Geoscience Reference
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Aerobic performance decreased by 37 and 47%,
respectively. This i nding contrasts with that in
Atlantic cod (
G. morhua
) where there was no signii -
cant change in swimming capacity after 12 months
of exposure to both 3000 and 6000 μatm (Melzner
et al
. 2009b). This difference may indicate a higher
sensitivity of warm-water i shes but may also relate
to the different durations of the respective acclima-
tion periods (12 months versus 1 week). It remains
to be investigated whether long-term acclimation to
elevated seawater
p
CO
2
would alleviate the response
found in the two tropical i sh species. It is also
important to consider that the cod study was con-
ducted well within the thermal range of the species;
this may be one reason why exposure to elevated
CO
2
did not affect the resting and active oxygen
consumption rates. Unchanged aerobic scope at
intermediate temperatures coincides with the gen-
eralized conclusion that i sh growth is unaffected
until
p
CO
2
becomes much higher than 15 000 μatm
( Ishimatsu
et al.
2008). Overall, most studies indi-
cate that adult temperate zone teleosts, if in the
midst of their thermal range, will not be sensitive to
increases of seawater
p
CO
2
of about 1000 μatm
above normocapnic control values.
Nearly all experiments examining the sensitivity
of marine teleosts to ocean acidii cation have been
conducted with temperate i shes. Therefore, the
i nding of a surprisingly high sensitivity to ocean
acidii cation in the two species of tropical cardinal
i shes emphasizes the need to conduct studies in a
broad range of habitats and climate zones. The
decrease in aerobic scope of the coral reef i shes
occurred especially at very low or high tempera-
tures. Also, the acclimation temperature was very
different (~5°C for cod and 29 to 32°C for the tropi-
cal cardinal i shes). These differences probably coin-
cide with different capacities for acid-base
regulation and cardiovascular or ventilatory sys-
tems. At 5°C, metabolic rates are low and the sea-
water oxygen concentration is more than 30%
higher than in tropical waters. In light of the princi-
ples of the concept of OCLTT, tropical i shes live
closer to the edge of oxygen limitation than temper-
ate i shes. This may render tropical i shes more sen-
sitive to environmental change in general, and even
more so to the combined effects of temperature
extremes and ocean acidii cation. In fact, the data of
Munday
et al.
(2009a) illustrate that sensitivity to
elevated CO
2
is strongly enhanced at the low and
high ends of the thermal window, similar to earlier
i ndings in crustaceans (Walther
et al.
2009 ), thereby
coni rming the projections by Pörtner and Farrell
(2008) for the effect of elevated
p
CO
2
on marine
fauna. It remains to be explored to what extent the
combined effects of temperature and elevated CO
2
can be compensated for by acclimatization. It is con-
ceivable that at the limits of thermal acclimatization
capacity, the capacity to acclimatize to elevated
p
CO
2
levels is also reduced, and vice versa.
8.3 Effects of ocean acidii cation
on cephalopods
Among nektonic animals the oceanic squids are the
most active cephalopods, competing with active
pelagic i shes such as tuna and marlin. These mus-
cular squid display a very high oxygen demand
( Seibel 2007 ; Seibel and Drazen 2007 ) and are
hypothesized to live near the limit to oxygen avail-
ability. This high demand rel ects high activity lev-
els, as dictated by the pelagic environment, as well
as the low energy efi ciency that characterizes the
squid's jet propulsion relative to other forms of
locomotion (O'Dor and Webber 1986). The meta-
bolic capacity of these muscular squids is surpris-
ing if one considers the inherent constraints on their
metabolism and oxygen transport system. The oxy-
gen-carrying capacity of squid blood is low relative
to similarly active i shes due to viscosity-related
constraints associated with an extracellular respira-
tory protein. Although squid are rather eurythermal
due to their mode and rate of metabolism, they are
considered vulnerable to the combined impact of
climate-related variables (Pörtner and Reipschläger
1996 ; Pörtner and Zielinski 1998 ; Pörtner 2002 ; Rosa
and Seibel 2008). However, information on the sen-
sitivity to ocean acidii cation is much more limited
in cephalopod molluscs than in teleost i shes.
Oxygen transport in the blood of cephalopods
occurs via the extracellular pigment haemocyanin,
which is highly responsive to pH changes (Pörtner
1990 ; Bridges 1994 ), a property that facilitates the
delivery of oxygen to demanding tissues. In ommas-
trephid squids, such pH sensitivity is maximized
and i ne-tuned to support the full oxygen loading
