In ectotherms, the aerobic scope for activity is defined as
the difference between the basal (or resting) and maximum
oxygen consumption rate (Brett 1971) with the maximum
aerobic scope occurring at the optimal temperature. Deviations
from the thermal optimum leads to a reduction in aerobic
scope, and at critically low or elevated temperatures,
aerobic scope disappears and anaerobic metabolism is
employed (Po¨ rtner & Farrell 2008). The limits of thermal tolerance
in ectotherms are likely linked to the inability of the
ventilation and circulation systems to uptake and distribute
oxygen to meet the oxygen demand of the tissues (Po¨ rtner
2001; Po¨ rtner & Knust 2007). The oxygen requirements of
the tissues not being met leads to hypoxaemia, a decrease in
aerobic scope and eventually the onset of anaerobic metabolism
(Po¨ rtner & Knust 2007). Beyond critical temperature,
any additional energetic demands must be supplied by anaerobic
pathways, the consequences being depletion of anaerobic
fuels (i.e. muscle glycogen, adenosine triphosphate and
phosphocreatine) and accumulation of anaerobic end products
(i.e. lactate).