Comparative water relations of four species of scorpions in Israel: evidence for phylogenetic differences
Department of Zoology, Tel Aviv University, Ramat Aviv 69978, Israel
* Author for correspondence (e-mail: erangef{at}post.tau.ac.il)
Accepted 31 December 2003
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Summary |
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When sampled in the laboratory following their capture, B. judaicus (548±38 mOsm l1; mean ± S.D.) and L. quinquestriatus (571±39 mOsm l1) had higher and less variable haemolymph osmolarities than the scorpionids occupying the same habitats (511±56 and 493±53 mOsm l1 for S. m. fuscus and S. m. palmatus, respectively).
In response to 10% mass loss when desiccated at 30°C, the haemolymph osmolarity of the two buthids increased by 59%, compared to ca. 23% in the two scorpionids. Buthids had lower water loss rates than scorpionids. The similar oxygen consumption rates, when converted to metabolic water production, imply a higher relative contribution of metabolic water to the overall water budget of buthids. This could explain why the osmoregulative capabilities exhibited by buthids are better than those of scorpionids.
We conclude that the observed interspecific differences in water and solute budgets are primarily phylogenetically derived, rather than an adaptation of the scorpions to environmental conditions in their natural habitat.
Key words: scorpion, haemolymph, osmolarity, desiccation, water budget, phylogenetic, osmoregulation
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Introduction |
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Scorpions in general have been reported to show some of the lowest
transpiration rates among arthropods
(Crawford and Wooten, 1973;
Edney, 1977
;
Hadley, 1990
). The nocturnal
Hadrurus arizonensis (Iuridae) loses water at a rate ten times lower
than that of the tenebrionid beetle Eleodes armata, which can be
active during the hot daytime hours
(Hadley, 1970
). Edney
(1977
) found `good examples
of relationships between permeabilities and habitats in all classes of
arthropods', and Hadley
(1990
) pointed out a
`definite trend for lower transpiration rates in the more xeric
species' in scorpions.
Interspecific differences in water loss rates of scorpions have been
reported in several studies. Relatively high water loss rates were reported
for the tropical Pandinus imperator (Scorpionidae) compared with the
more xeric Buthus hottentotta hottentotta (Buthidae)
(Toye, 1970). A comparison of
four scorpion species captured in the Mediterranean region of Northern Israel
revealed higher water loss rates for Scorpio maurus fuscus
(Scorpionidae) and Nebo hierichonticus (Diplocentridae) than those of
Leiurus quinquestriatus and Buthotus judaicus (both
Buthidae) (Warburg et al.,
1980
). The xeric Parabuthus villosus (Buthidae) had
significantly lower water loss rates in comparison with the mesic
Opistophthalmus capensis (Scorpionidae)
(Robertson et al., 1982
).
Most scorpion species have been reported to simply tolerate increased
haemolymph osmotic and ionic concentrations as a result of dehydration
(Hadley, 1974;
Riddle et al., 1976
;
Warburg et al., 1980
;
Punzo, 1991
). The xeric South
African buthid, P. villosus, was reported as an exception to this
trend, showing good osmoregulative capacity in comparison with the mesic
scorpionid O. capensis (Robertson
et al., 1982
) and other previously studied species, and comparable
to that of tenebrionid beetles, successful desert-inhabiting insects. They
view these capabilities, together with the scorpion's large body size and low
metabolic and water loss rates, as a `...very useful adaptation to a
desert existence'.
Metabolic water, produced during food oxidation and entering the body
general reserves, is essential to the water budget of dry-living arthropods.
Generally, metabolic water constitutes a small portion of the arthropods'
total water needs (Hadley,
1994). However, the importance of metabolic water increases when
other water sources are not available, e.g. in pupae or during long flights,
or in animals feeding on dry food (flour moth larvae)
(Edney, 1977
). Robertson et al.
(1982
) calculated metabolic
water production for P. villosus, based on oxygen consumption rates.
They concluded, assuming oxidation of lipids, that metabolic water production
rate accounts for only
5% of the total water loss rate (WLR). However,
this ratio was based on short-term water loss, and the authors suggest that
the fraction of metabolic water from WLR could be higher if transpiration
rates decrease significantly during prolonged desiccation.
Scorpions are represented in Israel by three families, Buthidae,
Scorpionidae and Diplocentridae, consisting of 19 species and subspecies from
9 genera (Levy and Amitai,
1980). Buthidae and Scorpionidae are represented in Israel by more
than one species (or subspecies). Within each of the two families there are
species/subspecies of distinct geographical distribution, which are thus faced
with different environmental conditions. L. quinquestriatus
(Buthidae) and Scorpio maurus palmatus (Scorpionidae) are
predominantly xeric species, whereas B. judaicus (Buthidae)
(previously named Hottentotta judaica) and S. m. fuscus
(Scorpionidae) mainly occupy mesic environments. B. judaicus occurs
in areas where annual rainfall is at least 350400 mm, while the Judea
mountains constitute the northern and southern distribution borders of S.
m. palmatus and S. m. fuscus, respectively
(Levy and Amitai, 1980
).
Previous studies referred to B. judaicus as a xeric species, which correlated well with the species' low transpiration rates and fitted the habitatpermeability accepted relationship. However, the actual mesic distribution of B. judaicus led us to hypothesize that the observed interspecific differences could be phylogenetically derived. The above four species were used in an attempt to determine whether interspecific differences in water budgets and osmotic responses to desiccation can be viewed as physiological adaptations to environmental conditions, or stem from phylogenetic constraints.
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Materials and methods |
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The two buthids were found under stones, whereas the two scorpionids were captured mostly by digging their burrows. Of the two Scorpio maurus subspecies, the mesic S. m. fuscus was often captured at its burrow entrance, under stones.
Haemolymph osmolarity
Haemolymph samples were taken from the scorpions within 48 h of collection.
The osmolarity values recorded from scorpions within 24 h and 48 h of
collection were similar. After weighing the scorpions (to ±0.1 mg),
asharpened glass capillary was inserted into the pericardial sinus by
puncturing the dorsal intersegmental membrane. The tapered tip of the glass
capillary enabled an immediate closure of the wound. The haemolymph volume
withdrawn was usually 1015 µl, of which 8 µl were required for
measuring osmolarity (5100C Vapour Pressure Osmometer, Wescor, Logan,
USA).
The scorpions were then held at room temperature (25°C) in round
(9 cm diameter) transparent plastic boxes with soil from their respective
collection sites. Food (adult crickets) was supplied ad libitum for
14 days in order to monitor the effect of feeding on haemolymph osmolarity,
and to minimise variation between individuals that could have resulted from
their energetic or hydration status upon capture.
A second haemolymph sample was taken following feeding, when the mean
initial masses of the four species were 2.164 g (range 1.0283.623 g),
2.294 g (1.0234.404 g), 1.745 g (1.1002.603 g) and 2.023 g
(1.1363.284 g) for B. judaicus, L. quinquestriatus, S. m.
fuscus and S. m. palmatus, respectively. Preliminary
measurements of haemolymph volume revealed a 2030% volume:body
mass ratio (E. Gefen and A. Ar, unpublished data), thus a 10 µl sample from
a 1 g scorpion did not constitute more than 5% of the total haemolymph volume.
In case of persistent bleeding as a result of haemolymph withdrawal, scorpions
were discarded from further investigations.
After allowing 24 h for recovery the scorpions were weighed again, and transferred to identical empty plastic boxes. The boxes were placed in a controlled temperature chamber (30.0±0.5°C, ambient humidity 4060%). The scorpions were weighed daily, and following losses of up to 26% of initial mass haemolymph osmolarity was measured again.
A separate group of scorpions was sampled for both total haemolymph osmolarity and ion concentrations. The scorpions were maintained as described above, and up to 30 µl of haemolymph were extracted following feeding and again after prolonged desiccation. 8 µl samples of haemolymph were used for osmolarity measurements and determination of chloride (CMT10 chloride titrator; Radiometer, Copenhagen, Denmark) and sodium/potassium content (Model 480 Flame Photometer, Corning, Medfield, USA).
Mass loss rate
Mass loss rates (MLR) were measured at 30°C and 50% relative humidity
(RH). The scorpions were fed ad libitum until 48 h before the
initiation of the measurements. After allowing 24 h for acclimation in the
temperature chamber (30.0±0.5°C, ambient humidity 4060%),
the scorpions were placed in individual plastic boxes with perforated bottoms,
which were put on a raised plastic grid in a sealed tank. Air supply (100 ml
min1) at 50%RH through the boxes was achieved by mixing dry
air with air saturated with water vapour at the experimental temperature. Flow
rates were controlled by flow controllers (5800 Series, Brooks, Veenendaal,
Holland), and the humidity level was validated using a humidity sensor
(±2%RH; Almemo, Holzkirchen, Germany). Animal weighing was performed
every second day, and MLR calculated by the difference in mass between
successive measurements (excluding dry mass of excretions), divided by the
elapsed time between measurements. Mass-specific MLR were calculated by
dividing MLR by the mass recorded in the previous weighing.
Gas exchange mass loss, based on measured oxygen consumption rates and RQ values, was used for estimation of water loss rates (WLR). The maximal mass loss rate (assuming carbohydrate catabolism) that may have resulted from gas exchange was subtracted from the total MLR, and the remainder taken as water loss rate (WLR).
Oxygen consumption
Oxygen consumption rates
(O2) were
calculated from a pressure drop in a closed system, comprising two 100 ml
glass syringes (cell volume 40 ml), which contained ascarite for absorption of
CO2 and water vapour. The tips of the two syringes were connected
to the two sides of a differential pressure transducer (model DP15TL,
Validyne, Northridge, USA). The scorpion was held in one syringe, while an
identical empty syringe was used as a reference cell. Both syringes were
placed in a temperature-controlled water bath (30±0.2°C). The
voltage output recorded from the pressure transducer was converted to oxygen
consumption rate by injecting known volumes of oxygen to the measurement cell
in order to restore initial pressure. Cell pressure was not allowed to drop by
more than 0.25%.
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Results |
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Fig. 1 presents the haemolymph osmotic change after 14 days of ad libitum feeding, as a function of the deviation in haemolymph osmolarity of the individual scorpion from its sample mean upon capture. In all species variability decreased following feeding; haemolymph osmolarity increased when initial values were lower than the mean, whereas relatively high initial osmolarities resulted in moderate increase or even decrease in osmolarities following feeding (Fig. 1). However, the results suggested that Buthidae and Scorpionidae differ in their osmotic response to ad libitum feeding following capture (Table 2). Within-family slopes did not differ significantly for either Scorpionidae (F1,262=0.585, P=0.45) or Buthidae (F1,186=0.013, P=0.91), but the between-families difference in slopes was significant (F1,452=8.687, P=0.003). Feeding resulted in significantly increased haemolymph osmotic concentrations in all species (t-test for dependent samples, P<0.01), but a milder increase was observed for B. judaicus (Fig. 1; Table 2).
|
|
Fig. 2 shows the effect of desiccation at 30°C and 4060% RH (expressed as % mass loss, excluding dry excretions) on haemolymph osmolarity. S. m. palmatus and S. m. fuscus show a sharp increase in haemolymph osmolarity even at moderate desiccation levels (510% mass loss, excluding dry excretions). The two Buthidae, L. quinquestriatus and B. judaicus, show a certain capability to withstand higher water losses while maintaining relatively stable haemolymph osmotic concentrations. Moreover, buthids often displayed a decrease in osmotic concentration of the haemolymph with the onset of desiccation (Fig. 2). Such a decrease was not observed for either of the two scorpionid subspecies.
|
Most of the measured osmolarity values, before and after feeding, were
accounted for by sodium and chloride ions (>93% of total osmolarity). These
remained the main ions contributing to the total haemolymph osmolarity
following desiccation in all four species. However, at 570 mOsm
l1 sodium and chloride ions accounted for more than 90% of
the total osmotic concentration of the haemolymph, but their combined
contribution decreased to 80% of the >700 mOsm l1 of
desiccated scorpions (Fig. 3).
The concentration of potassium ions appeared to increase with desiccation, but
never exceeded 2% of the total haemolymph osmolarity.
|
Table 3 presents the results of haemolymph osmotic change (% of initial), after desiccating scorpions of the four species to 10% loss of their initial mass under similar experimental conditions. Interspecific comparison (ANCOVA, initial mass as covariate) of arcsine-transformed % osmolarity changes (decreased osmolarity was designated as 0% change) matched the overall picture reflected in Fig. 2. The haemolymph osmolarities of both L. quinquestriatus and B. judaicus were significantly (P<0.05) less affected by a 10% body mass loss, than were those of the two Scorpio maurus subspecies. The moderate increase in haemolymph osmolarity of B. judaicus is further highlighted by the observation that one specimen had an uncharacteristic increase of 27% in haemolymph osmolarity, while the other 11 showed an increase of 3.05±2.53% following a 10% mass loss. Furthermore, of all scorpions desiccated to 10% mass loss, only five B. judaicus specimens exhibited a decreased haemolymph osmolarity compared to initial values. These results are in agreement with the general osmotic pattern presented in Fig. 2, and confirm that buthids show a relatively moderate osmotic concentration increase in response to desiccation, regardless of their geographic distribution.
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Mass loss rate
Mass-specific mass loss rates (MLR) at 30°C and 50%RH are shown, at
2-day intervals over a 2-week measurement period, in
Fig. 4. The two buthids show a
steady MLR throughout the measuring period, while those of the scorpionids
decrease during the first week and reach steady-state values towards the
second week. While the xeric S. m. palmatus had a seemingly lower MLR
than its mesic subspecies S. m. fuscus (though without statistical
significance between steady state values), it was the mesic B.
judaicus with the lower MLR among the buthids. Interspecific differences
in MLR were more evident following a short desiccation period as a result of
the decreasing MLR of scorpionids (Fig.
4). Comparison of average steady-state MLR (days 814)
revealed a significant difference (P<0.05) between the two mesic
species only, B. judaicus and S. m. fuscus (one-way ANOVA of values
adjusted to initial body mass, followed by Tukey's HSD test).
|
Lipid catabolism (RQ 0.7) results in negligible mass change as a
result of gas exchange, because the molecular weight ratio of consumed
O2 to emitted CO2 is also
0.7. However, when
carbohydrates are metabolised, and RQ is 1.0, this molecular mass ratio
results in net dry mass loss. Dry mass loss is maximal when carbohydrates are
the sole metabolic fuel, whereas values are intermediate when both lipids and
carbohydrates are catabolised. Therefore, the filled area in
Fig. 4 represents the estimated
maximal dry-mass loss rate of the scorpions, based on carbohydrate catabolism
and an
O2 of 0.1
ml g1 h1. Assuming the four species
included in this study use similar proportions of lipids and carbohydrates as
metabolic fuels, the measured differences in mass loss rates represent
differences in the water loss rates of the species.
Oxygen consumption rate
Oxygen consumption rates
(O2) of resting
scorpions at 30°C (mean ± S.D.), and their
body mass range are given in Table
4. ANCOVA (body mass as covariate) confirmed a lack of any
interspecific difference in
O2
(F(3,26)=0.324, P=0.81). The combined effect of
body mass on the oxygen consumption of the four scorpion species was:
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Discussion |
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The haemolymph osmolarity of scorpionids appear to be more variable than
that of buthids (Table 1;
Kimura et al., 1988). The
calculated coefficients of variance for haemolymph osmolarities of scorpionids
and buthids are 10.8% and 6.8%, respectively. This may result from the
scorpionids' higher WLR (Fig.
4), which is more challenging for maintaining a balanced water
budget, reflected in the haemolymph osmotic concentrations measured
immediately following capture (Table
1). This osmotic variability decreases in all four species after
feeding, but the significantly higher slopes found for buthids
(Fig. 1,
Table 2) provide further
evidence of their better haemolymph osmoregulative capabilities compared with
scorpionids. It is interesting to note that following a 14-day ad
libitum feeding period the haemolymph osmolarity of B. judaicus
remained fairly stable, while that of the other three species showed an
increase of 5070 mOsm l1
(Table 3). The `osmotically
favourable' state in which specimens of B. judaicus are found in the
field could be attributed to their lower WLR.
Both Fig. 2 and
Table 3 highlight the fact that
the two buthids have a better capability to osmoregulate following water loss,
compared to the two Scorpio maurus subspecies. It has been suggested
that most scorpion species respond to dehydration by `...simply tolerating
increases in haemolymph osmolality and ionic concentrations...'
(Hadley, 1994). Nevertheless,
our results (Fig. 2), together
with those from a previous study
(Robertson et al., 1982
)
suggest that scorpions vary in their ability to osmoregulate in response to
water loss.
Fig. 3 shows that the
contribution of sodium and chloride ions to total haemolymph osmolarity
decreases with increasing total osmolarity. The decrease in the relative
combined contribution of these two ions to the total haemolymph osmolarity of
desiccated scorpions correlates with an observed decrease in excretion rate
with time during prolonged desiccation (E. Gefen and A. Ar, unpublished data).
Scorpions reaching haemolymph osmolarities of >700 mOsm
l1 (Fig. 3;
N=11), and averaging 757 mOsm l1, had an initial
mean value of 586 mOsm l1. The decrease in the relative
sodium and chloride ion contribution from the initial 93% (545 mOsm
l1) to 81% following desiccation (613 mOsm
l1) means that these two ions are responsible for 40%
of the total increase in the haemolymph osmolarity of the desiccated
scorpions. It has been shown that dehydration results in an increase in solute
content in the haemolymph of beetles, probably due to the accumulation of
excretory products such as allantoin and urea
(Cohen et al., 1986
; Naidu,
1998
,
2001
). It appears possible
that at least some of the remaining unknown accumulated solutes found in the
haemolymph of the desiccated scorpions are excretory metabolites. These are
usually excreted in the hydrated organism, but may be retained during
desiccation in an attempt to reduce excretory water loss.
Fig. 2 shows that the
osmotic response of scorpions to desiccation is not linear. Polynomial rather
than linear regression lines are best fitted to data of all four species. This
is particularly evident for L. quinquestriatus and B.
judaicus, with linear regression r2 values of 0.34
and 0.23, respectively. The non-linear osmotic response is further supported
by the decreasing osmolarities recorded for buthids in the early stages of
desiccation. A similar pattern is evident in the osmotic response of P.
villosus to prolonged desiccation (figure 2 in
Robertson et al., 1982). A
non-linear osmotic response to desiccation was also reported for the isopod
Porcellio scaber (Horowitz,
1970
). The author suggests movement of water from tissues to the
haemolymph, which later increases in osmolarity when the water supply does not
meet demand. A decreased haemolymph osmolarity following mild desiccation has
also been reported for P. scaber, with the hepatopancreas being
mentioned as a possible source for water movement to the haemolymph
(Lindqvist and Fitzgerald,
1976
). It has also been shown
(Woodring, 1974
) that
osmoregulative capacity in the millipede Pachydesmus crassicutis
depends on desiccation rates.
Buthids lose body water at a relatively low rate (Fig. 4) and thus may allow compensatory mechanisms, in the form of water stores other than the haemolymph, to keep haemolymph osmolarity levels stable during mild dehydration. In comparison, scorpionids lose the first 10% of their initial mass within 610 days of high WLR (Fig. 4; Table 3). These rates may be higher than the rate of water movement to the haemolymph, which could explain why scorpionids do not exhibit the initial decrease in haemolymph osmolarity. Following prolonged desiccation water stores may be exhausted, and cannot keep haemolymph osmolarity stable under these experimental conditions.
Robertson et al. (1982)
described the osmoregulative capabilities of P. villosus (Buthidae),
together with its low WLR and large body size, as a possible adaptation to its
xeric habitat. This was in contrast to the mesic scorpionid O.
capensis, which had higher WLR, and did not exhibit similar ability to
osmoregulate its haemolymph following desiccation. The difference in WLR of
P. villosus and O. capensis is in agreement with the
habitatwater permeability relationship reported for arthropods in
general (Edney, 1977
), as is
that between B. hottentota (Buthidae) and P. imperator
(Scorpionidae) (Toye, 1970
).
Another study concluded that among four scorpion species found in Northern
Israel, those inhabiting mesic habitats lose water at a higher rate than xeric
species (Warburg et al.,
1980
).
However, none of the previous comparative studies
(Toye, 1970;
Warburg et al., 1980
;
Robertson et al., 1982
)
distinguished between the suggested ecologically adaptive nature of the
physiological phenomenon and its possible phylogenetic origin. It is therefore
important to note that the lower WLR reported by all the above authors are
those of Buthidae. Likewise, reference to B. judaicus as a xeric
species (Warburg et al.,
1980
), despite its distribution being limited to areas of at least
350400 mm annual rainfall (Warburg
and Ben-Horin, 1978
; Levy and
Amitai, 1980
), could be just as misleading.
The results of the present study, together with previously available
comparative data, lead us to suggest that interspecific differences in water
relations of scorpions are likely to be largely the result of phylogenetic
constraints rather than simply an adaptation of the organism to environmental
stress in its natural habitat. The importance of minimising water loss for the
overall water budget of terrestrial arthropods, coupled with the low WLR
reported for buthids in general, could explain the better osmoregulative
capabilities of scorpions of this family
(Robertson et al., 1982; this
study). Many buthids have a range of life history characteristics that differ
from scorpions of other families, e.g. accelerated life history and
developmental plasticity (Polis,
1990
; Lourenco et al.,
2003
). These characteristics, together with lowered integument
permeability, correlate well with the extreme and unpredictable environmental
conditions encountered by the surface-dwelling buthids.
Robertson et al. (1982)
calculated a metabolic water production rate (metH2O) to
transpiration rate ratio of 5% for P. villosus at 25°C, though
they stress that this seemingly small contribution applies to short-term (6 h
exposure) desiccation. Scorpions in general have relatively low metabolic
rates, compared to other arthropods of similar body size
(Lighton et al., 2001
), and as
a result the total amount of metabolic water produced is low. However, the
highly waterproof integument of scorpions contributes to their water budget by
lowering WLR, and thus increases the relative contribution of metabolic water
to the overall water budget. Lighton et al.
(2001
) calculated the
metabolic rate of scorpions to be 24% that of typical terrestrial arthropods,
while the transpiration rates of six buthid species calculated by Hadley
(1994
) are at least an order of
magnitude lower than those of most xeric insects. Therefore, the significance
of metabolic water in the water budget of scorpions is expected to be
relatively high in comparison with other arthropods. This should also be
reflected in osmoregulative capabilities correlated with WLR of the respective
species.
The results of our measurements of oxygen consumption rates
(O2)
(Table 4) are comparable to
previously reported values for scorpions
(Withers and Smith, 1993
).
O2 converted to
metabolic water production (1.89 ml O2 mg1
H2O for lipid oxidation) yielded higher percentages of the total
water loss for all four species included in this study, compared to the value
reported by Robertson et al.
(1982
). The calculated
metH2O:WLR ratio for scorpionids was 918% in the
early stages of desiccation, and up to 22% in the second week. In comparison,
WLR values of buthids are lower, and as a result a higher percentage
(2034%) of their WLR is accounted for by
metH2O.
Glycogen levels in the hepatopancreas of scorpions have been shown to
decrease during starvation (Sinha and
Kanungo, 1967). Assuming carbohydrate metabolism, and accounting
for the resultant mass loss as a result of gas exchange
(Fig. 4), the
metH2O:WLR ratios were
30% for L.
quinquestriatus throughout the experiment, and as high as 50% for B.
judaicus. These high ratios for buthids contribute to their better
osmoregulative capabilities in response to desiccation. In fact, of the four
species in this study, B. judaicus appears to be the best
osmoregulator (Table 3), which
is well reflected in its metH2O:WLR ratio. In
comparison, until reaching 10% mass loss, the contribution of
metH2O to the overall water turnover of S. m.
fuscus and S. m. palmatus is 1317% and 1527%,
respectively, as a result of their high initial WLR
(Fig. 4). Furthermore, glycogen
binds water, estimated at 35 times its own mass
(Schmidt-Nielsen, 1990
), which
is made available when glycogen is catabolised. Thus, for low WLR, rates of
release and transfer of water (bound and metabolic) to the haemolymph may
exceed evaporation rate. Therefore, glycogen oxidation and the low WLR
recorded for buthids (Fig. 4)
could account for the observed initial decrease in the haemolymph osmolarity
in species of this family (Fig.
2). However, availability of this water source may diminish with
depleting glycogen stores.
In conclusion, interspecific differences in the osmoregulative capabilities of scorpions occur, and we suggest that phylogenetic constraints play a major role in the ability of scorpions to resist desiccation. The enhanced osmoregulative capacity of buthids is likely to have evolved in response to the surface-dwelling existence of scorpions of this family. The results of this study suggest that the contribution of metabolic water to the overall water budget of scorpions is higher than previously thought, and influences their osmoregulative capabilities.
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Acknowledgments |
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References |
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