Abstract
Metabolic rate is a key component of energy budgets that scales with body size and varies with large-scale environmental geographical patterns. Here we conduct an analysis of standard metabolic rates (SMR) of marine ectotherms across a 70° latitudinal gradient in both hemispheres that spanned collection temperatures of 0–30 °C. To account for latitudinal differences in the size and skeletal composition between species, SMR was mass normalized to that of a standard-sized (223 mg) ash-free dry mass individual. SMR was measured for 17 species of calcified invertebrates (bivalves, gastropods, urchins and brachiopods), using a single consistent methodology, including 11 species whose SMR was described for the first time. SMR of 15 out of 17 species had a mass-scaling exponent between 2/3 and 1, with no greater support for a 3/4 rather than a 2/3 scaling exponent. After accounting for taxonomy and variability in parameter estimates among species using variance-weighted linear mixed effects modelling, temperature sensitivity of SMR had an activation energy (Ea) of 0.16 for both Northern and Southern Hemisphere species which was lower than predicted under the metabolic theory of ecology (Ea 0.2–1.2 eV). Northern Hemisphere species, however, had a higher SMR at each habitat temperature, but a lower mass-scaling exponent relative to SMR. Evolutionary trade-offs that may be driving differences in metabolic rate (such as metabolic cold adaptation of Northern Hemisphere species) will have important impacts on species abilities to respond to changing environments.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-013-2767-8) contains supplementary material, which is available to authorized users.
Highlights
The physiological plasticity of ectotherms is correlated with large-scale geographic patterns in environmental conditions (e.g. Gaston et al 2009)
Metabolic rate is a key component of energy budgets and investigations into its scaling with morphology and temperature have been extensive (Heusner 1985; Bergmann 1847), with global comparisons being used to identify geographic patterns of metabolic rate and potential mechanisms underlying these patterns (e.g. Clarke and Johnston 1999)
Even unitary organisms whose metabolic rate is directly proportional to the energy requirements of activity and minimal tissue maintenance should have a mass-scaling exponent close to 1, whereas organisms whose metabolic rate is restricted by the flux of resources across surfaces should have a 2/3 mass-scaling exponent (Glazier 2010)
Summary
The physiological plasticity of ectotherms is correlated with large-scale geographic patterns in environmental conditions (e.g. Gaston et al 2009). Significant differences in the scaling of metabolic rate with body mass are known across morphologically distinct marine taxa, resulting in scaling exponents largely between 2/3 and 1 (Glazier 2010). Modular organisms, such as clonal bryozoans, can have a mass-scaling exponent for metabolic rate that is close to unity (M1; Hughes and Hughes 1986; Peck and Barnes 2004). A number of other restrictions on the allometry of metabolic rates, such as the evolutionary diversification of cell and genome size (which predicts a scaling exponent between 2/3 and 1) are thought to add to the variation of scaling exponents (Kozlowski et al 2003)
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