobin content scaled positively (b=0.7). Capillary to fibre ratio of red muscle was higher in larger (1.42±0.15) than smaller (1.20±0.15)fish, suggesting progressive angiogenesis. By contrast, capillary density decreased (1989±161 vs 2962±305 mm–2)as a result of larger fibre size (658±31 vs 307±24μm2 in 1595 g and 22.9 g fish, respectively). Thus, facilitated and convective delivery of O2 show opposite allometric trends. Relative mitochondrial content of red muscle (an index of O2demand) varied little with body mass overall, but declined from ∼40% fibre volume in the smallest to ∼30% in the largest fish. However, total content per fibre increased, suggesting that mitochondrial biogenesis supported aerobic capacity during fibre growth. Heterogeneous fibre size indicates both hypertrophic and hyperplastic growth, although positive scaling of fibre myofibrillar content (b=0.085) may enhance specific force generation in larger fish. Modelling intracellular PO2distribution suggests such integrated structural modifications are required to maintain adequate oxygen delivery (calculated PO2 5.15±0.02 kPa and 5.21±0.01 kPa in small and large fish, respectively).

The long and winding road: influences of intracellular metabolite diffusion on cellular organization and metabolism in skeletal muscle

A fundamental principle of physiology is that cells are small in order to minimize diffusion distances for O(2) and intracellular metabolites. In skeletal muscle, it has long been recognized that aerobic fibers that are used for steady state locomotion tend to be smaller than anaerobic fibers that are used for burst movements. This tendency reflects the interaction between diffusion distances and aerobic ATP turnover rates, since maximal intracellular diffusion distances are ultimately limited by fiber size. The effect of diffusion distance on O(2) flux in muscle has been the subject of quantitative analyses for a century, but the influence of ATP diffusion from mitochondria to cellular ATPases on aerobic metabolism has received much less attention. The application of reaction-diffusion mathematical models to experimental measurements of aerobic metabolic processes has revealed that the extreme diffusion distances between mitochondria found in some muscle fibers do not necessarily limit the rates of aerobic processes per se, as long as the metabolic process is sufficiently slow. However, skeletal muscle fibers from a variety of animals appear to have intracellular diffusion distances and/or fiber sizes that put them on the brink of diffusion limitation. Thus, intracellular metabolite diffusion likely influences the evolution of muscle design and places limits on muscle function.

In vivored blood cell sickling and mechanism of recovery in whiting,Merlangius merlangus

Haemoglobin concentrations in vertebrate red blood cells are so high that in human sickle cell disease a single surface amino acid mutation can result in formation of large insoluble haemoglobin aggregates at low oxygen levels,causing peculiar cell deformations or `sickling'. This may cause vascular occlusion and thereby severe pain, organ failure and death. Here, using light and transmission electron microscopy, we demonstrate extensive in vivo sickling of whiting red blood cells after capture stress without any apparent haemolysis and show its subsequent recovery. We show exceptionally high cooperative proton binding during the sickling process in vitroand identify the reduction of extracellular pH below resting values as the primary cause for in vivo sickling, although the response is modulated to a lesser extent also by oxygen tension. Using isotope tracer fluxes, we further show that β-adrenergic hormones, which are released under capture stress, activate a powerful endogenous Na/H exchanger in these fish red blood cells, which is known to elevate intracellular pH.β-adrenergic treatment further leads to a marked reduction of acid-induced in vitro sickling, which is impaired when Na/H exchange is inhibited by amiloride. We propose that this mechanism protects red blood cells of some fishes against the problem of haemoglobin aggregation and red blood cell sickling, except under most severe acidosis. This system offers a unique example of how, over evolutionary time, nature may have overcome what is still a deadly disease in humans.

Periodic cooling of bird eggs reduces embryonic growth efficiency

For many bird embryos, periodic cooling occurs when the incubating adult leaves the nest to forage, but the effects of periodic cooling on embryo growth, yolk use, and metabolism are poorly known. To address this question, we conducted incubation experiments on eggs of zebra finches (Taeniopygia guttata) that were frequently cooled and then rewarmed or were allowed to develop at a constant temperature. After 12 d of incubation, embryo mass and yolk reserves were less in eggs that experienced periodic cooling than in controls incubated constantly at 37.5 degrees Celsius. Embryos that regularly cooled to 20 degrees Celsius had higher mass-specific metabolic rates than embryos incubated constantly at 37.5 degrees Celsius. Periodic cooling delayed development and increased metabolic costs, reducing the efficiency with which egg nutrients were converted into embryo tissue. Avian embryos can tolerate periodic cooling, possibly by adjusting their physiology to variable thermal conditions, but at a cost to growth efficiency as well as rate of development. This reduction in embryo growth efficiency adds a new dimension to the fitness consequences of variation in adult nest attentiveness.

The effects of sustained exercise and hypoxia upon oxygen tensions in the red muscle of rainbow trout

Teleost fish possess discrete blocks of oxidative red muscle (RM) and glycolytic white muscle, whereas tetrapod skeletal muscles are mixed oxidative/glycolytic. It has been suggested that the anatomy of RM in teleost fish could lead to higher intramuscular O2 partial pressures (PO2) than in mammalian skeletal muscles. This study provides the first direct experimental support for this suggestion by using novel optical fibre sensors to discover a mean (+/- S.E.M., N=6) normoxic steady-state red muscle PO2 (PrmO2) of 61+/-10 mmHg (1 mmHg=133.3 Pa) in free-swimming rainbow trout Oncorhynchus mykiss. This is significantly higher than literature reports for mammalian muscles, where the PO2 never exceeds 40 mmHg. Aerobic RM powers sustained swimming in rainbow trout. During graded incremental exercise, PrmO2 declined from 62+/-5 mmHg at the lowest swim speed down to 45+/-3 mmHg at maximum rates of aerobic work, but then rose again to 51+/-5 mmHg at exhaustion. These measurements of PrmO2 during exercise indicated, therefore, that O2 supply to the RM was not a major limiting factor at exhaustion in trout. The current study found no evidence that teleost haemoglobins with a Root effect cause extremely elevated O2 tensions in aerobic tissues. Under normoxic conditions, PrmO2 was significantly lower than arterial PO2 (119+/-5 mmHg), and remained lower when the arterial to tissue PO2 gradient was reduced by exposure to mild hypoxia. When two sequential levels of mild hypoxia (30 min at a water PO2 of 100 mmHg then 30 min at 75 mmHg) caused PaO2 to fall to 84+/-2 mmHg then 61+/-3 mmHg, respectively, this elicited simultaneous reductions in PrmO2,to 51+/-6 mmHg then 41+/-5 mmHg, respectively. Although these hypoxic reductions in PrmO2 were significantly smaller than those in PaO2, the effect could be attributed to the sigmoid shape of the trout haemoglobin-O2 dissociation curve.

Time course of the response of mitochondria from oxidative muscle during thermal acclimation of rainbow trout, Oncorhynchus mykiss

The time course of changes in the properties of mitochondria from oxidative muscle of rainbow trout was examined during warm (15 degrees C) and cold (5 degrees C) acclimation. Mitochondrial oxidative capacities showed a biphasic response during thermal acclimation: at a given assay temperature, capacities first increased and then decreased during warm acclimation and showed the inverse pattern during cold acclimation. This was most apparent for maximal rates of state 3 oxygen consumption expressed per mg mitochondrial protein. Rates expressed per nmol ADP-ATP translocase (ANT) showed this pattern during cold acclimation. A biphasic pattern was also apparent for state 4 and oligomycin-inhibited (state 4(ol)) rates of oxygen uptake expressed per mg protein. Changes in states 4 and 4(ol) were smaller during cold than warm acclimation. Warm acclimation reduced the proportion of cytochrome c oxidase and citrate synthase needed during mitochondrial substrate oxidation. Phospholipid concentrations per mg mitochondrial protein changed little with thermal acclimation. Mitochondrial properties changed more quickly during warm than cold acclimation. While the biochemical modifications during thermal acclimation may eventually compensate for the thermal change, compensation did not occur at its onset. Rather, the initial changes of mitochondrial oxidative capacity in response to temperature change accentuated the functional impact of the thermal change, and prolonged exposure to the new temperature was required to attain a degree of thermal compensation.

The rainbow trout skeletal muscle beta-adrenergic system: characterization and signaling

The presence and functionality of beta-adrenoceptors (beta-ARs) were examined in red (RM) and white muscle (WM) membranes isolated from the rainbow trout Oncorhynchus mykiss. Specific binding assays revealed the presence of a single class of binding sites with similar affinities in both muscle types (K(d) in nM: 0.14 +/- 0.03 and 0.18 +/- 0.03 for RM and WM, respectively) but with a significantly higher number of binding sites in RM compared with WM (B(max) in fmol/mg protein: 3.22 +/- 0.11 and 2.60 +/- 0.13, respectively). Selective and nonselective beta-adrenergic agonists (beta-AAs) and antagonists indicated an atypical beta-AR pharmacology. This result may represent a nonmammalian beta-AR classification or, more likely, the presence of more than one beta-AR subtype in trout muscles with similar affinities that could not be kinetically resolved. Adenylyl cyclase (ACase) assays showed a dose-dependent increase in cAMP production as concentrations of beta(2)-AAs increased in both muscle membranes with significantly higher basal cAMP production in RM compared with WM (cAMP production in pmol cAMP. mg protein(-1). 10 min(-1): 24.67 +/- 3.06 and 9.64 +/- 3.45, respectively). The agonist-induced increase in cAMP production was blocked by the beta-adrenergic antagonist propranolol, while the ACase activator forskolin increased cAMP production by 7- to 14-fold above basal and approximately 3-fold above all beta-AAs tested. This study demonstrated the presence of atypical beta(2)-ARs on RM and WM membranes of trout, suggesting that beta(2)-AAs may be a tool to enhance protein accretion through this signaling pathway.

Muscle fine structure may maintain the function of oxidative fibres in haemoglobinless Antarctic fishes

Muscle fine structure and metabolism were examined in four species of Antarctic fishes that vary in their expression of haemoglobin (Hb). To determine how locomotory pectoral muscles maintain function, metabolic capacity, capillary supply and fibre ultrastructure were examined in two nototheniid species that express Hb (Notothenia coriiceps and Gobionotothen gibberifrons) and two species of channichthyid icefish that lack Hb (Chaenocephalus aceratus and Chionodraco rastrospinosus). Surprisingly, icefish have higher densities of mitochondria than red-blooded species (C. aceratus, 53+/-3% of cell volume; C. rastrospinosus, 39+/-3%; N. coriiceps, 29+/-3%; G. gibberifrons, 25+/-1%). Despite higher mitochondrial densities the aerobic metabolic capacities per g wet mass, estimated from measurements of maximal activities of key metabolic enzymes, are lower in icefish compared to red-blooded species. This apparent incongruity can be explained by the significantly lower mitochondrial cristae surface area per unit mitochondrion volume in icefishes (C. aceratus, 20.8+/-1.6 microm(-1); C. rastrospinosus, 25.5+/-1.8 microm(-1)) compared to red-blooded species (N. coriiceps, 33.6+/-3.0 microm(-1); G. gibberifrons, 37.7+/-3.6 microm(-1)). Consequently, the cristae surface area per unit muscle mass is conserved at approximately 9 m(2)g(-1). Although high mitochondrial densities in icefish muscle do not enhance aerobic metabolic capacity, they may facilitate intracellular oxygen movement because oxygen is more soluble in lipid, including the hydrocarbon core of intracellular membrane systems, than in aqueous cytoplasm. This may be particularly vital in icefish, which have larger oxidative muscle fibres compared to red-blooded nototheniods (C. aceratus, 2932+/-428 microm(2); C. rastrospinosus, 9352+/-318 microm(2); N. coriiceps, 1843+/-312 microm(2); G. gibberifrons, 2103+/-194 microm(2)). These large fibres contribute to a relatively low capillary density, which is partially compensated for in icefish by a high index of tortuosity in the capillary bed (C. aceratus=1.4, N. coriiceps=1.1).

Physiological basis of temperature-dependent biogeography: trade-offs in muscle design and performance in polar ectotherms

Polar, especially Antarctic, oceans host ectothermic fish and invertebrates characterized by low-to-moderate levels of motor activity; maximum performance is reduced compared with that in warmer habitats. The present review attempts to identify the trade-offs involved in adaptation to cold in the light of progress in the physiology of thermal tolerance. Recent evidence suggests that oxygen limitations and a decrease in aerobic scope are the first indications of tolerance limits at both low and high temperature extremes. The cold-induced reduction in aerobic capacity is compensated for at the cellular level by elevated mitochondrial densities, accompanied by molecular and membrane adjustments for the maintenance of muscle function. Particularly in the muscle of pelagic Antarctic fish, among notothenioids, the mitochondrial volume densities are among the highest known for vertebrates and are associated with cold compensation of aerobic metabolic pathways, a reduction in anaerobic scope, rapid recovery from exhaustive exercise and enhanced lipid stores as well as a preference for lipid catabolism characterized by high energy efficiency at high levels of ambient oxygen supply. Significant anaerobic capacity is still found at the very low end of the activity spectrum, e.g. among benthic eelpout (Zoarcideae).

In contrast to the cold-adapted eurytherms of the Arctic, polar (especially Antarctic) stenotherms minimize standard metabolic rate and, as a precondition, the aerobic capacity per milligram of mitochondrial protein,thereby minimizing oxygen demand. Cost reductions are supported by the downregulation of the cost and flexibility of acid—base regulation. At maintained factorial scopes, the reduction in standard metabolic rate will cause net aerobic scope to be lower than in temperate species. Loss of contractile myofilaments and, thereby, force results from space constraints due to excessive mitochondrial proliferation. On a continuum between low and moderately high levels of muscular activity, polar fish have developed characteristics of aerobic metabolism equivalent to those of high-performance swimmers in warmer waters. However, they only reach low performance levels despite taking aerobic design to an extreme.