JOURNAL OF EXPERIMENTAL ZOOLOGY 309A (2008)
A Journal of Integrative Biology
Plasticity of Growth Rate and Metabolism in Daphnia
magna Populations From Different Thermal Habitats
J. CHOPELET, P.U. BLIER, AND F. DUFRESNE
De´partement de Biologie, Université du Que´bec à Rimouski, Rimouski,
Que´bec, Canada
ABSTRACT
The aim of this study was to evaluate the effect of temperature on growth and
aerobic metabolism in clones of Daphnia magna from different thermal regimes. Growth rate
(increment in size), somatic juvenile growth rate (increment in mass), and oxygen consumption were
measured at 15 and 251C in 21 clones from one northern and two southern sites. There were no
significant differences in body size and growth rate (increase in length) at both 15 and 251C among
the three sites. Clones from southern site 2 had a higher mass increment than clones from the other
two sites at both temperatures. Clone had a significant effect on growth (body length) and body size
at both temperatures. As expected, age at maturity was lower at 251C (4.5 days) than at 151C, (11.6
days) and body sizes, after the release of the third clutch, were larger at 151C than at 251C. Northern
clones had higher oxygen consumption rates and specific dynamic action (SDA) than southern clones
at 151C. By contrast, southern clones from site 1 had a higher oxygen consumption and SDA than
subarctic clones at 251C. Clones from southern site 2 had high oxygen consumption rates at both
temperatures. Our results reveal important differences in metabolic rates among Daphnia from
different thermal regimes, which were not always reflected in growth rate differences. J. Exp. Zool.
r 2008 Wiley-Liss, Inc.
309A, 2008.
How to cite this article: Choplet J, Blier PU, Dufresne F. 2008. Plasticity of growth rate
and metabolism in Daphnia magna populations from different thermal habitats. J. Exp.
Zool. 309A:[page range].
Phenotypic plasticity of organisms is a highly
relevant biological character to study in the face of
a changing world and challenged ecosystems.
Levins (’69) first introduced the term countergradient variation (CgV) that refers to a geographic pattern of variation in which genetic
influences on a trait oppose environmental influences across an environmental gradient, thereby
reducing phenotypic change across the gradient.
These compensations allow ectotherms from higher latitudes to carry out their life cycle in stressful
environments with low temperatures and short
growing seasons (Conover, ’90; Conover and
Schultz, ’95; Conover et al. ’97). Several physiological studies comparing ectotherm populations
from different latitudes support this CgV hypothesis (Wohlschlag, ’60; Holeton, ’74; Clarke, ’83;
Guderley and Blier, ’88; Addo-Bediako et al.,
2002). For example, in the lugworm Arenicola
marina, individuals from colder environments in
the White Sea have a higher mitochondrial activity
along with higher oxygen consumption rates
compared with their counterparts from warmer
r 2008 WILEY-LISS, INC.
environments in the North Sea (Sommer and
Pörtner, 2002). These studies reveal significant
levels of plasticity in these phenotypic characters
allowing partial compensation to adverse environmental conditions. Investigation of the evolution
of phenotypic plasticity requires integration of the
different physiological components of an organism
(from molecular biology to whole organism physiology) to identify the tools and constraints of
adaptation that could be ecologically relevant
(for a review, see Pörtner et al., 2006). The first
step in the identification of key genetic, metabolic,
or physiological attributes allowing adaptations or
Grant sponsors: Natural Sciences and Engineering Research
Council of Canada (NSERC); Fonds Québécois de la Recherche sur
la Nature et les Technologies (FQRNT); Northern scientific training
program.
Correspondence to: F. Dufresne, Département de Biologie, Université du Québec à Rimouski, 300 Allée des Ursulines, Rimouski,
Québec, Canada G5L 3A1. E-mail: [email protected]
Received 26 October 2007; Revised 17 March 2008; Accepted 29
June 2008
Published online in Wiley InterScience (www.interscience.wiley.
com). DOI: 10.1002/jez.488
2
J. CHOPELET ET AL.
acclimations is the identification of divergences
among species or populations, which could lead to
advantageous phenotypes in specific environments. Growth rates and metabolic rates are good
candidates as ecologically relevant characters
because they have been associated with life history
traits and fitness (Anguilletta et al., 2004), and
both can be strongly affected by environmental
conditions, offering the grip to selective process to
work on. Comparative approach is usually the
most convenient one to identify phenotypic
changes in different environments but here the
choice of animal model is critical to ensure that
the observed correlations are not dictated by
phylogenetic constraints and therefore could not
reflect local adaptations. Therefore, the study of
widely distributed and closely related taxon should
prevail in such comparative studies.
Species of zooplankton are often widespread
over large latitudinal distributions. Freshwater
populations of the same species may inhabit
contrasting environments, from deep permanent
lakes to shallow temporary rock pools, experiencing large temperature variations. Differences in
developmental rate, size, and growth rate have
been measured among copepod species from
different latitudes with higher performances at
temperatures prevailing in their specific environment (Londsale and Levington, ’85). Clones from
Daphnia magna collected in winter have higher
survival and fecundity at a low temperature than
those collected in summer (Carvalho, ’87; Carvalho and Crisp, ’87). D. carinata collected in summer
have a higher tolerance to warm temperatures
than D. carinata collected in winter (Wiggins and
Frappell, 2000). Lamkemeyer et al. (2003) found
that D. magna from Germany acclimated to cold
temperatures has a higher aerobic metabolism
compared with warm acclimated individuals. Both
developmental and metabolic adjustments are
involved in adaptation to thermal environments,
whereas their relationships and there respective
role are still enigmatic.
The objectives of this study were to assess
intraspecific variation in phenotypic responses
(body size, growth rate, and aerobic metabolism)
to temperature in populations of D. magna
inhabiting different climatic zones (subarctic and
temperate). If growth rate adjustments to different thermal regimes are modulated through
metabolic adaptations, we expect that differences
in growth performance or its thermal sensitivity
among populations will be reflected at the metabolic level. Therefore, we hypothesized that
J. Exp. Zool.
subarctic clones will have higher growth and
metabolic rates than clones from more southern
sites at 151C and lower rates at 251C. Clonal
reproduction (production of genetically identical
individuals) and wide distribution in different
habitats make D. magna a suitable model to study
the physiological mechanisms dictating growth
and metabolic plasticity. We herein document for
the first time intraspecific variability for aerobic
metabolism in a daphniid experiencing different
thermal regimes.
MATERIAL AND METHODS
D. magna origins
Subarctic clones were collected from Churchill,
Manitoba, Canada (581440 N; 941030 W) in July
and August 2004. They were found in small
oligotrophic rockbluff ponds in close proximity
to Hudson’s Bay. Temperate clones were collected
in Gimlet Lake, in the Nebraska Crescent
Lake National Wildlife Refuge, Nebraska, USA
(411470 19N; 1021260 W). We also obtained Daphnia
from a second southern site in Israel. Daphnia was
collected in January 2004 and February 2004 in a
permanent flooded ditch near Lake Hula (33160 N;
351370 W) and in a temporary rainpool in Dora
(321170 N; 341500 W). The climate at this site can be
classified as mediterranean i.e. long rainy winters
and short dry summers.
Clonal identification
To identify clones, DNA was extracted with the
6% Chelex method and polymerase chain reaction
(PCR) was performed using three microsatellite
loci available from Genebank: dma11 (AF291911),
dma12 (AF291912), and dma14 (AF291913). PCR
products were migrated on 6% polyacrylamide gels
and subsequently stained with Syber Green. Gels
were then scanned and allele identified on a
Hitachi FMBIO III (Yokohama, Japan). As these
loci were monomorphic in the Churchill population, four allozymes (PGM, GPI, GOT, MDH) were
additionally screened on all populations on cellulose acetate plates according to Hebert and Beaton
(’93). Nine multilocus genotypes from Nebraska,
five from Israel, and seven from Churchill were
identified using both allozyme and microsatellite
markers.
Maintenance conditions
Clones (n 5 21) were raised at 15 and 251C under
a 16:8 hr L:D photoperiod for three generations to
3
GROWTH RATE AND METABOLISM IN DAPHNIA MAGNA
remove any maternal effect (Lynch and Ennis, ’83;
Mousseau and Fox, ’98). Five individuals per clone
were grown together in 100 mL of filtered lake
water. Algae (Selenastrum capricornotum) were
added to the 100 mL of water in order to obtain a
concentration of approximately 3 105 cells mL1.
We replaced the water of the containers twice a
week with water at the right algal concentration.
Growth rate
We used neonates from the first clutch of newly
mature females to measure sizes and growth rates
as maternal size and age significantly affect
neonate size. Size at birth and at first and third
clutches was measured using a BQ nova prime
software (Bioquant, Nashville) interfaced to a
microscope. Size was measured from the top of
the eye to the base of the caudal spine. Growth
rate (increment in body length, IBL) was calculated as the difference between sizes at first
reproduction and birth divided by the number of
days to maturity. Size vs. time was plotted and
growth curve was derived by curve fitting. We also
measured the somatic juvenile growth rate (Gj),
which is defined as Gj 5 (ln W2ln W1)/t2t1 where
W1 and W2 are the masses of individual at times t1
and t2 and t1 is the neonate stage and t2 is
maturity. This parameter has been shown to be
linearly correlated with the per capita rate of
increase (r) when differences in food concentration, temperature, and clonal differences are
considered (Lampert and Trubetskova, ’96). It is
a good proxy for fitness (Lampert and Trubetskova, ’96) and can serve as an index of clonal
performance when size may be affected by predation (Hairston et al., 2001).
Oxygen consumption
Oxygen consumption was measured on five
individuals per clone for each population with a
spectrophotometer following a modified Winkler
titration (Broenkow and Cline, ’69; Peck et al., ’86;
Roland et al., ’99; Lemos et al., 2003). A single
Daphnia without eggs in the brood chamber
(rinsed with sterile water) was introduced for
24 hr in a 1.5-mL eppendorf tube filled with sterile
water (no air inside) and streched with parafilm.
Another tube filled with the same sterile water but
without D. magna was used as control. Preliminary experiments revealed that after 24 hr at 251C,
oxygen concentration in eppendorf tubes containing large individuals was well above values leading
to oxygen stress in Daphnia (50%; Paul et al., ’97;
Hodkinson, 2003). We also controlled for oxygen
diffusion inside the tubes during the 24-hr period.
Eppendorf tubes were filled with anoxic water and
kept at 15 or 251C for 24 hr. Oxygen content was
measured before and after the 24-hr period. No
significant diffusion was observed. The water was
subsequently sampled with a syringe (without
opening the eppendorf) and mixed with reagent
solutions : 7 mL of MnCl2 (3 M), 7 mL of NaOH (8 N)
and INa (4 M), and with 7 mL of H2SO4 (10 N). The
absorbance of the final yellow solution was read at
a 440 nm wavelength with a Perkin Elmer spectrophotometer (lambda 11, Waltham). The relationship between absorbance of the solution and
oxygen concentration was determined with the
following equation:
O2 ðmg L1 Þ ¼ 13:682 Absorbance 0:5769
ðr2 ¼ 0:998; Po0:001Þ:
The relationship between absorbance at 440 nm of
the yellow solution and the oxygen concentration
was constructed by thiosulfate titration. Solutions
with different oxygen concentrations were
obtained by bubbling water with nitrogen.
The previous reagent solutions (7 mL of MnCl2
(3 M), 7 mL of NaOH (8 N) and INa (4 M), and with
7 mL of H2SO4 (10 N)) were then added to 1.5 mL
of water. The volume of thiosulfate added to
ensure that the solution became translucid
(absorbance null) is proportional to the oxygen
concentration:
O2 ðmg L1 Þ
¼ ð5; 600Nthio ðVthio Vblk ÞÞ=ð0:7ðV sample Vreagent ÞÞ:
Vthio (mL) is the volume of thiosulfate added
and Vblk (0.2 mL) a correction to subtract the
volume of thiosulfate used to titrate oxygen
introduced by reagents, 5,600 cm3 is the volume
of oxygen corresponding to 1 equivalent of
thiosulfate at standard temperature pressure
(STP), and 0.7 cm3 is the volume of 1 mg of
oxygen. Vsample and Vreagent are, respectively, the
volume of sample (1.5 mL) and reagents
(0.021 mL) added. The exact normality of the
thiosulfate solution (Nthio 5 0.159) was evaluated
by titration with 0.02 N potassium iodate.
In order to measure the apparent specific
dynamic action (SDA), oxygen consumption was
measured on fed (3 105 cells mL1) and unfed
individuals. Measures were first taken on fed
individuals. These were subsequently starved
for 24 hr at 251C and 48 hr at 151C (Schmocker
J. Exp. Zool.
4
J. CHOPELET ET AL.
and Hernandez-Leon, 2003) and oxygen consumption measurements were therefore taken on
the same individuals. Oxygen consumption rates
were measured on a wide range of individual body
sizes for each clone. The dry weight of each
individual was measured to the nearest 0.1 mg on
a microbalance following a 24-hr desiccation
period at 601C.
Because it is not possible to measure standard
metabolic rate in Daphnia as limb movements are
required for respiration, we measured oxygen
consumption of unfed animals, which reflects the
sum of maintenance plus activity costs (SMRA).
The apparent SDA is defined as the energy used
during digestion, assimilation, and storage in
active organisms and is calculated as the arithmetic difference between fed and unfed oxygen
consumption rates (Fry, ’47). As we were unable to
calculate the true factorial scope, which is usually
defined as the ratio of maximal oxygen consumption to standard metabolic rate, we calculated
an ‘‘apparent factorial scope’’ that we defined as
the ratio between fed and unfed oxygen consumption rates. The aerobic scope provides a good
estimation of the aerobic capacity available for
either activities, growth processes, or reproduction, whereas the factorial scope indicates the
proportion of maximal aerobic capacity that is
devoted to life maintenance processes. The later
parameter can be a good metabolic indicator
of the stress faced by an organism in a given
environment.
Statistical analyses
Comparisons of neonate size, size and age at first
reproduction, juvenile and somatic growth rates
were carried out using a mixed-model nested
analysis of variance. Terms of the model were
temperature, population, clone nested in population, their interaction, and error. When normality
and homogeneity of variances of nontransformed
and log-transformed data residuals were rejected,
nonparametric tests were used. When parametric
and nonparametric statistical conclusions were
similar, only parametric results were presented.
Tukey post hoc tests were used to identify groups
showing the significant differences. Comparisons
of oxygen consumption–mass and SDA–mass
regressions were carried out using analysis of
covariance followed by pairwise comparisons.
Interactions of covariate term (mass) with temperature, population, and feeding status were
tested in the first model then removed when not
J. Exp. Zool.
significant to compare elevation of regressions.
All statistical analyses were performed using
general linear model procedure of SYSTAT
9.0 (SPSS Inc., ’99).
RESULTS
Body size and growth rate
No significant differences in somatic juvenile
growth rates (Gj) were found among Churchill,
Nebraska, and Israel populations at 15 and 251C
(F2; 44 5 1.13, P 5 0.332) (Fig. 1). There was a
significant effect of clone on growth rate (IBL) at
151C (F 5 10.79, df 5 18, P 5 0.001) and at 251C
(F 5 12.31, df 5 18, P 5 0.001). At birth, clones
from Nebraska had larger sizes, whereas Churchill
clones had smaller sizes (F2; 160 5 7.09, P 5 0.023)
(Fig. 1). Although all populations had similar sizes
at first (F2; 160 5 0.83, P 5 0.452) and third
clutches (F2; 39 5 0.35, P 5 0.706), Israel clones
had higher growth rates (length) than clones
from Churchill and Nebraska (F2; 160 5 893,
P 5 0.002) owing to their shorter time to reach
the first clutch (age at first clutch, F2; 167 5 9.58,
P 5 0.001) (Fig. 1).
Sizes at birth did not differ at both temperatures
but were significantly smaller at 251C than
at 151C at first (F1; 18 5 27.2, Po0.001) and third
reproduction (F1; 39 5 13.786, P 5 0.001) (Fig. 1).
Age at first reproduction decreased with increasing temperature from an average of 11.6 days at
151C to 4.5 days at 251C (Figs. 1 and 2). D. magna
had a higher somatic juvenile growth rate (Gj) at
251C than at 151C (F1; 44 5 28, Po0.001) (Fig. 1).
Significant differences among clones were found
for the following parameters: size at birth with
significant interactions between clones and temperature (F18; 160 5 6.45, Po0.001), size at first
reproduction (F18; 160 5 3.38, Po0.001), age at
first reproduction with significant interactions
between clones and temperature (F18; 167 5 10.1,
Po0.001), and growth rate (IBL) with significant
interactions between clones and temperature (F18;
160 5 9.85, Po0.001). No significant interactions
between population and temperature were found
for Gj (F2; 44 5 1.2, P 5 0.3), growth rate in length
(F2; 18 5 1.8, P 5 0.194), size at birth (F2; 18 5 0.5,
P 5 0.643) and at first maturity (F2; 18 5 3.1,
P 5 0.068), and age at first maturity (F2; 18 5 2.3,
P 5 0.130).
We pooled all individuals at each temperature
to plot body size vs. time (Fig. 2). At 251C,
curve based on the von Bertalanffy equation was
fitted (Size 5 3,963.85(1e0.312(Time)), r2 5 0.94,
5
GROWTH RATE AND METABOLISM IN DAPHNIA MAGNA
b
16
ab
Age at first clutch (day)
a
1200
1000
800
600
400
200
2
6000
0.8
Juvenile growth rate
(µm day-1)
Body size at first clutch (µm)
0
Somatic juvenile growth rate
(Gj, day -1)
3000
2000
1000
0
Churchill
Nebraska
Israel
b
4
2000
4000
a
6
700
5000
a
8
4000
2500
b
10
0
3000
a
12
0
3500
a
14
Body size at third clutch (µm)
Body size at birth (µm)
1400
600
500
400
300
200
100
0.6
0.4
0.2
0.0
Churchill
Nebraska
Israel
Fig. 1. Growth characteristics (7SD) at 151C (undashed) and 251C (dashed) in seven clones from Churchill (dark gray), nine
clones from Nebraska (light gray), and five clones from Israel (white). Populations with same letters do not show significant
differences. Standard errors express variability among clones (five individuals per clone).
5000
15˚C
25˚C
(Fig. 2). Juvenile growth rate was twice higher at
than
at
151C
251C
(475730 mm day1)
(215742 mm day1). At 251C, after the first clutch
(around 4.5 days), growth rate decreased and
became lower than at 151C (Fig. 2). As a
consequence, for approximately 20 days, D. magna
individuals were larger at 251C than at 151C and
inversely after that threshold (Fig. 2).
Size (µm)
4000
3000
2000
1000
Oxygen consumption
0
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
Time (days)
Fig. 2. Growth curves for D. magna at 151C (Size 5 981.33
(Time)0.462 ) and at 251C (Size 5 3,963.85(1e0.312(Time))).
Po0.0001). In contrast, the curve was better
fitted at 151C when based on a power equation
(Size 5 981.33(Time)0.462, r2 5 0.94, Po0.0001)
Oxygen consumption rates ranged from 0.055
(70.026) mg O2 hr1 ind1 for juveniles (o10 mg)
to 0.390 mg O2 hr1 ind1. There were no interactions of feeding status with mass (F1; 1083 5 1.2,
P 5 0.266). Fed Daphnia had oxygen consumption
rates that were on an average twice higher than
food-deprived Daphnia (F1; 1,092 5 601.1, Po0.001)
(Table 1). There were significant interaction effects
J. Exp. Zool.
6
J. CHOPELET ET AL.
among temperature population mass when
individuals were fed (F2; 542 5 5.848, P 5 0.003).
Oxygen consumption of D. magna from the
three populations was affected by temperature
and feeding in different ways (F2; 1,092 5 34.7,
Po0.001) (Fig. 3, Table 1). When fed, Churchill
clones had significantly higher oxygen consumption rates at 151C than at 251C (F1; 180 5 24.2,
Po0.001). By contrast, clones from Nebraska had
higher oxygen consumption rates at 251C than at
TABLE 1. ANCOVA analysis of oxygen consumption in
Daphnia magna from Churchill, Nebraska, and Israel at two
temperatures (Temp) for fed and unfed Daphnia (Food) and
with mass as covariate
Source of variation
df
SS
F-ratio
P
Temp
Pop
Food
Mass
Pop Temp
Food Temp
Food Pop
Food Pop Temp
Pop Temp Mass
Error
1
2
1
1
2
1
2
2
2
1,092
0.9
0.8
97.1
188.9
13.9
0.9
4.0
5.9
11.2
176.4
5.8
2.4
601.0
1169.2
43.0
5.6
12.4
18.2
34.7
0.016
0.089
o0.001
o0.001
o0.001
0.018
o0.001
o0.001
o0.001
All data were Ln-transformed before analysis. Interactions with the
covariate were removed when not significant. Pairwise comparison
results are given in the text. ANCOVA, analysis of covariance.
Churchill
151C (F1; 253 5 25.1, Po0.001). Oxygen consumption rates of Israel clones were high and did not
vary significantly with temperature (Table 1).
When unfed, oxygen consumption rates were
significantly lower at 251C than at 151C in both
Nebraska (F1; 253 5 77.8, Po0.001) and Churchill
clones (F1; 181 5 29.0, Po0.001).
At 151C, the oxygen consumption of Churchill
clones was higher than the oxygen consumption of
clones from Nebraska and Israel (when fed
C4I4N and when unfed C4N 5 I). At 251C,
Churchill clones had a higher oxygen consumption
than the other populations only when unfed
(C4N 5 I). When fed, Nebraska clones had a
higher oxygen consumption than other clones at
251C (N4I 5 C).
SDA increased with mass (Fig. 4). At 151C,
clones from Nebraska had the lowest SDA values
(F2; 261 5 14.730, Po0.001; NoI 5 C). By contrast,
at 251C, Churchill clones had lower SDA values
than Israel and Nebraska clones (F2; 263 5 7.126,
P 5 0.001; CoI 5 N). Israel clones had high aerobic scopes at both temperatures. There were
significant interactions among temperaturepopulation mass for SDA (F2; 541 5 26.839,
Po0.001).
There was no relationship between mass and
apparent factorial scope in Nebraska and Churchill clones and a very small albeit significant
relationship in Israel clones. Factorial scope did
Nebraska
Israel
0.5
15ºC
15ºC
15ºC
25ºC
25ºC
25ºC
0.4
Oxygen consumption (µg O2 h-1 ind-1)
0.3
0.2
0.1
0.0
0.5
0.4
0.3
0.2
0.1
0.0
0
20
40
60
80
100
0
20
40
60
80
100
0
20
40
60
80
100
Dry mass (µg)
Fig. 3. Oxygen consumption at 15 and 251C for fed (filled) and unfed (empty) D. magna from Churchill, Nebraska, and
Israel.
J. Exp. Zool.
7
GROWTH RATE AND METABOLISM IN DAPHNIA MAGNA
Nebraska
Churchill
Israel
0.30
15˚C
15˚C
15˚C
25˚C
25˚C
25˚C
0.25
0.20
0.15
0.10
SDA (µg O h ind )
0.05
0.00
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
20
40
60
80
100
0
20
40
60
80
100
0
20
40
60
80
100
Dry mass (µg)
Fig. 4. SDA for D. magna from Churchill, Nebraska, and Israel at 15 and 251C. SDA, specific dynamic action.
not vary with temperature in Israel clones
(2.472.1 at 151C and 2.671.5 at 251C), whereas
it increased from 1.871.3 at 151C to 2.871.8 at
251C for clones from Nebraska and inversely
decreased from 2.171.3 at 151C to 1.772.1 at
251C for Churchill clones.
DISCUSSION
Metabolic adaptations to temperature
This study provides evidence for metabolic
adaptations to temperature in D. magna clones
from different thermal environments. Metabolic
responses to temperature variation were significantly different between subarctic and temperate
populations. Adaptation to colder environment in
subarctic clones was illustrated by a higher aerobic metabolism at lower temperatures (Fig. 3,
Table 1). This result is consistent with the
metabolic cold adaptation hypothesis, one of the
three paradigms in thermal physiology, namely
elevated metabolism in populations inhabiting
cold environments (Ege and Krogh, ’14, in Pörtner
et al., 2006). By contrast, the temperate
clones from Nebraska had higher oxygen consumption rates at 251C than subarctic clones.
Churchill clones encounter daily average air
temperatures between 6 and 121C, whereas
clones from Nebraska cope with daily average
temperatures between 15 and 231C during spring
and summer. Oxygen consumption rates did
not vary with temperature in the Israel population, suggesting that they are equally well adapted
at both temperatures and, therefore, that for
this species, metabolic adaptation to high
temperature is not necessarily associated with
the loss of ability at lower temperature regimes.
Comparisons of Israel clones with those of
Churchill is more difficult as these clones undergo
important daily and seasonal temperature fluctuations. More data on the presence of Daphnia
in the water column and water temperatures
in Israel are needed before we can conclude
on the apparent eurythermy of these clones.
Interestingly, the SDA scaled positively with
mass only at a low temperature for the Churchill
clones, at a high temperature for clones from
Nebraska, and at both thermal regimes for the
clones from Israel. This might suggest that the
adjustment of SDA with mass is a required
physiological trait in optimal environmental conditions and that collapses of this trait reflect
stressful environments.
Plasticity of growth
Subarctic clones had similar growth rates as
temperate clones at either 15 or 251C. This result
is consistent with a previous study on temperature
reaction norms of somatic juvenile growth rate of
D. magna. No significant differences in growth
rates (Gj) were found among populations ranging
J. Exp. Zool.
8
J. CHOPELET ET AL.
from southern Spain to Finland (Mitchell and
Lampert, 2000). Our results revealed significant
differences in growth rates (IBL) among clones
within each population as well as significant clone
by temperature interactions, suggesting that a
substantial variation for growth under different
temperatures is present in D. magna (see also
Mitchell and Lampert, 2000). The existence of
clones with different growth capacity may enable
populations to cope with seasonal alterations of
temperature through replacement by alternative
thermal specialist clones (Carvalho, ’87; Carvalho
and Crisp, ’87). Mitchell and Lampert (2004) did
not find differences in juvenile growth rates
among clones of D. magna collected at different
seasons, suggesting that clones were generalists
and able to optimize their phenotypes in a given
environment. Growth rates were measured under
optimal food conditions in the laboratory.
Although laboratory conditions were essential
to grow clones under similar environmental
conditions, we might have obtained different
results in the field with more stringent conditions
and scarcer resources. Future studies should
measure growth rates and temperature in situ
in the field. It is interesting to note that clones
from Israel had, on an average, higher somatic
juvenile growth rates than both temperate
and subarctic clones at both 15 and 251C.
The shorter time to reach first reproduction may
be an adaptation to Israel environment, characterized by unpredictable environmental conditions.
High predation pressures from fish may also
select for a rapid growth in these populations.
More studies are needed to identify factors
responsible for these genetic differences in
growth rates.
Direct effects of temperature on growth
Temperature has direct and opposed effect on
growth rate and asymptotic size (Atkinson, ’94;
Atkinson and Sibly, ’97). Our results are in
accordance with the temperature–size rule and
the model of Berrigan and Charnov (’94), which
states that growth rate and asymptotic size are
negatively correlated. The negative correlation
between size and temperature has already been
shown in D. magna with a smaller maturation
threshold size at 221C than at 121C (McKee and
Ebert, ’96). However, our results showed that this
negative correlation occurred only after the third
clutch. During the first 20 days, D. magna had a
larger body size at 251C than at 151C resulting
J. Exp. Zool.
from a higher growth rate before the first
reproduction at 251C. This therefore excludes
any temperature modulation of adult size by egg
or juvenile size (for example, see Stelzer, 2002)
and limits the explanation to a prolonged growth
period induced by delayed maturation at a low
temperature (Figs. 1 and 2).
Growth and metabolism decoupling
Growth is an energy-consuming trait and is
expected to depend on aerobic metabolism
(Jobling, ’85). Therefore, at 151C, subarctic
D. magna might be expected to grow faster than
temperate D. magna according to higher maximum aerobic capacity, which is not what we
observed. The unfed metabolism was also higher
in the subarctic population than in the temperate
one leaving a lower aerobic scope for growth. At
251C, the mediterranean population had the
highest fed metabolism, whereas its unfed metabolism was lower than the subarctic one and
equivalent to the temperate one, suggesting higher access to SDA for growth. Again this did not
translate into higher somatic juvenile growth
rates. This decoupling between growth and SDA
at different thermal regimes suggests that growth
rates in D. magna are not dictated by aerobic
capacity or SDA. In the thermal range covered by
this study, the overall metabolic capacity can cope
with energetic requirements imposed by growth
rate in optimal conditions, or in other words
growth processes are not limited by metabolic
capacity of an organism. This is in agreement with
observations on fast-growing fish larvae, which
suggest an excess in overall metabolic capacity to
support the growth requirements and limitation
at the level of the digestive capacity (see Blier
et al., ’97). Under natural conditions, energy
allocation and metabolism have to cope with
different physiological demands such as maintenance, locomotion, reproduction, or growth. An
apparent excess of metabolic capacity would
ensure minimum ability to perform different
physiological tasks without restricting growth
processes when resources are available. The higher aerobic capacity and SDA at a lower temperature in the subarctic population and at a higher
temperature in the temperate population with no
apparent correlation with growth rate could partly
support this interpretation. What appears counterintuitive and difficult to interpret in terms of
local adaptation is the increase of SMRA at a low
temperature for the subarctic population and at
GROWTH RATE AND METABOLISM IN DAPHNIA MAGNA
a higher temperature for the temperate population. Standard metabolism or the cost of maintenance has been perceived as a relatively passive
process and its optimization being obtained by
minimization, allowing higher allocation to active
process such as activity or growth rate. Much of
the standard metabolism is associated with protein turn-over and mitochondrial uncoupling.
Maintenance of a high protein turn-over rate
could be associated with the maintenance of highly
functional physiological machinery when organisms are highly active. The increase of mitochondrial uncoupling has been associated with
modulation of reactive oxygen species production
by active mitochondria. It is therefore reasonable
to suggest that optimization of standard metabolism cannot be restricted to only energy cost but
rather to optimization of the processes that
generate these costs. In this context, an increase
of protein turn-over rate and mitochondrial
uncoupling to meet physiological requirements at
an optimal temperature (high or low) could easily
be an adaptive response.
CONCLUSION
Our data clearly revealed that temperature
adaptations maximize SDA at temperatures experienced by clones from different localities. In
spite of clear metabolic compensations, growth
rates did not show the same level of divergences
among populations when measured at the same
temperature, questioning the function of these
metabolic adjustments. D. magna appears to be an
excellent model system to investigate physiological
basis of thermal sensitivity of metabolism. Crosses
between individuals from Churchill and Nebraska
would be of interest to enable a finer identification
of physiological mechanisms involved in metabolic
adaptations.
ACKNOWLEDGMENTS
This study was supported by a Natural Sciences
and Engineering Research Council of Canada
(NSERC) and Fonds Québécois de la Recherche
sur la Nature et les Technologies (FQRNT) grants
to France Dufresne. We thank Adam Petrusek for
providing us with samples from Israel, Marlin
French for his hospitality in the Crescent Lake
National Wildlife Refuge, and Marie-Eve Houde
for the sampling in Churchill. Northern scientific
training program provided funding for the sampling in Churchill. We thank the staff from the
9
Churchill Northern Study Centre for their help
during our stay.
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