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existence, and is therefore tightly coupled to the
animal's i tness. In practice, metabolic scope is
determined as the difference between energy dis-
sipation (or, more conventionally, oxygen con-
sumption) at maximum swimming activity (active
metabolic rate) and at rest (standard or basal
metabolic rate) ( Fry 1971 ). By dei nition, the term
metabolic scope refers to the sum of all energy-
demanding biological activities, not exclusively to
mechanical work. There is evidence of a signii cant
positive relationship between metabolic scope,
estimated from increments in activity costs, and
other types of performance such as food intake
(Mallekh and Lagardère 2002) and growth
(Claireaux and Lefrançois 2007; Pörtner and Knust
2007), supporting the usefulness of the concept.
Available data indicate that the characteristics of
aerobic performance, the associated functional
capacity of tissues, not least those supplying oxy-
gen to mitochondria, shape the i tness of the organ-
ism and its role at the ecosystem level. The
underlying OCLTT concept has been used success-
fully to explain the effects of climate-induced
increases in temperature on species abundance
and survival in the i eld (Pörtner and Knust 2007).
The ecological relevance of the OCLTT concept in
the context of ongoing warming supports its use as
a matrix for studying the concomitant and syner-
gistic effects of other factors (Pörtner 2010).
In animals, energy is partitioned between func-
tions required for basal maintenance and the per-
formance of those required for additional activities
such as locomotion, immune defences and stress
resistance, digestion, and biosynthesis (e.g. growth
and reproduction). Loss of performance at the bor-
ders of the thermal envelope rel ects the earliest
level of thermal stress, caused by either insufi cient
functional capacity or hypoxemia (reduced oxygen
partial pressure in the blood), or both, and by the
resulting mismatch of oxygen supply and demand.
In other words, under extreme conditions a limita-
tion in aerobic scope may result from unfavourable
shifts in energy budget towards maintenance.
Oxygen dei ciency limits one or more of the aerobic
activities mentioned above and elicits transition
from the sustenance of i tness to time-limited pas-
sive tolerance and associated systemic and cellular
stress signals. These include hormonal responses or
oxidative stress, as well as the use of protective
mechanisms such as heat shock proteins at thermal
extremes (Anestis
et al.
2007 ; Feidantsis
et al.
2009 ;
Kyprianou
et al.
2010 ; Tomanek and Zuzow 2010 ;
see Box 8.1). In general, limitations in the response
of an organism to environmental factors i rst become
effective at the highest levels of biological organiza-
tion, the intact organism, which displays a higher
sensitivity than any of the subordinate, cellular and
molecular, functions ( Pörtner 2002 ). Nevertheless,
whole-organism limitations are ultimately based on
the integration of molecular functions into func-
tional and regulatory networks. Consequently,
when studying the adaptation of organisms to a
changing environment one needs to consider the
function of individual molecules and their integra-
tion into higher organizational levels, up to the
whole organism.
Evidence available for crustaceans and i shes
indicates that the thermal window of aerobic per-
formance is indeed affected by the synergistic
impact of elevated CO
2
(Metzger
et al.
2007 ; Munday
et al.
2009a ; Walther
et al.
2009 ) or hypoxia (see
Pörtner 2010 ). Recent i ndings by Findlay
et al
.
(2010) can be interpreted in similar ways. Thermal
acclimatization between seasons, or evolutionary
adaptation to a climate regime, has implications for
metabolic rate with consequences for capacity, per-
formance, and probably for tolerance to hypoxia or
elevated CO
2
. The relationships between energy
turnover, capacities for activity, and the width of
thermal windows, leads to a fundamental under-
standing of adaptation and specialization to cli-
mate, and, in turn, of sensitivity to climate change
( Pörtner 2006 , 2010 ). Insufi cient functional and res-
piratory capacity at the edges of the thermal win-
dow not only leads to oxygen dei ciency but also to
an accumulation of endogenous CO
2
produced by
metabolism, which may then contribute to the exac-
erbation of the response to elevated ambient CO
2
and be involved in narrowing the thermal window.
As a corollary, hypoxia or elevated CO
2
elicit strate-
gies of passive tolerance to environmental extremes
(e.g. metabolic depression) but lead the organism
earlier to its limits of functional capacity. Such
effects of climate-related stressors on functional
relationships might also underpin any climate-
induced changes in species interactions, and thus
