The relationship between metabolic rate and latitude in marine ectotherms has
received much interest, especially for increasing our understanding of the metabolic
specialisations shown by polar species. Studies have been undertaken to examine
natural variations in metabolic rate ( oxygen uptake) and rates of protein synthesis in
an ecologically important group of marine ectothems, the gammarid amphipods, with
a wide latitudinal distribution along the coasts of the NE Atlantic and Arctic Oceans.
Comparisons between populations revealed relatively low whole animal rates
of oxygen uptake and protein synthesis in the subarctic populations of Gammarus
setosus and G. oceanicus from Svalbard (79°N). Despite an increase in rates of both
variables in G. oceanicus as latitude decreased, the energetic unit-costs of protein
synthesis remained the same. Consequently, costs of protein synthesis, at least in the
subarctic-temperate G. oceanicus, are fixed and independent of temperature. In sharp
contrast, the temperate species, G. locusta, shows no variation in rates of oxygen
uptake or protein synthesis across its latitudinal range, despite a variation in habitat
temperature of 8°C. G. locusta is also characterised by decreased Arrhenius activation
energies for metabolism and increased RNA efficiencies with latitude, suggesting
optimisation of key enzymes to lower temperatures in the northern population (53°N).
The cold-temperate upper shore species, G. d. duebeni, also shows no variation in
rates of oxygen uptake or protein synthesis with latitude, probably to impart some
independence from its highly variable environment and to compensate for the effects
of shorter seasons on growth and development. Unexpectedly, G. d. duebeni exhibited
comparatively low rates of protein synthesis, suggesting low rates of energy turnover.
This may be due to resource limitation and energy conservation in an unstable
environment.
It appears that warm-temperate species compensate for temperature-related
changes in metabolic rate and protein synthesis, whereas the Arctic/boreal species do
not. Such differences could be related to their ancestral origins and thermal histories.
Global-scale meta-analysis of rates of oxygen uptake in 48 species of amphipod
indicates an overall lack of metabolic compensation with latitude between species,
with low energetic costs ofliving observed in polar species. Low rates of energy
consumption and expenditure may be advantageous in low energy polar
environments; and may underlie K-selected traits such as slow rates of growth,
development and reproduction reported in most polar ectotherms, including
amphipods. Such traits favour stable environments and may limit adaptation to long
term variations in temperature associated with climate change.