ICES Journal of Marine Science (2011), 68(6), 1147–1154. doi:10.1093/icesjms/fsq188
Synergistic effects of elevated CO2 and temperature on the
metabolic scope and activity in a shallow-water coastal decapod
(Metapenaeus joyneri; Crustacea: Penaeidae)
Awantha Dissanayake* and Atsushi Ishimatsu
Institute for East China Sea Research, Nagasaki University, Taira-machi 1551-7, Nagasaki 851-2213, Japan
*Corresponding Author: tel: +81 95 850 7311; fax: +81 95 840 1881; e-mail: [email protected].
Dissanayake, A., and Ishimatsu, A. 2011. Synergistic effects of elevated CO2 and temperature on the metabolic scope and activity in a shallowwater coastal decapod (Metapenaeus joyneri; Crustacea: Penaeidae). – ICES Journal of Marine Science, 68: 1147 – 1154.
Received 27 May 2010; accepted 11 November 2010; advance access publication 4 February 2011.
The physical drivers of climate change (increased CO2; hypercapnia and temperature) are causing increasing warming of the earth’s
oceans, elevating oceanic CO2 concentrations, and acidity. Elucidating possible climate change impacts on marine biota is of paramount importance, because generally, invertebrates are more sensitive to hypercapnia than fish. This study addresses impacts of synergistic factors; hypercapnia and temperature on osmoregulation, acid – base balance, and resting and active metabolism (assessed as
oxygen consumption rates) and behavioural performance in a model nektonic crustacean. Metapenaeus joyneri exposed to both
hypercapnia (1 kPa) at two temperatures (15 and 208C) demonstrated significant physiological effects, i.e. new regulatory set
points (lower haemolymph osmolality and higher pH, i.e. alkalosis) and reduced metabolic scope (MS), compared with control individuals (normocapnia, 0.04 kPa). Behavioural effects included a significant 30% reduction in swimming ability and may be the result of
reduced MS (i.e. difference between active and routine metabolism). Synergistic factors may cause organisms to shift energy utilization
towards up-regulation of maintenance functions (i.e. osmoregulatory ability) resulting in a decrease in both aerobic scope and energydemanding activities. Laboratory-derived evidence elucidating the impacts in key model groups is of paramount importance, if we are
to improve our knowledge of physiological effects of synergistic climate change factors.
Keywords: crustacea, hypercapnia, metabolic scope, physiology, temperature.
Introduction
The current trend of global anthropogenic climate change (i.e.
ocean warming) and future predicted ocean CO2 scenarios
(IPCC, 2001; Caldeira and Wickett, 2003; IPCC, 2007) are
thought to cause widespread effects on the marine fauna and ecosystem processes (Fabry et al., 2008). The rapid rise in ocean acidification (because of hypercapnia; i.e. elevated CO2 levels) may
have profound effects on organisms, of which critical limits and
long-term effects are currently unknown (Pörtner et al., 1998;
Seibel and Walsh, 2003; Widdicombe and Spicer, 2008). It is
likely that changes in seawater carbonate chemistry would affect
the internal physiology of marine organisms, such as acid–base
balance (Raven et al., 2005). Mechanisms of acid–base regulation
require ion exchange with seawater where hydrogen [H+] and
bicarbonate ions [HCO3– ] are exchanged for sodium [Na+] and
chloride [Cl2], respectively (Cameron and Mangum, 1983;
Mantel and Farmer, 1983; Wheatly and Henry, 1992). Calcifying
organisms are generally considered more sensitive than fish,
because fish are efficient acid–base regulators (Marshall and
Grosell, 2006); however, crustaceans have also been demonstrated
to be efficient regulators (Melzner et al., 2009b). Shallow-water
coastal nektonic organisms, such as crustaceans, are considered
at risk, because of increased sensitivity to the synergistic effects
of temperature and hypercapnia, which have consequences for
# 2011
hypercapnia, hypoxia, and thermal tolerance (Metzger et al.,
2007; Pörtner and Farrell, 2008; Walther et al., 2009); however,
there is a lack of knowledge of the synergistic effects of these
factors.
The current study was designed to elucidate the effects of both
elevated CO2 and temperature in a model crustacean group
(Decapoda: Penaeidae). Metapenaeus joyneri was chosen as a
species indicative of the nektonic ability of other organisms occupying various ecosystems (shallow-water coastal and open ocean),
as well as its economic importance as a commercial fishery species
(Asia). Specifically, this study tested the hypothesis that both elevated CO2 and temperature results in an alteration in internal physiological function (osmoregulation and acid–base balance) and
decrease in organism physiological performance, evaluated using
energetically demanding behaviour (swimming ability) and
aerobic performance [active and routine metabolic rates (AMR
and RMR)]. Hypercapnia is purported to reduce organism performance, especially at higher temperatures, coupled with a thermally limited mismatch between oxygen supply (in water) and
demand (in tissues; Pörtner and Knust, 2007; Pörtner and
Farrell, 2008). The difference between AMR and RMR is defined
as aerobic or metabolic scope (MS); it therefore estimates the
capacity of an individual for aerobic metabolism beyond that of
biological maintenance (Fry, 1971). Aerobic performance
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1148
therefore provides an ecologically relevant measure of physiological consequences of the synergistic effects of hypercapnia and
increased temperature exposure.
Material and methods
Animal collection and maintenance
Metapenaeus joyneri (n ¼ 60) were collected with nets by a local
fisher (Ariake Bay, Saga Prefecture, Japan; 32858′ 35′′ N
130813′ 09′′ E) during February 2010. Only adults were used in
the experiments described below; each individual was measured
[body length (BL), mm: rostrum to telson); Dall, 1964; mean +
s.d.; BL 94 + 5.2] and weighed (wet weight, g: mean + s.d.;
6.3 + 0.8). Prawns were maintained in two flow-through
holding aquaria (volume ¼ 250 l), each containing filtered
(10 mm carbon-filtered), continually aerated seawater [salinity of
32 psu, 15 + 18C, pH (NBS scale) 8.2 + 0.01] under a 12 h
light:12 h dark photoperiod.
Experimental design
Prawns (n ¼ 10 per treatment, per time-point) were exposed to
either CO2-enriched hypercapnic seawater (1 kPa: to elucidate
hypercapnia-associated changes in acid–base regulation as previously used in crustaceans; Cameron, 1978, 1985; Cameron and
Batterton, 1978; Cameron and Iwama, 1987) or control seawater
(normocapnic: 0.04 kPa CO2) for either 0, 1, 2, or 10 d acclimated
to various seasonal temperatures within their habitat range (spring
mean: 158C; autumn mean: 208C; and summer mean: 258C;
Minagawa et al., 2000). Osmolality and extracellular pH were
assessed before exposure (0 d) and in individuals exposed for 1
and 2 d; behavioural and physiological parameters were assessed
only in individuals exposed for 10 d.
The CO2-enriched seawater for the experimental period was
prepared as follows. Ambient (outdoor) air was desiccated, filtered, and mixed with CO2 using a Gas Blender (Kofloc, Japan)
to produce a nominal CO2 concentration (PCO2: 1 kPa). The
resultant gas mixture was supplied to the seawater in the experimental tanks. The normocapnic (ambient) air (PCO2: 0.04 kPa)
was supplied to the seawater in the experimental tanks without
any added CO2. The gas flow rate to each tank (irrespective of
treatment) was 0.2 l min21. Details of seawater physicochemical
parameters for the 10-d exposure period are presented in
Table 1. Prawns were held individually in aquaria (volume ¼ 2 l)
and fed artificial feed pellets (0.19 g + 0.001) equivalent to ca.
3% body mass for crustaceans (McGaw and Reiber, 2000;
Dissanayake et al., 2008) every 2 d; the water was changed
within 18 h of feeding.
Behavioural analysis: critical swimming speed (Ucrit)
Following 10 d exposure, the critical swimming speed (Ucrit) of
individuals (Brett, 1964; defined as swimming endurance and
expressed in cm s21) and relative critical swimming speed
(RCSS; Ucrit divided by BL and expressed in BL s21) was evaluated.
A series of behaviours was assessed on behavioural categories
based on direction and net movement (Roast et al., 2000).
Swimming trials were performed in a swimming chamber, as
described by Yu et al. (2009); swimming behaviour was tested
on individuals where the water velocity was adjusted manually
using a Kofloc flowmeter. Before the start of swimming trials, individuals were given 30 min for acclimation (no water flow); trials
commenced (i.e. water flow) when individuals were stationary
A. Dissanayake and A. Ishimatsu
(i.e. resting/behavioural type 1) and the swimming behaviour
was then recorded. In general, penaeids were observed to exhibit
positive rheotaxic behaviour (type 2) at low current velocities
(33 cm s21), where individuals swim forward into the current.
As current velocity increases, individuals exhibit either positive
rheotaxis or neutral position (neutral rheotaxis: type 3). As
current velocity increases further (maximum velocity ¼
133.36 cm s21), individuals that cannot maintain positive or
neutral rheotaxis are swept with the current to the end of the
swim chamber [behaviour type 4 (facing into current) and 5
(facing away from current) but carried with the current], i.e. the
criteria for fatigue and limit of swimming endurance. The swimming endurance at the maximum velocity was recorded for each
individual using a video camera (Sony DCR-HC48). Individuals
were returned to their respective treatment aquaria before conducting physiological assessments (see below). The critical swimming speed of each penaeid was calculated where individuals
were subjected to a water velocity in the swimming chamber, at
an incremental rate of 16.7 cm s21 for 30 s intervals until the
maximum water velocity was reached (133 cm s21) and where
penaeids exhibited positive rheotaxis; the time taken for fatigue/
limit of swimming endurance was recorded [Equation (1)]:
Ucrit = U1 +
T1
× U2 ,
T2
(1)
where Ucrit is the critical swimming speed, U1 ¼ 133 cm s21, U2 ¼
16 cm s21, T1 the time taken for fatigue/limit of swimming
endurance (s), and T2 ¼ 30 s.
Physiological analysis: metabolic rates
Oxygen consumption rate (using respirometry) has been used as a
surrogate measure of metabolic rates (Fry, 1971); RMR (inactive
or basal) and AMR are described as the minimal and maximum
rates of aerobic metabolism (Walker et al., 2009). Oxygen consumption rates were determined using closed respirometry 1.9 l
respiration chambers with a “closed-cell” recirculating system
(temperature maintained +0.58C with an immersion heater, salinity of 32 psu). To measure RMRs, individuals were placed in
each chamber (30 min acclimation) with recirculating seawater
(open chamber). To determine oxygen concentrations, water
samples were taken at the start (t0) and after 40 min (t40) during
which the water was non-recirculating (closed chamber).
Following the measurement of RMR, oxygen levels were replenished for 30 min (i.e. oxygen-saturated seawater, so open
chamber), whereafter AMRs were measured. AMRs are described
as maximum sustained activity (Brett, 1972). To induce activity
in penaeids in a “closed-cell” respirometer, a combination of a
“feeding cue” [0.5 ml of food pellets diluted in seawater (0.08:1,
w/v)] and exercise (swimming) was used. Individuals (n ¼ 10)
were tethered to a nylon line using a “thoracic saddle” (plastic
saddle glued to the cephalothorax using cyanoacrylate glue).
Before withdrawing water samples (t0), 0.5 ml of “feeding cue”
was introduced into the respirometer via a syringe and needle
(closed chamber), and the nylon line was pulled taut and fixed,
thereby raising individuals vertically from the base to the top of
the respirometer chamber, and held in place, thus inducing swimming behaviour. AMRs are therefore described here and measured
by the induction of activity by a combination of a feeding cue and
exercise (representative of a sustained rate of oxygen consumption). The difference between AMR and RMR is defined as
1149
Synergistic effects of elevated CO2 and temperature on the metabolic scope of Metapenaeus joyneri
Table 1. Seawater physico-chemical and M. joyneri haemolymph parameters (mean + 1 s.e.) for both normocapnia (0.04 kPa) and
hypercapnia (1 kPa) treatments (n ¼ 10 for each parameter).
Treatment/parameter
Seawater temperature (8C)
Salinity (psu)
Seawater pHNBS
Seawater PCO2 (kPa)
SW total alkalinity (mmol kg – 1)
VCal (calcite saturation)
VAra (aragonite saturation)
Haemolymph PCO2 (kPa)
–1
Haemolymph [HCO2
3 ](mmol l )
Normocapnia (0.04 kPa)
15.0 + 0.02
19.9 + 0.06
24.9 + 0.07
32 + 0.1
32 + 0.1
32 + 0.1
8.14 + 0.01
8.16 + 0.01
7.94 + 0.01
0.04 + 0.001
0.04 + 0.005
0.07 + 0.020
2192 + 4.31
2173 + 4.37
2288 + 7.37
3.25 + 0.03
3.99 + 0.04
3.01 + 0.13
2.07 + 0.02
2.58 + 0.02
1.97 + 0.09
0.27 + 0.02
0.40 + 0.03
0.18 + 0.06
3.96 + 0.35
3.99 + 0.38
2.87 + 0.06
“scope for activity” or aerobic scope or MS (Fry, 1971). Oxygen
concentrations were measured (model 782 Strathkelvin
Instruments, Glasgow, Scotland) and standardized against individual weight (wet weight) to allow for the weight-specific expression
of oxygen consumption rates (expressed as mg O2 l21 g21 h21).
The thermal coefficient (Q10) of RMR and AMR for both normocapnia and hypercapnia treatments were calculated [Equation (2)],
where K1 and K2 are the metabolic rates at temperatures t1 and t2
(i.e. 15 and 208C), respectively (Spanopoulos-Hernandez et al.,
2005):
Q10 =
10/t2 −t1
K2
.
K1
15.0 + 0.02
32 + 0.1
6.91 + 0.02
0.92 + 0.004
2281 + 21.96
0.24 + 0.01
0.16 + 0.01
1.00 + 0.01
18.47 + 4.80
Hypercapnia (1 kPa)
19.9 + 0.06
24.9 + 0.07
32 + 0.1
32 + 0.1
6.90 + 0.01
6.69 + 0.03
0.92 + 0.005
1.52 + 0.005
2191 + 9.63
2188 + 1.72
0.27 + 0.01
0.18 + 0.01
0.17 + 0.01
0.12 + 0.01
1.00 + 0.05
0.84 + 0.01
16.54 + 0.44
36.11 + 0.20
Statistical analysis
Analysis of variance tests were performed to test for differences [as
revealed by the post hoc Student–Newman –Keuls tests] regarding
(i) the CO2 exposure level [normocapnia (control) or hypercapnia] or (ii) temperature (15, 20, or 258C) using GMAV for
Windowsw (Underwood, 2005). Before analysis, data were tested
for normality (Cochran’s test) to adhere to guidelines of the parametric test.
Results
(2)
Haemolymph sampling
Before sampling haemolymph, the cephalothorax was blotted
thoroughly using the absorbent paper. Haemolymph was withdrawn using an ice-chilled Hamilton microsyringe (volume ¼
100 ml); the microsyringe needle was inserted directly into the
pericardial cavity between the thorax and first abdominal
segment (Campbell and Jones, 1989). The sampled haemolymph
was then treated as follows.
Cellular endpoints: haemolymph and seawater
All measurements were taken within 10 s of haemolymph
sampling from each individual. Haemolymph osmolality was estimated for samples (volume ¼ 8 ml) using a vapour pressure osmometer (Wescor 5500, USA: Campbell and Jones, 1989; Spicer
et al., 2007). Total carbon dioxide content of haemolymph
(CCO2 mmol l21) was measured using a CO2 analyser
(Capni-con model 5, Cameron Instrument Company, USA;
Cameron, 1985). Dissolved inorganic carbon was measured in seawater using a total alkalinity titrator (ATT-05, Kimoto Japan).
Haemolymph partial pressure of CO2 (PCO2) was then calculated
using measured values for CCO2 and pH using a modified form of
the Henderson –Hasselbalch equation (Spicer et al., 1988) and as
used by Miles et al. (2007). Haemolymph (extracellular) pH and
seawater pH (NBS scale) was measured using a micro pH electrode
(Orion 8220BNWP) equilibrated at the experimental temperatures
(Thermo Scientific, USA) and inserted directly into the sample in
vitro within seconds of collection. Using the seawater pH and
CCO2 data, values for bicarbonate concentration [HCO2
3 ] and
the saturation states for calcium and aragonite were calculated
using the CO2SYS program (Pierrot et al., 2006) and presented
in Table 1.
High acclimation temperature (258C) caused moulting frequency
to increase in both normocapnia- and hypercapnia-exposed individuals, but hypercapnic individuals failed to survive past moulting (Figure 1), resulting in the high observed mortality, compared
with control (normocapnic) individuals. All individuals in the
lower acclimation temperatures (15 and 208C) survived.
Hypercapnia exposure, irrespective of temperature (15 and
208C), resulted in a significant average 27% reduction in critical
swimming speed (Ucrit: 30 and 25% at 15 and 208C, respectively:
F1,44 ¼ 10.28, p , 0.01: Figure 2), which is equivalent to a
reduction of 70 cm s21 and 8 BL s21 in Ucrit and RCSS, respectively. A significant concomitant reduction in MS was observed in
response to hypercapnia (irrespective of temperature; F1,36 ¼ 4.79,
p , 0.05; Table 2). Although normocapnic RMRs increased
significantly with temperature (Q10 ¼ 2.68; F1,36 ¼ 283.96, p ,
0.05), no differences were observed between hypercapnia (Q10 ¼
2.62) and control individuals (at either 15 or 208C: F1,36 ¼ 1.07,
p ¼ 0.31). The observed significant reduction in MS between
hypercapnic and normocapnic penaeids was because of a average
30% reduction in AMRs in hypercapnic prawns, irrespective of
temperature (36 and 22% reduction at 15 and 208C, respectively;
Q10 ¼ 1.18 and Q10 ¼ 0.72 for normocapnic and hypercapnic
prawns, respectively; Table 2). No data for RMR or AMR for
hypercapnia-exposed individuals at 258C were available because
of significant mortality (85%) during the 10 d exposure period
(Figure 1a).
Hypercapnia exposure over 10 d resulted in a significant lowering of haemolymph osmolality (F1,36 ¼ 75.62, p , 0.001), with
concomitant increase in haemolymph pH, compared with
control levels at all acclimation temperatures (F1,36 ¼ 15.40, p ,
0.001: Figure 3). A 12% decrease in abdominal muscle mass (percentage body mass) of hypercapnic individuals was observed compared with normocapnic individuals at 208C, but not at 158C
(Table 2).
1150
Figure 1. Survival (a) and moulting frequency (b) (mean + 1 s.e.) in
M. joyneri individuals acclimated to 258C. Open and filled circles
represent control (normocapnic: 0.04 kPa CO2) and hypercapnic
(1 kPa) treatments, respectively (n ¼ 10).
Figure 2. Swimming behaviour (mean + 1 s.e.) of M. joyneri
individuals at various acclimation temperatures. Open and hatched
bars represent control (normocapnic: 0.04 kPa CO2) and hypercapnic
(1 kPa CO2) treatments, respectively (n ¼ 10, except 258C
hypercapnic treatment where n ¼ 2). Significance symbols NS and
double asterisks indicate no significance and p , 0.01, respectively.
Table 2. Summary of physiological effects (mean + 1 s.e.) between normocapnia (0.04 kPa CO2) and hypercapnia-exposed (1 kPa CO2) M. joyneri individuals acclimated to various
temperatures (n ¼ 10).
1588 C
Normocapnia
(0.04 kPa)
15.48 + 2.53
44.86 + 0.60
Hypercapnia
(1 kPa CO2)
9.45 + 1.21
45.50 + 1.02
Statistical
significance
F1,36 ¼ 4.79, p , 0.05
NS
NS, ND, and NT indicate no statistical significance, no data, and not tested, respectively.
Normocapnia
(0.04 kPa)
11.34 + 1.76
45.95 + 1.48
Hypercapnia
(1 kPa CO2)
8.84 + 2.51
40.56 + 0.46
2588 C
Statistical
significance
F1,36 ¼ 4.79, p , 0.05
F1,36 ¼ 11.76, p , 0.01
Normocapnia
(0.04 kPa)
8.33 + 2.12
ND
Hypercapnia
(1 kPa CO2)
ND
ND
Statistical
significance
NT
NT
A. Dissanayake and A. Ishimatsu
Parameter
MS (DAMR– RMR)
Muscle mass (% of
body mass)
2088 C
Synergistic effects of elevated CO2 and temperature on the metabolic scope of Metapenaeus joyneri
1151
Figure 4. Schematic diagram of a reduction in aerobic scope [i.e.
difference between AMR and RMR] with hypercapnia exposure
(1 kPa CO2 for 10 d). Open and filled symbols represent
normocapnic (0.04 kPa CO2) and hypercapnic metabolic rates,
respectively. Regression lines are plotted for normocapnic RMR and
AMR values. Polygon B depicts the aerobic scope for
hypercapnia-exposed penaeids with increasing temperature. Polygon
A depicts the decrease in the aerobic scope of ca. 30%.
Figure 3. Time course (d) of internal osmolality and haemolymph
(extracellular) pH (mean + s.e.) of M. joyneri at (a) 158C, (b) 208C,
and (c) 258C depicting haemolymph acid-base alterations over a
10-d hypercapnic exposure. Open and closed symbols represent
control (normocapnic: 0.04 kPa CO2) and hypercapnic (1 kPa)
treatments, respectively (n ¼ 10).
Discussion
Swimming behaviour and aerobic scope
The results demonstrated several physiological and behavioural
impacts of hypercapnia exposure in a penaeid species representative
of shallow-water coastal decapod crustaceans. This study is the first
to describe a reduction in swimming behaviour in a nektonic
crustacean postulated to occur because of a reduction in aerobic
scope/MS. Results suggest that the effects of climate change
factors (increased temperature and hypercapnia) may be revealed
when organisms are engaging in energetically demanding behaviours, such as swimming. A significant reduction in swimming
ability (ca. 30%) may be linked to the reduction in AMRs and
hence a concomitant reduced MS or aerobic scope (ca. 30%: cf.
Figure 4). RMRs increase with increasing acclimation temperature.
However, AMRs are compressed at higher temperatures, corroborating Pörtner and Farrell’s (2008) hypothesis of decreased aerobic
scope and performance at temperatures past the thermal
optimum (assumed in this study as between 15 and 208C for
M. joyneri), as well as the loss in aerobic performance with exposure
to a synergistic stressor (in this case hypercapnia). The effects of
elevated CO2 on swimming performance have been recently examined for several species of fish, Atlantic cod Gadus morhua (Melzner
et al., 2009a), and two species of coral reef fish (Ostorhinchus
doederleini and O. cyanosoma; Munday et al., 2009), with conflicting
results. Atlantic cod demonstrated no significant change in
aerobic scope or critical swimming speed after 12-month exposure
to elevated CO2 levels (3080 and 5792 matm), whereas in coral
reef fish, aerobic scope decreased significantly (between 30 and
50%) when exposed to 1000 matm PCO2 after 1 week of
acclimation, largely because of increased RMR. No such increases
were observed in either RMR or AMR in cod (Melzner et al.,
2009a; Munday et al., 2009). The apparent difference in effects on
aerobic scope, as observed in these fish species, may be because of
the different thermal niches being occupied (29–338C for coral
reef fish and 58C for Atlantic cod). For aquatic animals, tropical
species are considered living close to thermal limits compared
with temperate species (Tewksbury et al., 2008), so significant
effects of temperature and hypercapnia could be exerted close to
these limits.
In the current study, the apparent significant reduction in
aerobic scope of penaeids was observed at higher hypercapnia
1152
levels than tested in earlier studies; however, it provides a mechanistic insight into both physiological (RMR and AMR) and behavioural (swimming ability) effects of climate change in a decapod
crustacean. The reduced swimming ability observed in the
current study was relatively high, because first, a high hypercapnia
level was evaluated, and second, individual penaeids were subjected to high water velocities (therefore accounting for the high
relative swimming speeds) to test limits to fatigue following energetically demanding behaviour.
Physiological effects
At a higher temperature (258C), which is close to the summer
maximum temperature experienced by the penaeids (Minagawa
et al., 2000), a high mortality (85%) rate was observed in the
hypercapnic treatment, compared with the normocapnic treatment where all individuals survived. Observations indicate that
this acclimation temperature triggered individuals of both treatments to moult. However, hypercapnic individuals did not
survive post moult, because they did not complete the moulting
process and were presumably encapsulated within the exoskeleton,
also known as moult death syndrome (Shields et al., 2006). This is
commonly observed in lobsters at high temperature, although the
exact mechanism is unclear. Physiological effects of both temperature and hypercapnia exposure include a lowering of haemolymph
osmolality, with a concomitant shift in haemolymph pH (alkalosis). Previous studies of several other decapod crustaceans have
revealed that hypercapnia exposure results in haemolymph pH
acidosis and that it is linked to ionic regulatory ability
(Cameron, 1978, 1985; Cameron and Batterton, 1978; Cameron
and Iwama, 1987). Strong ionic regulators can actively compensate
for lowering of body fluid pH, because of elevated PCO2, which
involves either outward transport of hydrogen [H+] ions or
inward transport of bicarbonate [HCO2
3 ] ions, and is linked to
exchanges for sodium [Na+] and chloride [Cl2] ions, respectively
(Cameron, 1985; Henry and Wheatly, 1992). The mechanism for
“overcompensation” (as observed by significant alkalosis) is
unknown, but it may be linked to ion exchange. When crustaceans
are returned to normoxic conditions following hyperoxic or
hypercapnic exposure, [HCO2
3 ] or anion exchange is important
to the restoration of haemolymph pH after acid–base imbalances
(i.e. acidosis and/or alkalosis; Truchot, 1979; Wheatly, 1989).
Whiteley et al. (2001) found that intracellular (leg muscle) alkalosis results in a transient metabolic acidosis and subsequent alkalosis in the blood in a euryhaline crustacean exposed to low salinity.
Regarding salinity, alterations in acid–base status are considered
secondary to the adjustments required for ionic and osmotic regulation and, therefore, cell volume regulation (Whiteley et al.,
2001), and it may account for the changes in osmotic regulation
observed in penaeids. The significant decrease in osmolality of
around 350 mosmol kg21 (from 800 to 450 mosmol kg21)
observed in all three acclimation temperatures (Figure 3a–c),
suggests an alteration in internal physiological functioning
(osmotic regulation) consistent over the 10-d exposure period.
The long-term energetic consequences for this possible
up-regulation are unknown, but may account for the possible
muscle loss (wastage or changes in cell volume regulation)
observed here at a higher temperature (208C).
Muscle wastage has been observed as a compensatory function
in echinoderms (brittlestars) exposed to hypercapnia as maintenance functions are altered (increased calcification and metabolism;
Wood et al., 2008). Our findings suggest that this shallow-water
A. Dissanayake and A. Ishimatsu
coastal species compensates for hypercapnia by lowering osmolality and a subsequent shift in internal pH. Any up-regulation of
internal functioning (osmotic regulation: hypothesized to occur
by up-regulation of the iono-regulatory enzyme Na+/K+
ATPase) is an active and hence energy-consuming process (Lucu
and Towle, 2003; Freire et al., 2008). There is a direct link
between energetic costs for energy-demanding behaviour (swimming) and maintenance (osmotic regulation; Gibbs and Somero,
1990), and it could be assumed that under hypercapnia, energetic
costs are diverted to internal functioning, so affecting energy available for aerobic performance (Seibel and Walsh, 2003).
Conclusions
Our findings imply that the synergistic effects of both elevated CO2
and temperature could compromise the aerobic scope and swimming ability of penaeids. Although the hypercapnia levels used
here are higher than future forecasted surface water CO2 scenarios
(Caldeira and Wickett, 2003), it may be relevant for elucidating
possible effects from CO2 geological sequestration leaks in shallowwater coastal ecosystems, especially in enhanced oil recovery techniques (IPCC, 2005) and possible subsequent leaks, e.g. Deepwater
Horizon oil leak in the Gulf of Mexico (April 2010). High CO2 concentrations have also been recorded in estuaries where high PCO2
values have been recorded globally [380–7800 matm (reviewed in
Raymond et al., 2000)]. Nektonic organisms, such as penaeid
prawns, inhabit shallow-water coastal habitats and utilize these
habitats as important nursery grounds and feeding areas
(Haywood et al., 1998; Webb and Kneib, 2002). Our findings establish that the synergistic effects of both hypercapnia and temperature
affect aerobic performance by reducing MS, as hypothesized by
Pörtner and Farrell (2008). Synergistic effects of both high temperatures and hypercapnia may serve to “narrow” thermal windows, as
observed here, and corroborate previous evidence in other species
(Metzger et al., 2007; Melzner et al., 2009a; Munday et al., 2009),
the ecological consequences of which are currently unknown.
Acknowledgements
This study was supported by a Japan Society for the Promotion of
Science postdoctoral fellowship awarded to AD.
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