Geoscience Reference
In-Depth Information
with respect to CaCO
3
( Millero 1996 ; Fabry
et al.
2008 ).
In the eastern Tropical Pacii c
D. gigas
migrates from
warm shallow waters at night into a layer of cooler,
but oxygen-depleted, hypercapnic water at a depth of
about 300 m during the day (Gilly
et al.
2006 ). The
hypoxia they experience during these vertical excur-
sions is sufi cient to exclude many top vertebrate
predators (Carey and Robison 1981; Prince and
Goodyear 2006). In contrast, the Humboldt squid
appears to thrive in OMZs. Over the last few years,
D.
gigas
has greatly extended its tropical/subtropical
range as far north as Canada and Alaska, a range
expansion correlated with the expanding OMZ and
with the decline of some commercial i sh stocks
( Zeidberg and Robison 2007 ).
Some authors have proposed using naturally
acidii ed water, such as that found near hydrother-
mal vents or in OMZs, as a natural laboratory for
studying the mechanisms that organisms may use
to adjust to ocean acidii cation (Fabry
et al.
2008 ;
Tunnicliffe
et al.
2009 ). However, Rosa and Seibel
(2008, 2011) demonstrated that the effects of ele-
vated
p
CO
2
on activity and metabolism in the OMZ
are confounded by, and negligible in comparison to,
the effects of the hypoxia. Metabolism and activity
are suppressed at temperatures and oxygen levels
consistent with the daytime habitat depth of
D. gigas
. While in some organisms metabolic sup-
pression (Hand 1991) is acutely triggered by high
CO
2
tensions (10 000 μatm) and the associated low
level of extracellular pH (Reipschläger and Pörtner
1996 ), a
p
CO
2
of 1000 μatm had no effect on routine
or resting metabolism or on activity levels of
D.
gigas
at 10°C (Rosa and Seibel 2008). However, after
a 12-h acclimation period and a 24-h exposure
period to a
p
CO
2
of 1000 μatm, active metabolism
decreased at higher activity levels and higher tem-
peratures (e.g. 25°C), from 70 to 48 μmol O
2
g
-1
h
-1
on average, a reduction by 31%. Although the OMZ
environment is characterized by both hypoxia and
high CO
2
levels these results indicate that for the
squid in its cold, hypoxic daytime habitat at depth
the effect of oxygen availability on metabolic rate
overwhelms the more subtle CO
2
effect. However,
at warmer temperatures and higher activity levels
in the squid's night-time habitat, CO
2
has an impor-
tant inl uence on the metabolic rate of
D. gigas
. This
pattern matches the projection developed above
that CO
2
effects become stronger at the edges of the
thermal window.
To date, the response of acid-base regulation to
elevated
p
CO
2
has been examined in a single nekto-
benthic cephalopod species, the cuttlei sh,
Sepia
ofi cinalis
. This species exhibits active compensation
of blood pH during exposure to a
p
CO
2
of 6000
μatm. Unlike teleosts, pH compensation is only par-
tial (Gutowska
et al.
2010a). Despite a decrease in
extracellular pH by 0.2 units (Fig. 8.4),
S. ofi cinalis
maintained control standard metabolic rates during
acute exposure, and control growth rates over a
6-week period (Fig. 8.5). The oxygen saturation of
the arterial blood was not signii cantly modii ed by
a pH decrease of 0.2 units. The tolerance of the cut-
tlei sh to hypercapnia may thus partly be attributed
to its high capacity to maintain blood pH, which is
higher than in other invertebrates, and the compar-
atively low oxygen demand and insensitivity of its
haemocyanin to acidosis.
To increase our understanding of the sensitivity
of cephalopods to ocean acidii cation, the mecha-
nisms of acid-base regulation and ion transport in
their gills need to be characterized at the same level
of detail as in i sh (see Section 8.2). In
S. ofi cinalis
,
the transporter Na
+
/K
+
-ATPase has been localized
in the basolateral membranes of the gills and dis-
plays activity levels similar to those in teleosts
( Schipp
et al.
1979 ; Hu
et al.
2010 ). A recent study
found a transient elevation (15%) in maximum
activity of Na
+
/K
+
-ATPase during the i rst 10 days
of exposure to a
p
CO
2
of 3000 μatm in juvenile
S.
ofi cinalis
(M. Y. Hu
et al.
pers. comm.). Activity
returned to control values after 6 weeks of expo-
sure. Furthermore, there were no signii cant CO
2
-
related expression responses in transcripts of 29
proteins from various gene ontology classes in the
gills (e.g. ion transporters, metabolic enzymes,
stress-response proteins, and transcription factors).
The electrogenic Na
+
/HCO
3
-
cotransporter NBC1
may play an important role in the hypercapnic
response of cephalopods, similar to its role in i shes.
The presence and electrogenic nature of NBCs have
recently been demonstrated in the giant i bre lobe of
squid (
Loligo pealei
; Piermarini
et al.
2007 ). Whether
this or other isoforms contribute to the hypercapnic
response in the branchial tissues of cephalopods
remains to be established. Similar to teleosts, it can
