Muscle fibre types and metabolism in post-larval and adult stages of notothenioid fish

A comparative and ontogenetic examination of mitochondrial function in Antarctic notothenioid species

Notothenioidei fishes have evolved under stable cold temperatures; however, ocean conditions are changing globally, with polar regions poised to experience the greatest changes in environmental factors, such as warming. These stressors have the potential to dramatically affect energetic demands, and the persistence of the notothenioids will be dependent on metabolic capacity, or the ability to match energy supply with energy demand, to restore homeostasis in the face of changing climate conditions. In this study we examined aerobic metabolic capacity in three species, Trematomus bernacchii, T. pennellii and T. newnesi, and between two life stages, juvenile and adult, by assessing mitochondrial function of permeabilized cardiac fibers. Respiratory capacity differed among the adult notothenioids in this study, with greater oxidative phosphorylation (OXPHOS) respiration in the pelagic T. newnesi than the benthic T. bernacchii and T. pennellii. The variation in mitochondrial respiratory capacity was likely driven by differences in the mitochondrial content, as measured by citrate synthase activity, which was the highest in T. newnesi. In addition to high OXPHOS, T. newnesi exhibited lower LEAK respiration, resulting in greater mitochondrial efficiency than either T. bernacchii or T. pennellii. Life stage largely had an effect on mitochondrial efficiency and excess complex IV capacity, but there were little differences in OXPHOS respiration and electron transfer capacity, pointing to a lack of significant differences in the metabolic capacity between juveniles and adults. Overall, these results demonstrate species-specific differences in cardiac metabolic capacity, which may influence the acclimation potential of notothenioid fishes to changing environmental conditions.

Impacts of Climate Variability and Change on (Marine) Animals: Physiological Underpinnings and Evolutionary Consequences

Understanding thermal ranges and limits of organisms becomes important in light of climate change and observed effects on ecosystems as reported by the IPCC (2014). Evolutionary adaptation to temperature is presently unable to keep animals and other organisms in place; if they can these rather follow the moving isotherms. These effects of climate change on aquatic and terrestrial ecosystems have brought into focus the mechanisms by which temperature and its oscillations shape the biogeography and survival of species. For animals, the integrative concept of oxygen and capacity limited thermal tolerance (OCLTT) has successfully characterized the sublethal limits to performance and the consequences of such limits for ecosystems. Recent models illustrate how routine energy demand defines the realized niche. Steady state temperature-dependent performance profiles thus trace the thermal window and indicate a key role for aerobic metabolism, and the resulting budget of available energy (power), in defining performance under routine conditions, from growth to exercise and reproduction. Differences in the performance and productivity of marine species across latitudes relate to changes in mitochondrial density, capacity, and other features of cellular design. Comparative studies indicate how and why such mechanisms underpinning OCLTT may have developed on evolutionary timescales in different climatic zones and contributed to shaping the functional characteristics and species richness of the respective fauna. A cause-and-effect understanding emerges from considering the relationships between fluctuations in body temperature, cellular design, and performance. Such principles may also have been involved in shaping the functional characteristics of survivors in mass extinction events during earth's history; furthermore, they may provide access to understanding the evolution of endothermy in mammals and birds. Accordingly, an understanding is emerging how climate changes and variability throughout earth's history have influenced animal evolution and co-defined their success or failure from a bio-energetic point of view. Deepening such understanding may further reduce uncertainty about projected impacts of anthropogenic climate variability and change on the distribution, productivity and last not least, survival of aquatic and terrestrial species.

The zebrafish myotome contains tonic muscle fibers: morphological characterization and time course of formation

It is long known that the skeletal muscle of teleost fish contains muscle fibers which are in all probability of a tonic type according to morphological criteria. However, the evidence for the existence of teleost tonic fibers is still confined to a very small number of species, and knowledge concerning their ontogeny and possible functions is even more restricted. A remarkable deficit in this context is that it is not even exactly known whether the zebrafish, which is widely used to study vertebrate developmental biology, has such fibers, or how they arise. The present study demonstrates the existence of tonic fibers in the zebrafish myotome. They are identical with a fiber population previously termed "red muscle rim" fibers. A combined histochemical, immunocytochemical, and ultrastructural approach is used to characterize the morphology and development of these fibers. This study provides a basis for using the zebrafish model system in the future research on the developmental regulation and the functions of tonic fibers.

Mitochondrial function in Antarctic nototheniids with ND6 translocation

Fish of the suborder Notothenioidei have successfully radiated into the Southern Ocean and today comprise the dominant fish sub-order in Antarctic waters in terms of biomass and species abundance. During evolution in the cold and stable Antarctic climate, the Antarctic lineage of notothenioids developed several unique physiological adaptations, which make them extremely vulnerable to the rapid warming of Antarctic waters currently observed. Only recently, a further phenomenon exclusive to notothenioid fish was reported: the translocation of the mitochondrial gene encoding the NADH Dehydrogenase subunit 6 (ND6), an indispensable part of complex I in the mitochondrial electron transport system.This study investigated the potential physiological consequences of ND6 translocation for the function and thermal sensitivity of the electron transport system in isolated liver mitochondria of the two nototheniid species Notothenia coriiceps and Notothenia rossii, with special attention to the contributions of complex I (NADH DH) and complex II (Succinate DH) to oxidative phosphorylation. Furthermore, enzymatic activities of NADH:Cytochrome c Oxidoreductase and Cytochrome C Oxidase were measured in membrane-enriched tissue extracts.During acute thermal challenge (0-15°C), capacities of mitochondrial respiration and enzymatic function in the liver could only be increased until 9°C. Mitochondrial complex I (NADH Dehydrogenase) was fully functional but displayed a higher thermal sensitivity than the other complexes of the electron transport system, which may specifically result from its unique amino acid composition, revealing a lower degree of stability in notothenioids in general. We interpret the translocation of ND6 as functionally neutral but the change in amino acid sequence as adaptive and supportive of cold stenothermy in Antarctic nototheniids. From these findings, an enhanced sensitivity to ocean warming can be deduced for Antarctic notothenioid fish.

Fish muscle: the exceptional case of Notothenioids

Fish skeletal muscle is an excellent model for studying muscle structure and function, since it has a very well-structured arrangement with different fiber types segregated in the axial and pectoral fin muscles. The morphological and physiological characteristics of the different muscle fiber types have been studied in several teleost species. In fish muscle, fiber number and size varies with the species considered, limiting fish maximum final length due to constraints in metabolites and oxygen diffusion. In this work, we analyze some special characteristics of the skeletal muscle of the suborder Notothenioidei. They experienced an impressive radiation inside Antarctic waters, a stable and cold environment that could account for some of their special characteristics. The number of muscle fibers is very low, 12,700-164,000, in comparison to 550,000-1,200,000 in Salmo salar of similar sizes. The size of the fibers is very large, reaching 600 microm in diameter, while for example Salmo salar of similar sizes have fibers of 220 microm maximum diameter. Evolutionary adjustment in cell cycle length for working at low temperature has been shown in Harpagifer antarcticus (111 h at 0 degrees C), when compared to the closely related sub-Antarctic species Harpagifer bispinis (150 h at 5 degrees C). Maximum muscle fiber number decreases towards the more derived notothenioids, a trend that is more related to phylogeny than to geographical distribution (and hence water temperature), with values as low as 3,600 in Harpagifer bispinis. Mitochondria volume density in slow muscles of notothenioids is very high (reaching 0.56) and since maximal rates of substrate oxidation by mitochondria is not enhanced, at least in demersal notothenioids, volume density is the only means of overcoming thermal constraints on oxidative capacity. In brief, some characteristics of the muscles of notothenioids have an apparent phylogenetic component while others seem to be adaptations to low temperature.

Fishes of southern South America: a story driven by temperature

The latitudinal extension of southern South America imposes a thermal gradient that affects the structure of marine and freshwater fish assemblages and the biology of the species through direct exposure to the temperature gradients or by means of a web of historical and ecological relationships. We have reviewed biological and ecological data of marine and freshwater fishes from the southern Neotropics, including Patagonia, and report several examples of dependence on temperature, from glacial times to today's climate change. We were able to identify historic and present effects on the diversity of fish assemblages, isolation, southern limits for the distribution of species, and morphological variation among populations. There is a wide range of characteristics that exemplify an adaptation to low temperatures, including biochemical peculiarities, physiological adjustments, and alternative life history patterns, and these appear in both freshwater and marine, and native and exotic fishes. The consequences of stable temperature regimes in both the ocean and thermal streams deserve special mention as these shape specialists under conditions of low selective pressure. At present, habitat use and interactions among species are being subject to changes as consequences of water temperature, and some of these are already evident in the northern and southern hemispheres.

Oxygen limited thermal tolerance in fish?--Answers obtained by nuclear magnetic resonance techniques

In various phyla of marine invertebrates limited capacities of both ventilatory and circulatory performance were found to set the borders of the thermal tolerance window with limitations in aerobic scope and onset of hypoxia as a first line of sensitivity to both cold and warm temperature extremes. The hypothesis of oxygen limited thermal tolerance has recently been investigated in fish using a combination of non-invasive nuclear magnetic resonance (NMR) methodology with invasive techniques. In contrast to observations in marine invertebrates arterial oxygen tensions in fish were independent of temperature, while venous oxygen tensions displayed a thermal optimum. As the fish heart relies on venous oxygen supply, limited cardio-circulatory capacity is concluded to set the first level of thermal intolerance in fish. Nonetheless, maximized ventilatory capacity is seen to support circulation in maintaining the width of thermal tolerance windows. The interdependent setting of low and high tolerance limits is interpreted to result from trade-offs between optimized tissue functional capacity and baseline oxygen demand and energy turnover co-determined by the adjustment of mitochondrial densities and functional properties to a species-specific temperature range. At temperature extremes, systemic hypoxia will elicit metabolic depression, thereby widening the thermal window transiently sustained especially in those species preadapted to hypoxic environments.

Muscle metabolism and growth in Antarctic fishes (suborder Notothenioidei): evolution in a cold environment

The radiation of notothenioid fishes (order Perciformes) in the Southern Ocean provides a model system for investigating evolution and adaptation to a low temperature environment. The Notothenioid fishes comprising eight families, 43 genera and 122 species dominate the fish fauna in Antarctica. The diversification of the clade probably began 15-20 million years ago after the formation of the Antarctic Polar Front. The radiation was, therefore, associated with climatic cooling down to the present day temperature of -1.86 degrees C. Origins and Evolution of the Antarctic Biota Geological Society Special Publication No. 47, Geological Society of London. pp. 253-268). The success of the group has been closely linked with the evolution of glycopeptide and peptide antifreezes, which are amongst the most abundant proteins in blood and interstitial fluid. The radiation of the clade has been associated with disaptation (evolutionary loss of function) and recovery. For example, it is thought that the icefishes (Channichyidae) lost haemoglobin through a single mutational event leading to the deletion of the entire beta-globin gene and the 5' end of the linked alpha-globin gene, resulting in compensatory adaptations of the cardiovascular system. Phylogenetically based statistical methods also indicate a progressive and dramatic reduction in the number of skeletal muscle fibres (FN(max)) at the end of the recruitment phase of growth in basal compared to derived families. The reduction in FN(max) is associated with a compensatory increase in the maximum fibre diameter, which can reach 100 microm in slow and 600 microm in fast muscle fibres. At -1 to 0 degrees C, the oxygen consumption of isolated mitochondria per mg mitochondrial protein shows no evidence of up-regulation relative to mitochondria from temperate and tropical Perciform fishes. The mitochondria content of slow muscle fibres in Antarctic notothenioids is towards the upper end of the range reported for teleosts with similar lifestyles, reaching 50% in Channichthyids. High mitochondrial densities facilitate ATP production and oxygen diffusion through the membrane lipid compartment of the fibre. Modelling studies suggest that adequate oxygen flux in the large diameter muscle fibres of notothenioids is possible because of the reduced metabolic demand and enhanced solubility of oxygen associated with low temperature. At the whole animal level size-corrected resting metabolic rate fits on the same temperature relationship as for Perciformes from warmer climates. It seems likely that the additional energetic costs associated with antifreeze synthesis and high mitochondrial densities are compensated for by reductions in other energy requiring processes: a hypothesis that could be tested with detailed energy budget studies. One plausible candidate is a reduction in membrane leak pathways linked to the loss of muscle fibres, which would serve to minimise the cost of maintaining ionic gradients.

Reduction in muscle fibre number during the adaptive radiation of notothenioid fishes: a phylogenetic perspective

The fish fauna of the continental shelf of the Southern Ocean is dominated by a single sub-order of Perciformes, the Notothenioidei, which have unusually large diameter skeletal muscle fibres. We tested the hypothesis that in fast myotomal muscle a high maximum fibre diameter (FD(max)) was related to a reduction in the number of muscle fibres present at the end of the recruitment phase of growth. We also hypothesized that the maximum fibre number (FN(max)) would be negatively related to body size, and that both body size and size-corrected FN(max) would show phylogenetic signal (tendency for related species to resemble each other). Finally, we estimated ancestral values for body size and FN(max). A molecular phylogeny was constructed using 12S mitochondrial rRNA sequences. A total of 16 species were studied from the Beagle Channel, Tierra del Fuego (5-11 degrees C), Shag Rocks, South Georgia (0.5-4 degrees C), and Adelaide Island, Antarctic Peninsula (-1.5 to 0.5 degrees C). The absence of muscle fibres of less than 10 micro m diameter was used as the criterion for the cessation of fibre recruitment. FD(max) increased linearly with standard length (SL), reaching 500-650 micro m in most species. Maximum body size was a highly significant predictor of species variation in FN(max), and both body size and size-corrected FN(max) showed highly significant phylogenetic signal (P<0.001). Estimates of trait values at nodes of the maximum likelihood phylogenetic tree were consistent with a progressive reduction in fibre number during part of the notothenioid radiation, perhaps serving to reduce basal energy requirements to compensate for the additional energetic costs of antifreeze production. For example, FN(max) in Chaenocephalus aceratus (12 700+/-300, mean +/- S.E.M., N=18) was only 7.7% of the value found in Eleginops maclovinus (164 000+/-4100, N=17), which reaches a similar maximum length (85 cm). Postembryonic muscle fibre recruitment in teleost fish normally involves stratified followed by mosaic hyperplasia. No evidence for this final phase of growth was found in two of the most derived families (Channichthyidae and Harpagiferidae). The divergence of the notothenioids in Antarctica after the formation of the Antarctic Polar Front and more recent dispersal north would explain the high maximum diameter and low fibre number in the derived sub-Antarctic notothenioids. These characteristics of notothenioids may well restrict their upper thermal tolerance, particularly for Champsocephalus esox and similar Channichthyids that lack respiratory pigments.

Muscle growth in Nile tilapia (Oreochromis niloticus): histochemical, ultrastructural and morphometric study

Muscle growth in Nile tilapia (Oreochromis niloticus) was studied focusing on histochemical, ultrastructural, and morphometric characteristics of muscle fibers. Based on body length (cm), we studied four groups: G1=1.36+/-0.09, G2=3.38+/-0.44, G3=8.90+/-1.47, and G4=28.30+/-3.29 (mean+/-S.D.). All groups showed intense reaction to NADH-TR in subdermal fibers and weak or no reaction in deep layer fibers. In G3 and G4, an intermediate layer was also observed with fibers presenting weak reaction; in G4, groups of fibers with intense reaction were observed in the subdermal region. The myosin ATPase (m-ATPase) activities were acid-stable and alkali-labile in subdermal fibers; most deep layer fibers were alkali-stable and acid-labile. Intermediate fibers were acid-labile and alkali-stable. Two fiber populations were observed near deep muscle layer: one large presenting weak acid- and alkali-stable and the other small alkali-stable. During growth, muscle fiber hypertrophy was more evident in intermediate and white fibers for G3 and G4. However, in these groups, the presence of fiber diameters < or =21 microm suggested that there is still substantial fiber recruitment, confirmed by ultrastructural study, but hypertrophy is the main mechanism contributing to increase in muscular mass.

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.