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First published online September 19, 2008
Journal of Experimental Biology 211, 3139-3146 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.021907
Investigating onychophoran gas exchange and water balance as a means to inform current controversies in arthropod physiology
Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa
* Author for correspondence (e-mail: sct333{at}sun.ac.za)
Accepted 3 August 2008
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Summary |
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CO2) at 20°C
in P. capensis is 0.043 ml CO2 h–1, in
keeping with other onychophoran species; suggesting that low metabolic rates
in some arthropod groups are derived. Continuous gas exchange suggests that
more complex gas exchange patterns are also derived. Total water loss in
P. capensis is 57 mg H2O h–1 at 20°C,
similar to modern estimates for another onychophoran species. High relative
respiratory water loss rates (
34%; estimated using a regression
technique) suggest that the basal condition in arthropods may be a high
respiratory water loss rate. Relatively high Pc values
(5–10% O2) suggest that substantial safety margins in insects
are also a derived condition. Curling behaviour in P. capensis
appears to be a strategy to lower energetic costs when resting, and the
concomitant depression of water loss is a proximate consequence of this
behaviour.
Key words: metabolism, hypoxia, respiratory water loss, cuticular water loss, discontinuous gas exchange, invertebrate, velvet worm, respirometry
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Westneat et al., 2003;
Hetz and Bradley, 2005;
White et al., 2007), several
major controversies remain. Most prominent among these are the extent to which
selection for reduction of water loss might alter both metabolic rate and gas
exchange pattern (e.g. Lighton,
1998; Chown, 2002;
Gibbs and Johnson, 2004;
Lighton and Turner, 2008), the
evolutionary origins of discontinuous gas exchange (DGE)
(Marais et al., 2005), the
relationship between life-history strategies and their mean metabolic rates
(Reinhold, 1999;
Lighton et al., 2001;
Klok et al., 2002;
Terblanche et al., 2004), and
the likelihood that gas exchange abilities might limit overall size, so
accounting for gigantism during the hyperoxic Palaeozoic
(Greenlee et al., 2007;
Kaiser et al., 2007).
Typical of these debates are calls for a broader phylogenetic coverage,
especially of groups that are thought to possibly contribute to resolving the
question at hand. For example, Kaiser et al.
(Kaiser et al., 2007) called
for investigations of tracheal proportions in taxa closely related to those
that experienced Palaeozoic gigantism, whereas Greenlee et al.
(Greenlee et al., 2007) argued
that reduced safety margins for gas exchange under hypoxia might be detected
in the largest of insects. Lighton et al.
(Lighton et al., 2001)
concluded that the number of published metabolic measurements is sparse, and
that, in consequence, the direction of the relationship between low metabolic
rates and cannibalism cannot be established. Similarly, Marais et al.
(Marais et al., 2005) argued
that limited investigations of gas exchange patterns across the Arthropoda
constrain comparative investigations of the reasons for the origin of DGE. In
a different vein, Chown (Chown,
2002) argued that a null expectation for cuticular versus
respiratory water loss had not been articulated. Therefore, comparisons
amongst particular taxa without some reference to a baseline expectation, such
as for species that have continuously open spiracles during rest, might prove
to be unhelpful for resolving the significance of the contribution of
respiratory water loss to overall water balance.
From the perspective of a comparative approach to addressing these
controversies (e.g. Blomberg et al.,
2003; Garland et al.,
2005), perhaps the most obvious gap is the absence of information
for taxa basal to the arthropods. That the Tardigrada should not have been
investigated in this respect is unsurprising as a consequence of their small
size and experimental intractability. However, the relative absence of data on
the Onychophora is surprising, especially given their phylogenetic position
basal to the arthropods (Giribet et al.,
2001; Grimaldi and Engel,
2005; Dunn et al.,
2008) and renewed interest in the group (e.g.
Monge-Nájera, 1995;
Sunnucks et al., 2000;
Reinhard and Rowell, 2005).
The few studies of onychophoran metabolism and water balance undertaken to
date (Manton and Ramsay, 1937;
Morrison, 1946;
Bursell and Ewer, 1950;
Mendes and Sawaya, 1958;
Woodman et al., 2007) suggest
that their gas exchange is mediated via a simple open tracheal system
with large numbers of non-closable spiracles, and limited or no branching
(Lavallard and Campiglia-Reimann,
1966; Bicudo and Campiglia,
1985). In consequence, information on gas exchange and water
balance in the Onychophora would substantially inform debates about the likely
basal pattern of gas exchange in the arthropods, baseline expectations for
cuticular versus respiratory water loss, whether low metabolic rates
in ticks, scorpions, centipedes and whip-spiders are a basal condition, and
perhaps also the extent to which low critical oxygen partial pressure in
insects (reviewed by Hoback and Stanley,
2001; Schmitz and Harrison,
2004; Harrison et al.,
2006) can be considered derived.
Thus, the principal aim of this paper is to investigate gas exchange and
water balance of the onychophoran Peripatopsis capensis Grube 1866,
to address these questions. Specifically, we characterize the pattern of gas
exchange, determine the relative contributions of cuticular and respiratory
transpiration to total water loss, document standard metabolic rate for
comparison with other taxa, and measure metabolic and water loss rates under
declining oxygen partial pressures (PO2) to
identify the critical PO2 for resting
metabolism (Pc). Recently, Woodman et al.
(Woodman et al., 2007)
suggested that the curling behaviour displayed by the Australian species
Euperipatoides rowelli restricts water loss. Therefore, we also
determine whether such behaviour in P. capensis affects water loss at
different temperatures.
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MATERIALS AND METHODS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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100 known species from temperate regions of
the southern hemisphere and the tropics
(Grimaldi and Engel, 2005).
They are typically found in dark, moist microhabitats in forests, have a
highly malleable body covered by numerous sensory papillae, and possess glands
that secrete an adhesive slime used for defence and prey capture
(Hamer et al., 1997;
Benkendorff et al., 1999;
Barclay et al., 2000;
Walker et al., 2006). The
study species, Peripatopsis capensis (Peripatopsidae), is confined to
the southern parts of the Western Cape Province of South Africa and is usually
found in humid indigenous forests and bushy ravines of mountain slopes
(Hamer et al., 1997).
P. capensis individuals were collected in September 2006
(N=8) and February 2007 (N=7) from a milkwood
(Sideroxylon inerme) forest (140 m altitude, exact location withheld
for conservation reasons). Individuals were found under bark of fallen logs
where temperature and relative humidity (AZ Instruments, Taichung, Taiwan),
measured at the time of collection, were 13.4±5.5°C and
74.3±7.6% (mean ± s.e.m.; N=10), respectively.
Individuals were placed, on the day of collection, in a climate-controlled
chamber (Labcon, Johannesburg, South Africa) at 14.4±0.4°C with a
12 h:12 h L:D photoperiod, and they remained at this temperature for 10 days
before experiments commenced. One or two individuals were placed in single
non-air-tight plastic containers (17 cmx12 cmx6 cm) with a layer
of sand covered by leaf litter and bark. Moisture was maintained at 80%
relative humidity (RH) by regularly spraying the litter with distilled water
and by placing vials containing saturated salt (NaCl) solution within the
containers (Winston and Bates,
1960). Individuals were fed isopods, springtails and fruit flies
(one to four, depending on prey size) once per week, and none of the
individuals showed a consistent decline in body mass over the course of the
study. Food items were removed at least 24 h prior to experiments to ensure
that animals were post-absorptive during respirometry.
Respirometry
Flow-through respirometry was undertaken over several days between 08.00 h
and 18.00 h in humidified air to measure CO2 production in
conditions similar to the natural environment. Data were obtained at five
different temperatures randomized as 15, 10, 20, 25, 5°C, and the order of
individuals was consistent on each day to avoid diurnal effects that might
confound temperature effects within individuals. Individuals were placed at
15°C for 2 days between temperature trials. One week after the completion
of these trials, the same protocol was repeated, but dry air was used to
enable simultaneous measurement of CO2 production and water loss.
For each trial, individuals were weighed to 0.1 mg (Mettler Toledo AX-504
electronic balance; Columbus, OH, USA) and placed in a darkened 9 ml glass
cuvette kept at the temperature of the trial using a climate-controlled
chamber (±0.4°C; Labcon). Individuals were allowed to settle for 15
min prior to recordings. Dry, CO2-free bottled air (21%
O2, balance N2) scrubbed using soda lime, silica gel and
Drierite® (Xenia, OH, USA), passed through a mass flow control valve
(Sierra Instruments, Monterey, CA, USA) set at a STP-corrected flow rate of
100 and 200 ml min–1 for dry and humidified trials,
respectively. The air flow was increased in the humidified trials to enhance
stabilization and response times of the
CO2 (rate of
carbon dioxide release) reading. Dry air was directed through the cuvette or
in the case of the humidified trials, to a bubbler (flask containing distilled
water placed inside a water bath adjusted to the temperature required to
obtain 70% RH) before reaching the cuvette. The air was then guided to a
calibrated CO2/H2O analyzer (Li 7000 infrared gas
analyzer; LiCor, Lincoln, NE, USA). The temperature inside the cuvette (using
a 36 SWG T-type thermocouple), activity pattern (infrared AD-1 activity
detector; Sable Systems International, Las Vegas, NV, USA),
CO2 and
H2O (rate of
water loss) were recorded and stored via the LiCor and its software. Baseline
readings for the empty cuvette were taken before and after each individual
trial, which lasted 1–2 h for humid and 30–60 min for
dry trials. Sex was not determined because finding the male papilla
(Hamer et al., 1997) requires
prolonged handling which induces additional water loss.
To characterize interactions among behaviour and gas exchange traces, the
behaviour of six individuals acclimated at 15°C was filmed with a webcam
(Logitech, Fremont, CA, USA) while measuring
CO2 and
H2O at 15°C
in 21% O2 dry air and using a dim light to enhance the clarity of
the image. Because of their photonegative behaviour
(Newlands and Ruhberg, 1978),
individuals moved frequently in the chamber and variations in patterns of gas
exchange with changing behaviour were identified.
To measure the effects of PO2 on gas
exchange, the same respirometry methods were used to record
CO2,
H2O and activity
of six individuals at 15°C. Each individual was exposed to five
pre-determined PO2 (21, 15, 10, 5 and 2.5%) for
30 min in descending order to prevent an increase in metabolic rate that is
associated with prior exposure to hypoxia. Individuals were held at 15°C
(other conditions as above) for 3 days between measurements at each
PO2. The 15% and 2.5%
PO2 were obtained by mixing dry
CO2-free 21% or 5% O2 with the appropriate ratio of pure
N2. The other concentrations originated from purchased dry
CO2-free bottled air. All PO2 were
verified with a calibrated O2 analyzer (Ametek S-3A/II, AEI
Technologies, Pittsburgh, TN, USA).
Data analysis and statistics
Data were initially analyzed using ExpeData software version 1.0.24 (Sable
Systems International, Las Vegas, NV, USA). Differential CO2 (in
parts per million) and H2O (in parts per thousand) data were
corrected for baseline drift and transformed to
CO2 (in ml
h–1) and
H2O (in
mgh–1) using standard transformations
(Lighton, 1991). For all
trials, we assumed that standard metabolic rate (SMR) equalled the mean
CO2 for periods
of zero activity (i.e. resting) with lowest stable data (usually lasting
2–10 min). For water loss rates (WLR), the mean
H2O was
calculated from when the water trace stabilized until the end of the
recording. Metabolic rate, WLR and body mass data distributions were
log10 transformed prior to analyses to obtain, or in a few cases
improve, the normality of the data. The SMR– and WLR–temperature
and SMR– and WLR–body mass relationships were investigated using
ordinary least-squares regressions. Repeated measures ANOVA and ANCOVA were
performed in SAS version 8.0 (SAS Institute, Cary, NC, USA) to investigate the
extent to which variation in
CO2 (and
H2O for dry
trials) could be explained by variation in temperature, body mass and their
interaction. An unstructured covariance matrix was used in proc-mixed with a
reduced maximum-likelihood estimation method
(Littell et al., 1996).
Individual and temperature were entered as categorical variables and body mass
as a continuous variable in the model. Relative humidity was included in an
additional model to identify the effect of humidity treatment (70%
versus 0% RH) on SMR. We used a similar approach to test for an
effect of gas exchange pattern (now adding this as a categorical variable to
the model described above; see the Results section) on SMR and resting
H2O (obtained
from the same resting period as for SMR). To assess the effect of
PO2 on
CO2 and
H2O, periods of
inactivity with continuous gas exchange were selected to avoid potentially
confounding effects of variation in gas exchange pattern across oxygen trials.
A repeated measures model was again used with individual and
PO2 entered as categorical variables and body
mass as a continuous variable. Critical PO2for
resting metabolism (Pc) was defined as the
PO2 below which metabolism could no longer be
sustained (Tang, 1933;
Prosser and Brown, 1961).
Following the method of Gibbs and Johnson
(Gibbs and Johnson, 2004), an
ordinary least-squares regression of
H2O (dependent
variable) and
CO2 (independent
variable) was undertaken for each individual–temperature trial (mean
r2 of 0.55; range: 0.20–0.98). Cuticular water loss
(CWL) was estimated as the
H2O where
CO2 equalled
zero (y-intercept) assuming that in the absence of CO2
exchange, water loss must be entirely or predominantly cuticular
(Gibbs and Johnson, 2004).
Although the regression method provides CWL estimates that are generally as
repeatable (e.g. Chown et al.,
2006; Gray and Chown,
2008) as those found using other methods [e.g. the hyperoxic
switch (Lighton et al.,
2004)], this technique is not without its problems. In species
with continuous gas exchange, the estimation of CWL is obtained by
extrapolating beyond the measured
CO2 data to the
y-intercept, which might bias respiratory water loss (RWL) estimates
(see Gray and Chown, 2008).
However, since preliminary trials using the hyperoxic switch technique on
P. capensis revealed no spiracular response following exposure to
extreme hyperoxia, and several studies report a lack of spiracular control in
other onychophoran species (Manton and
Ramsay, 1937; Lavallard and
Campiglia-Reimann, 1966), which is a prerequisite for using the
alternative techniques, the regression method was the only option available
for estimating CWL. To this end, we used whole recording time periods minus
the first few minutes during which traces stabilized (total period 40
min). The difference between total water loss and CWL provided an estimate of
RWL (Gibbs and Johnson, 2004).
One negative value of CWL, considered biologically meaningless, was excluded
from the analysis (see Gray and Chown,
2008). Except for the repeated measures models, analyses were
performed using STATISTICA v.7 (Statsoft, Tulsa, OK, USA) and significance was
accepted at P<0.05.
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RESULTS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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CO2 (referred to
`downregulated' hereafter; Fig.
1A), where the
CO2 clearly
declines in an ongoing manner; (2) continuous CO2 exchange
interspersed with downregulated episodes (`interspersed';
Fig. 1B), where
CO2 clearly
declines and then increases in alternating episodes; and (3) continuous
CO2 exchange (`continuous'; Fig.
1C) during which gas exchange is continuous with no indication of
downregulation. Observed and imaged behaviour confirmed that the downregulated
pattern occurred when the animal assumed a curled position and remained
immobile (Fig. 2; this
behaviour can be seen in Movie 1 in supplementary material). Continuous gas
exchange occurred both during inactivity and activity
(Fig. 1C), although most
individuals remained inactive inside the darkened cuvettes. We quantified the
proportion of individuals with downregulated, interspersed and continuous
patterns across temperature trials and humidity treatments but did not compare
the latter since flow rates and sampling periods differed between these
treatments. The majority of individuals tested in humid conditions (from
5°C to 25°C) had interspersed patterns (60%) whereas 27% and 13% had
continuous and downregulated patterns, respectively
(Fig. 3A). Temperature had no
significant effect on the proportions of individuals with each pattern
[5x3 contingency test, G=15.2, P>0.05
(Sokal and Rohlf, 1995), p.
738]. However, in dry conditions, interspersed and continuous patterns were
most common (45% each), with downregulated patterns occurring more frequently
at 5°C and continuous patterns occurring more frequently at 25°C
(Fig. 3B; 5x3 contingency
test, G=22.6, P<0.05).
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During downregulation,
CO2 declined
continuously, whereas
H2O declined
initially and then remained constant (Fig.
1A,B). Mean
CO2 and
H2O of the last
3 min prior to and at the end of downregulated episodes were compared using
eight interspersed traces recorded at 15°C, and these indicated that
CO2 and
H2O were
significantly lower during downregulated periods than prior to them (paired
t-tests; mean
CO2:
t=11.06, d.f.=7, P<0.0001; mean
H2O:
t=4.37, d.f.=7, P<0.01). The percentage decline ranged
from 5 to 17% in
CO2 and from 3
to 9% for
H2O.
The standard, mass-adjusted
CO2 differed
significantly among patterns in dry trials and among all test temperatures
(pattern typextemperaturexbody mass,
F11,50=24.96, P<0.0001). The model indicated
that standard
CO2 during
interspersed was lower than during continuous recordings, but since the
downregulated pattern was only present at 5°C (N=6) and 10°C
(N=1), there was insufficient variation between test temperatures to
calculate a P value. Therefore, a sub-section of the data (from
15–25°C) was analyzed to exclude this pattern type and to determine
whether or not the former interaction effect was simply due to the presence of
downregulated patterns at 5°C and 10°C. The same significant
interaction (patternxtempxmass, F6,31=27.05,
P<0.0001) was found in the reduced dataset. Furthermore, to
clarify the effect of pattern type, data from the 5°C test temperature
were analyzed since this is the only temperature at which several individuals
show each of the three
CO2 patterns.
This analysis showed that mass-adjusted
CO2 was lower
during downregulated than during interspersed, which in turn had lower
CO2 than
continuous gas exchange (ANCOVA, F2,11=5.9,
P<0.018). In sum, these complementary analyses highlight clear
differences in mass-adjusted
CO2 among
pattern types irrespective of the dataset used. Therefore, in general, curling
behaviour led to a reduction of mass-adjusted
CO2.
The standard mass-adjusted
H2O also
differed significantly among pattern types in dry trials across all test
temperatures (patternxtemperaturexmass,
F11,50=4.58, P<0.0001). Even though the effect
of pattern was significant across the full dataset the model was unable to
resolve the estimate for the downregulated pattern. Regardless, the
interspersed pattern was characterized by a lower resting
H2O than the
continuous one. This result was confirmed by re-analysing the
15–25°C dataset, which showed the same significant interaction
(patternxtemperaturexmass; F6,31=4.14,
P=0.0036). Data from the 5°C test temperature, with all three
patterns represented, indicated that standard, mass-adjusted
H2O did not
differ significantly among the three patterns
(F2,11=1.034, P=0.39). Therefore, from the
available data, curling behaviour did not lower
H2O at 5°C,
but did so at higher temperatures (from 10 to 25°C).
Metabolism and water loss
Temperature and body mass had significant positive effects on standard
metabolic rate (SMR) in both dry and humidified trials (Tables
1,
2,
3). Relative humidity did not
have an effect on SMR overall, but a significant interaction between
temperature and humidity treatment (Table
3; Fig. 4)
demonstrated that responses to temperature differed among the two humidity
groups. Specifically, the increase in log
CO2 was larger
from 15 to 20°C in the 70% RH group than in the 0% RH group. This
difference was not caused by variation in the type of patterns across groups
since between 15 and 20°C the change in pattern in the 70% RH group (an
increase in the downregulation pattern)
(Fig. 3) was incompatible with
such a conclusion.
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Temperature (T) and body mass (M) had significant positive effects on total water loss rate (WLR; Tables 2 and 3) and the significant interaction of temperature and mass (Table 3) indicated that at different temperatures, the effect of mass on WLR varied. Indeed, a significant negative linear relationship (slope WLR–T relationship=0.0281–0.0068xlog10M; r2=0.394, F1,13=8.465, P=0.012) was found between the individual slopes of the WLR–T relationships and body mass, indicating that small individuals lost water relatively faster at higher temperatures than larger individuals. However, the relationship between individual slopes of SMR–T relationships and respective body masses was not significant (slope SMR–T=0.0257–0.0042xlog10M; r2=0.074, F1,13=1.041, P=0.33) indicating that small individuals were not more metabolically sensitive to temperature, nor was the opposite the case.
Cuticular water loss rates also increased significantly with temperature and body mass, although the interaction between temperature and mass was not significant (Table 4; T: F4,62-10.93, P<0.0001; M: F1,62=26.84, P=0.0001; TxM: F4,62=1.53, P=0.19). Similarly, temperature and body mass had a significant positive effect on respiratory water loss, and the interaction between temperature and mass was not significant (Table 4; T: F4,62=7.09, P<0.0001; M: F1,62=16.38, P=0.0001; TxM: F4,62=1.07, P=0.38).
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Oxygen partial pressure effects
During hypoxic trials, gas exchange was mostly continuous. Only two out of
24 recordings had downregulated episodes during hypoxia. Mean resting
CO2 remained
relatively constant from 21% to 10% O2, and declined at 5% and 2.5%
O2, by 19% and 41% relative to normoxia, respectively
(Fig. 5A). This decrease was
not significant when using a repeated measures model that included all
PO2 treatments (F4,20=1.19,
P=0.34). However, paired t-tests between treatments showed
that comparisons of log
CO2 between
21–15%, 15–10% and 21–10% were not significantly different
(t=0.32, P=0.76; t=0.24, P=0.82;
t=0.16, P=0.87, respectively with d.f.=5 in all tests)
whereas CO2 at
5% and 2.5% were significantly lower than all other
PO2 treatments (5–10%: t=3.12,
P=0.03; 5–15%: t=4.37, P=0.007; 5–21%:
t=2.11, P=0.04; 2.5–10%: t=3.26,
P=0.02; 2.5–15%: t=3.53, P=0.02;
2.5–21%: t=3.69, P=0.01; d.f.=5 in all tests). These
results suggest that the critical oxygen partial pressure
(Pc) for this species lies between 5% and 10%
O2. The use of a piecewise linear regression (non-linear regression
procedure in STATISTICA) resulted in a breakpoint at log
CO2=–1.7332,
also indicating a Pc between 5% and 10% O2.
Resting H2O did
not change across PO2 trials
(Fig. 5B;
F4,20=1.23, P=0.33; all paired t-tests
were non-significant, 0.10<P<0.91 for all).
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DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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), and
their phylogenetic position basal to the Arthropoda
(Giribet et al., 2001;
Dunn et al., 2008), suggest
that continuous gas exchange is likely the ancestral state for the arthropods.
Other forms of gas exchange pattern such as cyclic and discontinuous gas
exchange can therefore be considered more derived, and indeed probably evolved
several times within the Arthropoda
(Lighton, 1998;
Lighton and Joos, 2002;
Klok et al., 2002). A recent
study has suggested that either continuous or cyclic gas exchange is the
ancestral condition in the insects (Marais
et al., 2005). Although the current data cannot be used to resolve
this question, they support the idea that continuous gas exchange may be the
more likely basal state.
Given its body mass, the mean SMR of P. capensis at 25°C
(447.4±45 µW; N=15, mass of 488.1±78.8 mg and
assuming an RQ of 0.72) was not significantly different from the value
expected for a similar-sized arthropod [501.3 µW, RQ=0.72; equation 1 of
Lighton and Fielden (Lighton and Fielden,
1995); 526.5 µW, RQ=0.72
(Lighton et al., 2001)]. By
contrast, the SMR of P. capensis was significantly higher than the
values estimated from the scaling relationships of `anomalous' arthropods,
which have typically low SMR (Lighton and
Fielden, 1995; Klok et al.,
2002; Terblanche et al.,
2004). For example, the SMR values for a 488 mg animal derived
from the tick (Lighton and Fielden,
1995), scorpion (Lighton et
al., 2001), centipede (Klok et
al., 2002) and whip-spider
(Terblanche et al., 2004)
scaling relationships were 71.4, 128.3, 181.34 and 135.6 µW, respectively.
Assuming that onychophorans have a SMR similar to or higher than that recorded
for P. capensis (0.64µW mg–1 at 20°C), as is
the case for Epiperipatus brasiliensis (0.89 µW
mg–1) (Morrison,
1946), Peripatus acacioi (0.50 µW
mg–1) (Mendes and Sawaya,
1958) and Euperipatoides rowelli (1.27 µW
mg–1) (Woodman et al.,
2007), the basal phylogenetic position of Onychophora suggests
that the low metabolism of `anomalous' arthropods is a derived condition.
Further, explicit comparative studies incorporating information on
life-history strategies of the species used in such comparisons should be
undertaken to assess the extent to which low metabolic rates are indeed
associated with or constrained by variation in life histories
(Lighton and Fielden, 1995;
Reinhold, 1999;
Lighton et al., 2001). Such a
study would also contribute to further understanding of the significance of
such variation relative to that of body mass and temperature in determining
metabolic rate variation among species (e.g.
White et al., 2006).
Water loss
Despite the frequent reports that onychophorans are highly susceptible to
desiccation, few studies have quantified water loss rates (WLR) within the
group and undertaken comparisons among species or with other taxa
(Morrison, 1946;
Woodman et al., 2007). One
reason may be the difficulty of estimating area-independent WLR or cuticular
permeability (Loveridge, 1980)
of onychophorans given that they readily change body shape, confounding
estimates of body surface area. Here, we sought to overcome this problem using
an established framework (Zachariassen,
1996; Addo-Bediako et al.,
2001) that provides interspecific least-squares estimates of the
relationships between SMR and WLR for xeric and mesic insect species.
Peripatopis capensis lies well above the regression lines
(3x95% CI) for both mesic and xeric insects, indicating that for its
SMR, it has a much larger WLR (76.3 mg h–1 at 25°C) than
insects inhabiting a wide range of environments. Although comparisons with
other Onychophora show that P. capensis has a lower WLR than
Epiperipatus brasiliensis (Peripatidae; 222.2 mg h–1
at 24°C, 788 mg, placed in a dry-air container), Oroperipatus
corradi (Peripatidae; 167.5 mg h–1 at 24°C, 423 mg,
dry-air container) (Morrison,
1946), and a Peripatopsis spp. (263.3 mg
h–1 at 30°C, 303 mg, with 27.5% RH and 7 m
s–1 air flow) (Manton and
Ramsay, 1937), different methods and the likelihood of unaccounted
activity in the previous studies may explain the differences. Using
flow-through respirometry, Woodman et al.
(Woodman et al., 2007)
reported a WLR of 61.33 mg h–1 (20°C, mass=295 mg, flow
rate of 100 ml min–1) for Euperipatoides rowelli,
similar to the rate we documented. Thus, in keeping with claims in the
literature (Morrison, 1946;
Woodman et al., 2007), the
Onychophora can be considered a group with high WLR, or `extremely mesic'.
Although cuticular water loss (CWL) was found to be the largest avenue of
water loss in P. capensis, respiratory water loss (RWL; 34%)
also contributed substantially to total water loss. Given that onychophorans
have an open tracheal system with numerous non-occludible spiracles (e.g.
Bicudo and Campiglia, 1985), it
is unsurprising that RWL contributes substantially to water loss. However,
perhaps more importantly, the relative contributions of RWL estimated for this
species provide a baseline expectation against which the significance of RWL
in taxa that have occludible spiracles, and that in some cases show
discontinuous gas exchange, can be assessed. Although comparisons among
relative WLR expressed as percentages are problematic because they may fail to
assess adequately the extent to which CWL and RWL have been modulated
(Chown, 2002), they are widely
used as a first gauge of the significance of RWL. In this regard it is clear
that the null expectation for RWL is indeed for a high RWL, given that more
than 95% of the values reported in the literature [mostly for insects (see
Chown, 2002;
Johnson and Gibbs, 2004;
Lighton et al., 2004;
Schilman et al., 2005;
Gray and Chown, 2008)] lie
well below a RWL of 23%. However, it should be noted that complex changes to
RWL might evolve from such a basal state to the current condition. For
example, if CWL is reduced to a minimum, the relative contribution of RWL is
expected to be high (Zachariassen et al.,
1987; Zachariassen,
1991). In consequence, it may well be that the
RWL–environmental water availability relationship is non-linear or
U-shaped among arthropods and their allies.
Critical oxygen partial pressure
According to the repeated measures ANCOVA,
PO2 had no overall effect on SMR of P.
capensis. However, common tests used to identify critical oxygen partial
pressures (Pc) such as breakpoint regression and pairwise
t-test techniques (e.g. Ultsch et
al., 1978; Greenlee and
Harrison, 2004), indicated that the Pc of
P. capensis lies between 5% and 10% O2. These
discrepancies may have resulted from the fact that the repeated measures model
is a conservative approach given its high sensitivity to the within and
between group variances (and small degrees of freedom), whereas pairwise
t-tests and regression techniques are less conservative. Moreover,
the Pc of P. capensis is comparable to those
found in other Onychophora: Peripatus acacioi
(Mendes and Sawaya, 1958)
(10<Pc<15% O2 at 20°C) and
Euperipatoides rowelli [(Woodman
et al., 2007) re-analysis of their data in
Table 3,
5<Pc<10% at 10°C)]. Therefore, these results
suggest that onychophorans regulate metabolism down to intermediate levels of
hypoxia and are not oxyconformers (see
Schmitz and Harrison, 2004).
In addition, they have lower safety margins than most insects [range 2–5
kPa (Keister and Buck, 1964;
Greenlee and Harrison, 2004;
Schmitz and Harrison, 2004)]
while being consistent with most non-insect invertebrates [5–12 kPa
(Penteado and Hebling-Beraldo,
1991; Greenlee and Harrison,
2004; Schmitz and Harrison,
2004)]. Therefore, the high safety margins of insects may
represent a derived condition, and one which may have interacted with the
constraints set by the demands of metabolism during flight, or walking
activity, to determine body size (see
Dudley, 1998;
Kaiser et al., 2007). Explicit
comparative investigations of Pc and tracheal dimensions
across species in this group, and across other, seldom-investigated arthropod
taxa may provide additional, and much-needed insight into the mechanisms
underlying Palaeozoic gigantism in arthropods (see also
Berner et al., 2003).
Gas exchange, water loss and behaviour
The current study also demonstrated that behaviour has fundamental
consequences for gas exchange in P. capensis:
CO2 and
H2O declined
when individuals curled up. In E. rowelli this behaviour was rare (4
out of 80 respirometry trials) (Woodman et
al., 2007), whereas in P. capensis, periods of
downregulation were relatively common. Importantly, the declines in
CO2 and
H2O in P.
capensis differed in form, such that
CO2 declined
continuously while
H2O remained
relatively constant following an initial decline. If water loss restriction
was the primary role of curling, this behaviour might be expected most
commonly under high temperature and dry conditions, whereas the data indicate
that curling occurred less frequently as temperature increased. Thus, it
appears that curling may take place not necessarily to reduce water loss, but
as a precursor to metabolic downregulation. Thus, curling may be a strategy to
lower energetic costs when resting, whereas the depression of water loss is
likely a proximate consequence of this behaviour.
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Acknowledgments |
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References |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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