Abstract
Characteristics of temperature-dependent metabolic adaptation, as well as their impli- cations for climate-dependent energy budgets, biogeography and fitness are reviewed and analysed for populations of Atlantic cod Gadus morhua in relation to findings in other fish species from north- ern and southern hemispheres, especially various species of eelpout (Zoarcidae). The analysis builds on the recently posited concept of oxygen- and capacity-dependent thermal tolerance in aquatic ectotherms. Mechanistic physiological studies are used to explain both functional differences between populations and current observations (such as the northward movement of cod, or the changes in seasonal abundance of eelpout due to global warming). Available data support the hypothesis that natural selection favours individuals that maximize growth and energy efficiency at the expense of ranges of thermal tolerance. The levels of energy turnover are subject to the con- straints of resource availability and temperature variability. Temperature variability in the cold, such as in the sub-Arctic, causes a rise in maintenance costs at the expense of growth, but possibly in favour of motility, and thus of foraging capacity. These different trends are mirrored in higher capac- ities for gene expression of key enzymes involved in aerobic metabolism (during cold acclimation) in northern as compared to southern cod populations of the East Atlantic. However, detailed patterns, as well as many of the underlying mechanisms, remain underexplored, especially with respect to the suggested hierarchy of energy allocation to energy budget components.
Highlights
Characteristics of temperature-dependent metabolic adaptation, as well as their implications for climate-dependent energy budgets, biogeography and fitness are reviewed and analysed for populations of Atlantic cod Gadus morhua in relation to findings in other fish species from northern and southern hemispheres, especially various species of eelpout (Zoarcidae)
Temperature variability in the cold, such as in the sub-Arctic, causes a rise in maintenance costs at the expense of growth, but possibly in favour of motility, and of foraging capacity. These different trends are mirrored in higher capacities for gene expression of key enzymes involved in aerobic metabolism in northern as compared to southern cod populations of the East Atlantic
As a trade-off in eurythermal cold adaptation, standard metabolism and, in consequence, other aerobic functions like exercise capacity may increase in the cold (Pörtner 2002b), while temperature-specific growth performance is reduced, likely due to enhanced mitochondrial proton leakage and the associated shift in energy budget (Pörtner et al 2001)
Summary
Climate variability has long been seen to influence the structure and functioning of marine, terrestrial and freshwater ecosystems. Observed shifts in geographical distribution (Perry et al 2005) in species composition and in the population structure of different species may be explainable by the shifting frequencies of these extreme weather conditions, as has already been observed in the past for periods of ‘warmer winters’ (Cushing 1975) These general considerations indicate specialisation of marine fauna and their different life stages for climate-related temperature windows as one global reason for the sensitivity of marine fauna to temperature extremes and for associated processes on an ecosystem level (Pörtner & Knust 2007). The question needs to be asked how the degrees of thermal specialisation, as well as potential differences between life stages, influence the key processes shaping population size and structure, i.e. fecundity, growth and mortality All of these are related to the levels of organismal performance and fitness (i.e. the capacities to forage, migrate, grow, or reproduce), which, in turn, are influenced by temperature (Pörtner & Knust 2007). The principles and some of these implications will be elaborated on in this study
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