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Comparative study on the water economy of two sympatric species of desert snails (Sphincterochila zonata and S. prophetarum)
Journal of Arid Environments (1981) 4,115-121
Comparative study on the water economy of two sympatric species of desert snails (Sphincterochila sonata and
S. prophetarum) Moshe Shachak*
Accepted 1 October 1980 The two sympatric species S. zonata and S. prophetarum do not exhibit identical distribution in the desert regions of Israel and Sinai. Only S. prophetarum appears able to inhabit regions with average annual rainfall lower than 100 mm and with only 10-20 rain days from November through March. Experiments were designed to test the assumption that S. prophetarum is better able to gain and conserve water. It was found that the net water gained by S. zonata was 17-33 per cent of the initial body water pool, while S. prophetarum gained only 3-7 per cent. Therefore, the assumption was rejected. It seems that the explanation for the inverse relationship between physiological traits of water economy and the geographical distribution is related to the different habitats with which the two species are associated.
Introduction There is little information available on the water economy of desert snails (Machin, 1965; Schmidt-Nielsen, Taylor & Shkolnik, 1971, 1972; Yom-Tov, 1970). Most data have been obtained from experiments on water loss during summer dormancy. However, in order to understand the water balance of desert snails, the missing link of winter water input should be investigated. The following experiment was carried out on two parapatric desert snails S. zonata and S. prophetarum in order to compare water intake and loss under simulated winter conditions. Particular interest was shown in comparing water input of the snails, utilizing two resources available under field conditions, wet soil crust covered with algae and wet stones covered with lichens. During the dry periods between rains and during the summer, S. zonata lives under bushes or buried under the soil (Shachak, Orr and Steinberger, 1975), while S. prophetarum lives under stones on rocky hillsides (Steinberger, 1980). Sphincterochila zonata and S. prophetarum were chosen for the experiment because they are closely-related species exposed to similar climatic conditions in their sympatric zone. However, they do not exhibit identical distribution in the desert regions of Israel and Sinai (Bar, 1975). Of the two species only S. prophetarum appears able to maintain populations in regions with less than 100 mm average annual rainfall, with only 10-20 rain days from November through March. It would, therefore, seem reasonable to hypothesize that S. prophetarum is better able to conserve water. Thus, in addition to the study of influx and efflux of water in the two species, the validity of the above hypothesis was also tested. Methods Experimental animals
In December 1977, 150 adult snails (75 S. zonata+ 75 S. prophetarum were collected in the central Negev Desert, Israel (34°46'£, 30050'N). The snails were collected from their natural • The Institute for Desert Research, Ben Gurion University of the Negev, Sede Boqer, Israel. 0140-1963/81/020115 +07 $02.00/0
© 1981 Academic Press Inc. (London) Limited
M. SHACHAK
116
Table 1. Some morphometric data of the experimental animals (mean ± s.d.)
S. zonata S. prophetarum
Diameter (mm)
height (mm)
n
22·8 (±1·2) 14·6 (± 0'9)
15·6 (± 0'9) 8·1 (±6'7)
75 75
habitat. S. zonata from a loessial wadi and S. prophetarum from a rocky hill. Some morphometric data on the two species is given in Table 1. All snails in the experiment were taken from the field after seven months of summer dormancy. The snails were kept dormant for 5 days, in a controlled environment at 24 ± 1°C, until the onset of the experiment. Relative humidity in the chamber was 90-95 per cent. Experimental design
The water input and output of the two species of snails were investigated during three typical behavioral patterns that are exhibited under field conditions (Shachak, Orr & Steinberger, 1975; Steinberger, 1980): (1) active and feeding; (2) active, not feeding; (3) dormant. Feeding chambers made of plastic (30 x 20 x 12 em) were designed in order to stimulate state 1. Two different food sources taken from the area in which the snails were collected were available to the snails: soil crust algae and/or stone lichens. The only available free water was that around soil particles and stones, and this maintained a high humidity in the environment. The general layout of the experiment is summarized in Table 2. In order to distinguish between water and dry weight changes, three variables were measured: live body weight, dry body weight and water weight (water weight = live body weight-dry body weight). Details of the method are described in Shachak et al., (1980). The following determinations were made from the experiment: (1) Body water content of five snails taken from each group before and after each experiment detailed in Table 2. Each snail was oven dried (80 ± 2 0c) to a constant weight. (2) Changes in body water pool during each stage were calculated as follows: IBW·IWC-EBW·EWC. where IBW = initial body weight (at the onset of the stage) IWC = initial body water content EBW = body weight at the end of the experimental stage EWC = water content at the end of the experimental stage
Results In the experiment the snails were observed to feed on the soil algae. Grazing areas were identified, as the soil scraped by the radula is easily seen. The increase in body weight of the snails is, therefore, the sum of food and water input. In this paper only changes in the body water pool are reported. Body water
The changes in body water content of the snails under the experimental conditions are shown in Fig. 1. It can be-seen that after summer dormancy, when water is available, snails are able to increase their body water content. Comparison of the twospecies shows that S. sonata, which is active on soil crust, could increase its body water content to higher values than
l.SC= 2.ST= 3.SC+ST=
All 3 All 3
Active and feeding
Active Dormancy
20 15
25
n*
-
-
Soil crust algae'[ Stone lichens Algae + lichens
Source of food
-
-
Soil moisture Wet stones Soil moisture + wet stones
Source of water']
24±1 24± 1
14± 1 7±1 7±1
I 1 1 4 33
Temp. °C§
Duration (days)
12 12
0
12 0
Light (h)§ II
sprinkled on the soil crust and stones. :j: Soil crust was put into an incubator for 24 h at 14 ± 1 °C and light for algal growth. § Average temperature and light regime under natural conditions. II Fluorescent light 2-2'4 x 104 erg cm- 2 sec-I.
t Water was
60-80 60-80
95-100 95-100 95-100
Air humidity (%)
* The numbers are for each species and experimental group. At the end of each stage, five snails were taken for body water content determination.
Experimental group
Stage
Table 2. The experimental design for comparison of water input and output in the two species of desert snails (S. zonata and S. prophetarum)
::0
-.:J
-
~ 8en
en
~
trl
en
o"Ij otrl
><:
~
o z o
o
trl
~
M.
118
_
>-
A.,
0·80
0;.0
0·60
"
C .-
"5 ~
0·10 3
1·00 2
0
"0
-~
0'
E
040
~r
0·20
SHACHAK
~rf1
~l
~ ~
f/ V
SC
ST
:/ :/
~
0·80
/~
0·06
/
0·04
~
0·02 0010
SC +ST
0·008 0·006
meant s.d.
4
~
0·004
~
~
0·002
SC
ST
+SC ST
Figure 1. Changes in body water content of S. zonata 0 and S. prophetarum r2I under laboratory simulated conditions (see Table 2).1. After summer dormancy; 2. active and feeding; 3. active; 4. inactive.
S. prophetarum. However, S. prophetarum shows higher body water content when active on wet stones. As soon as its body water is high, S. zonata is able to maintain it while active under the experimental conditions without any additional water. Under the same conditions, the body water content decreases in S. prophetarum. During inactivity, the percentage of body water decreases in both snail species. Water input
There is a marked difference in the live body weight of the two species. In this study the initial average soft body weight of the five snails was 499 ± 30 mg and 1538 ± 44 mg for fram l to 2 10.-----------------------------,
90
~
(;
~
70f-
mean t s.d.
o
a. 0;
"5
50f-
~
i
.0
.s
30
on
~
C
~ u
0
-10
SC SC
ST
ST
ST
SC +ST
ST
fram 210 3
from 3 10 4
Figure 2. Changes in the body water pool of S. zonaia 0 and S. prophetarum ~ as related to the initial live body weight. 1, 2, 3, 4 (see Fig. 1). SC, ST (see Table 2).
WATER ECONOMY OF DESERT SNAILS 100
119
r------------------------------~
~
mean
± s.d.
~
.,
0
~ ,.,
~
90
"0 0
s.z.
1-... s.p.
~
lD
fi- ~~ ~
~ 80
;rfr.
~ ~
SC
3T
SC
SC
ST
SC +ST
SC
ST
SC +ST
SC
ST
SC
-2---- -3--- _4---
-I
Figure 3. Rates of water input and output in S. zonata 0 and S. prophetarum ~ under laboratory simulated conditions (see Table 2). 2, 3, 4 (see Fig. 1). SC, ST (see Table 2). (S.z. and S.p. average of the three experimental groups.)
S. prophetarum and S. zonata respectively. Thus, the net water input was analyzed as percentage increase from the initial body weight (Fig. 2) and as rates of water flow per unit weight (Fig. 3). In the experiments in which snails were provided with moist soil S. sonata was better than S. prophetarum in input rates and ability to control body water pool. However, when wet stones were used, the reverse results were obtained (Figs 2, 3). In spite of the inter-species differences in water input, the results show that both species have a very high capacity to increase their body water pool. In one day of water intake, their body water pool may increase from 75 to 104 per cent of their initial body weight. Water output The decrease in the body water pool of S. zonata and S. prophetarum in four days of activity without food and 33 days of dormancy is shown in Fig. 2. In the activity and dormancy stages, which are associated with water output only, the advantage of S. sonata and S. prophetarum is obvious. During the activity period, S. sonata lost 11-15 per cent of its body water pool, while S. prophetarum lost 21-25 per cent. The changes in the dormancy period 40
~
., '" (;
30
0
u;
.,
0
20
~
s
c:
0
<.:>
10
SC
ST
SC + ST
Figure 4. Comparison of net gain in body water pool in S. zonata 0 and S. prophetarum E2 under laboratory simulated conditions. SC, ST (see Table 2).
120
M. SHACHAK
were 21-24 per cent and 24-26 per cent for S. sonata and S. prophetarum respectively. The most pronounced difference in water output rate is during activity periods (Fig. 3). In periods of activity, water output rates of S. prophetarum are twice those of S. zonata. During activity, water loss is mainly from the soft body, while during dormancy high rates of water loss are assumed to be preventedby the shell. There is no significance in water loss between the two species during dormancy. In both species water output rates are similar (Fig. 3). Water balance
The net water balance of the two species of snails is shown in Fig. 4. Under the laboratory simulated conditions of one day of water input and 37 days of water output, both species increased their body water pool. Such net water balance is possible because of the input to output ratio. S. sonata is capable of utilizing water gained in one day for 40 days of activity and 123 days of dormancy. Under the same conditions S. prophetarum will exploit one day net water input for 14 and 107 days of activity and inactivity respectively. Discussion
This work supports previous studies which show that the basic strategy of water resource utilization in Sphincterochila spp. is by a very high rate of water input when water is available and low rate of water output during dry periods (Shachak et al., 1976; Steinberger, 1980). In natural conditions, both species become active a few hours after rain and move from their shelters to the wet soil crust. During the period of open-space activity, the wet soil crust covered by algae and the high humidity of the environment are the only sources of water for the snails. After a rain, the soil crust gradually dries and the snails move back to their shelters and become dormant (Shachak et al., 1975; Steinberger, 1980). Integration of their field activity pattern and water economy as shown in this study leads to the conclusion that, given the same rainfall pattern, the net gain in its body water pool by S. zonata is greater than that of S. prophetarum. Therefore, it can be assumed that the geographical distribution of S. zonata is associated with comparatively more extreme environments than that of S. prophetarum. In a study of the geographical distribution of the genus Sphincterochila in Israel and Sinai, Bar (1975) reports that S. prophetarum can be found in more arid areas than S. zonata. The two species are found in different ranges of annual average rainfall. The range of S. zonata is related to areas of 100-200 mm average annual rainfall, while the distribution of S. prophetarum is in the range of 50-100 mm. From Bar's study and this work, an inverse correlation between physiological traits of water economy and geographical distribution is found. The species which exhibits lower rates of water input and higher rates of water output-So prophetarum-is found in areas with less rainfall. It seems that the explanation for the inverse relationship is related to the habitats with which the two species are associated. In the investigations of the population dynamics of the two species in the Central Negev, it was found that S. zonata is associated mainly with the loessial plains and wadis ecosystems, while S. prophetarum inhabit only rocky hill ecosystems (Shachak, Orr & Steinberger, 1975; Steinberger, 1980). The typical temporal and spatial activity of S. zonata is by movement from shelters under bushes to the open spaces after rainfall. They return to the bushes after the soil starts drying. S. prophetarum shows the same pattern of activity, but use space under stones as their shelters instead of under bushes. After rainfall, both species utilize the soil moisture of the open spaces as their water resource. However, as soon as the open spaces start drying, the snails move to their inactivity shelters. S. sonata goes to bushes and stays under them or buried, while S. prophetarum stays under stones. Under bushes is a relatively drier environment than under stones (Yair, personal communication). Thus, S. zonata enters the dormancy state immediately after reaching the bush, in order to conserve water, while S. prophetarum may gain a few more days of water input from soil moisture under stones. Another factor which may prolong the number of days of water intake by S. prophetarum is the phenomenon of surface water runoff in rocky
WATER ECONOMY OF DESERT SNAILS
121
desert ecosystems. It was found by Evenari et al. (1971) that runoff water may increase water input to an area equivalent to an addition of 100-200 mm of rainfall. If this runoff water flows under stones it increases the water available to the snails under them. Thus S. prophetarum may compensate for its physiologically lower rates of water input capability by prolonging the water input periods. It may therefore be concluded from this study that the proximate factor in the geographical distribution of the two species of Sphincterochila is habitat selection, and not physiological characteristics. S. prophetarum is distributed in the more arid areas in spite of its physiological limitations, because it is able to utilize runoff water and moisture under stones as water resources. My thanks and appreciation to Dr Vera Fretter for reviewing the manuscript and for her valuable suggestions.
References Bar, Z. (1975). Distribution and habitat of the genus Sphincterochila in Israel and Sinai. Argamon, Israel Journal of Malacology, 5: 1-19. Evenari, M., Shanan, L. & Tadmor, N. (1971). The Negev-The Chailenge of a Desert. Harvard University Press. 124 pp. . Machin, J. (1965). Cutaneous regulation of evaporative water loss in the common garden snail Helix aspersa. Naturwissenschaften 52: 18. Schmidt-Nielsen, K, Taylor, C. R. & Shkolnik, A. (1971). Desert snails: problems of heat, water and food. Journal of Experimental Biology, 55: 385-398. Schmidt-Nielsen, K., Taylor, C. R. & Shkolnik, A. (1972). Desert Snails: problems of survival. In Maloiy G.G.O. (Ed.). Comparative Physiology of Desert Animals, pp. 1-14. Academic Press. 413 pp. Shachak, M., Orr, Y. & Steinberger, Y. (1975). Field observation on the natural history of Sphincterochila zonata, Argamon, Israel Journal of Malacology, 5: (1-4): 20-46. Shachak, M., Chapman, E. A. & Orr, Y. (1976). Some aspects of the ecology of the desert snail Sphincterochila boissieri (= zonata) in relation to water and energy flow. Israel Journal of Medical Sciences, 12: 887-891. Shachak, M. & Steinberger, Y. (1980). Desert snail food chain: energy flow and soil turnover. Oecologia (in press). Steinberger, Y. (1980). Some Aspects of the Ecology of the Desert Snail Sphincterochila prophetarum. Unpublished Ph.D. thesis, Bar Han University, Ramat Gan, Israel. 135 pp. Yom Tov, Y. (1970). Investigations in the ecology and survival of two snails in the Negev Desert. Unpublished Ph.D. thesis, University of Tel Aviv, Israel. 127 pp.
Comparative study on the water economy of two sympatric species of desert snails (Sphincterochila sonata and
S. prophetarum) Moshe Shachak*
Accepted 1 October 1980 The two sympatric species S. zonata and S. prophetarum do not exhibit identical distribution in the desert regions of Israel and Sinai. Only S. prophetarum appears able to inhabit regions with average annual rainfall lower than 100 mm and with only 10-20 rain days from November through March. Experiments were designed to test the assumption that S. prophetarum is better able to gain and conserve water. It was found that the net water gained by S. zonata was 17-33 per cent of the initial body water pool, while S. prophetarum gained only 3-7 per cent. Therefore, the assumption was rejected. It seems that the explanation for the inverse relationship between physiological traits of water economy and the geographical distribution is related to the different habitats with which the two species are associated.
Introduction There is little information available on the water economy of desert snails (Machin, 1965; Schmidt-Nielsen, Taylor & Shkolnik, 1971, 1972; Yom-Tov, 1970). Most data have been obtained from experiments on water loss during summer dormancy. However, in order to understand the water balance of desert snails, the missing link of winter water input should be investigated. The following experiment was carried out on two parapatric desert snails S. zonata and S. prophetarum in order to compare water intake and loss under simulated winter conditions. Particular interest was shown in comparing water input of the snails, utilizing two resources available under field conditions, wet soil crust covered with algae and wet stones covered with lichens. During the dry periods between rains and during the summer, S. zonata lives under bushes or buried under the soil (Shachak, Orr and Steinberger, 1975), while S. prophetarum lives under stones on rocky hillsides (Steinberger, 1980). Sphincterochila zonata and S. prophetarum were chosen for the experiment because they are closely-related species exposed to similar climatic conditions in their sympatric zone. However, they do not exhibit identical distribution in the desert regions of Israel and Sinai (Bar, 1975). Of the two species only S. prophetarum appears able to maintain populations in regions with less than 100 mm average annual rainfall, with only 10-20 rain days from November through March. It would, therefore, seem reasonable to hypothesize that S. prophetarum is better able to conserve water. Thus, in addition to the study of influx and efflux of water in the two species, the validity of the above hypothesis was also tested. Methods Experimental animals
In December 1977, 150 adult snails (75 S. zonata+ 75 S. prophetarum were collected in the central Negev Desert, Israel (34°46'£, 30050'N). The snails were collected from their natural • The Institute for Desert Research, Ben Gurion University of the Negev, Sede Boqer, Israel. 0140-1963/81/020115 +07 $02.00/0
© 1981 Academic Press Inc. (London) Limited
M. SHACHAK
116
Table 1. Some morphometric data of the experimental animals (mean ± s.d.)
S. zonata S. prophetarum
Diameter (mm)
height (mm)
n
22·8 (±1·2) 14·6 (± 0'9)
15·6 (± 0'9) 8·1 (±6'7)
75 75
habitat. S. zonata from a loessial wadi and S. prophetarum from a rocky hill. Some morphometric data on the two species is given in Table 1. All snails in the experiment were taken from the field after seven months of summer dormancy. The snails were kept dormant for 5 days, in a controlled environment at 24 ± 1°C, until the onset of the experiment. Relative humidity in the chamber was 90-95 per cent. Experimental design
The water input and output of the two species of snails were investigated during three typical behavioral patterns that are exhibited under field conditions (Shachak, Orr & Steinberger, 1975; Steinberger, 1980): (1) active and feeding; (2) active, not feeding; (3) dormant. Feeding chambers made of plastic (30 x 20 x 12 em) were designed in order to stimulate state 1. Two different food sources taken from the area in which the snails were collected were available to the snails: soil crust algae and/or stone lichens. The only available free water was that around soil particles and stones, and this maintained a high humidity in the environment. The general layout of the experiment is summarized in Table 2. In order to distinguish between water and dry weight changes, three variables were measured: live body weight, dry body weight and water weight (water weight = live body weight-dry body weight). Details of the method are described in Shachak et al., (1980). The following determinations were made from the experiment: (1) Body water content of five snails taken from each group before and after each experiment detailed in Table 2. Each snail was oven dried (80 ± 2 0c) to a constant weight. (2) Changes in body water pool during each stage were calculated as follows: IBW·IWC-EBW·EWC. where IBW = initial body weight (at the onset of the stage) IWC = initial body water content EBW = body weight at the end of the experimental stage EWC = water content at the end of the experimental stage
Results In the experiment the snails were observed to feed on the soil algae. Grazing areas were identified, as the soil scraped by the radula is easily seen. The increase in body weight of the snails is, therefore, the sum of food and water input. In this paper only changes in the body water pool are reported. Body water
The changes in body water content of the snails under the experimental conditions are shown in Fig. 1. It can be-seen that after summer dormancy, when water is available, snails are able to increase their body water content. Comparison of the twospecies shows that S. sonata, which is active on soil crust, could increase its body water content to higher values than
l.SC= 2.ST= 3.SC+ST=
All 3 All 3
Active and feeding
Active Dormancy
20 15
25
n*
-
-
Soil crust algae'[ Stone lichens Algae + lichens
Source of food
-
-
Soil moisture Wet stones Soil moisture + wet stones
Source of water']
24±1 24± 1
14± 1 7±1 7±1
I 1 1 4 33
Temp. °C§
Duration (days)
12 12
0
12 0
Light (h)§ II
sprinkled on the soil crust and stones. :j: Soil crust was put into an incubator for 24 h at 14 ± 1 °C and light for algal growth. § Average temperature and light regime under natural conditions. II Fluorescent light 2-2'4 x 104 erg cm- 2 sec-I.
t Water was
60-80 60-80
95-100 95-100 95-100
Air humidity (%)
* The numbers are for each species and experimental group. At the end of each stage, five snails were taken for body water content determination.
Experimental group
Stage
Table 2. The experimental design for comparison of water input and output in the two species of desert snails (S. zonata and S. prophetarum)
::0
-.:J
-
~ 8en
en
~
trl
en
o"Ij otrl
><:
~
o z o
o
trl
~
M.
118
_
>-
A.,
0·80
0;.0
0·60
"
C .-
"5 ~
0·10 3
1·00 2
0
"0
-~
0'
E
040
~r
0·20
SHACHAK
~rf1
~l
~ ~
f/ V
SC
ST
:/ :/
~
0·80
/~
0·06
/
0·04
~
0·02 0010
SC +ST
0·008 0·006
meant s.d.
4
~
0·004
~
~
0·002
SC
ST
+SC ST
Figure 1. Changes in body water content of S. zonata 0 and S. prophetarum r2I under laboratory simulated conditions (see Table 2).1. After summer dormancy; 2. active and feeding; 3. active; 4. inactive.
S. prophetarum. However, S. prophetarum shows higher body water content when active on wet stones. As soon as its body water is high, S. zonata is able to maintain it while active under the experimental conditions without any additional water. Under the same conditions, the body water content decreases in S. prophetarum. During inactivity, the percentage of body water decreases in both snail species. Water input
There is a marked difference in the live body weight of the two species. In this study the initial average soft body weight of the five snails was 499 ± 30 mg and 1538 ± 44 mg for fram l to 2 10.-----------------------------,
90
~
(;
~
70f-
mean t s.d.
o
a. 0;
"5
50f-
~
i
.0
.s
30
on
~
C
~ u
0
-10
SC SC
ST
ST
ST
SC +ST
ST
fram 210 3
from 3 10 4
Figure 2. Changes in the body water pool of S. zonaia 0 and S. prophetarum ~ as related to the initial live body weight. 1, 2, 3, 4 (see Fig. 1). SC, ST (see Table 2).
WATER ECONOMY OF DESERT SNAILS 100
119
r------------------------------~
~
mean
± s.d.
~
.,
0
~ ,.,
~
90
"0 0
s.z.
1-... s.p.
~
lD
fi- ~~ ~
~ 80
;rfr.
~ ~
SC
3T
SC
SC
ST
SC +ST
SC
ST
SC +ST
SC
ST
SC
-2---- -3--- _4---
-I
Figure 3. Rates of water input and output in S. zonata 0 and S. prophetarum ~ under laboratory simulated conditions (see Table 2). 2, 3, 4 (see Fig. 1). SC, ST (see Table 2). (S.z. and S.p. average of the three experimental groups.)
S. prophetarum and S. zonata respectively. Thus, the net water input was analyzed as percentage increase from the initial body weight (Fig. 2) and as rates of water flow per unit weight (Fig. 3). In the experiments in which snails were provided with moist soil S. sonata was better than S. prophetarum in input rates and ability to control body water pool. However, when wet stones were used, the reverse results were obtained (Figs 2, 3). In spite of the inter-species differences in water input, the results show that both species have a very high capacity to increase their body water pool. In one day of water intake, their body water pool may increase from 75 to 104 per cent of their initial body weight. Water output The decrease in the body water pool of S. zonata and S. prophetarum in four days of activity without food and 33 days of dormancy is shown in Fig. 2. In the activity and dormancy stages, which are associated with water output only, the advantage of S. sonata and S. prophetarum is obvious. During the activity period, S. sonata lost 11-15 per cent of its body water pool, while S. prophetarum lost 21-25 per cent. The changes in the dormancy period 40
~
., '" (;
30
0
u;
.,
0
20
~
s
c:
0
<.:>
10
SC
ST
SC + ST
Figure 4. Comparison of net gain in body water pool in S. zonata 0 and S. prophetarum E2 under laboratory simulated conditions. SC, ST (see Table 2).
120
M. SHACHAK
were 21-24 per cent and 24-26 per cent for S. sonata and S. prophetarum respectively. The most pronounced difference in water output rate is during activity periods (Fig. 3). In periods of activity, water output rates of S. prophetarum are twice those of S. zonata. During activity, water loss is mainly from the soft body, while during dormancy high rates of water loss are assumed to be preventedby the shell. There is no significance in water loss between the two species during dormancy. In both species water output rates are similar (Fig. 3). Water balance
The net water balance of the two species of snails is shown in Fig. 4. Under the laboratory simulated conditions of one day of water input and 37 days of water output, both species increased their body water pool. Such net water balance is possible because of the input to output ratio. S. sonata is capable of utilizing water gained in one day for 40 days of activity and 123 days of dormancy. Under the same conditions S. prophetarum will exploit one day net water input for 14 and 107 days of activity and inactivity respectively. Discussion
This work supports previous studies which show that the basic strategy of water resource utilization in Sphincterochila spp. is by a very high rate of water input when water is available and low rate of water output during dry periods (Shachak et al., 1976; Steinberger, 1980). In natural conditions, both species become active a few hours after rain and move from their shelters to the wet soil crust. During the period of open-space activity, the wet soil crust covered by algae and the high humidity of the environment are the only sources of water for the snails. After a rain, the soil crust gradually dries and the snails move back to their shelters and become dormant (Shachak et al., 1975; Steinberger, 1980). Integration of their field activity pattern and water economy as shown in this study leads to the conclusion that, given the same rainfall pattern, the net gain in its body water pool by S. zonata is greater than that of S. prophetarum. Therefore, it can be assumed that the geographical distribution of S. zonata is associated with comparatively more extreme environments than that of S. prophetarum. In a study of the geographical distribution of the genus Sphincterochila in Israel and Sinai, Bar (1975) reports that S. prophetarum can be found in more arid areas than S. zonata. The two species are found in different ranges of annual average rainfall. The range of S. zonata is related to areas of 100-200 mm average annual rainfall, while the distribution of S. prophetarum is in the range of 50-100 mm. From Bar's study and this work, an inverse correlation between physiological traits of water economy and geographical distribution is found. The species which exhibits lower rates of water input and higher rates of water output-So prophetarum-is found in areas with less rainfall. It seems that the explanation for the inverse relationship is related to the habitats with which the two species are associated. In the investigations of the population dynamics of the two species in the Central Negev, it was found that S. zonata is associated mainly with the loessial plains and wadis ecosystems, while S. prophetarum inhabit only rocky hill ecosystems (Shachak, Orr & Steinberger, 1975; Steinberger, 1980). The typical temporal and spatial activity of S. zonata is by movement from shelters under bushes to the open spaces after rainfall. They return to the bushes after the soil starts drying. S. prophetarum shows the same pattern of activity, but use space under stones as their shelters instead of under bushes. After rainfall, both species utilize the soil moisture of the open spaces as their water resource. However, as soon as the open spaces start drying, the snails move to their inactivity shelters. S. sonata goes to bushes and stays under them or buried, while S. prophetarum stays under stones. Under bushes is a relatively drier environment than under stones (Yair, personal communication). Thus, S. zonata enters the dormancy state immediately after reaching the bush, in order to conserve water, while S. prophetarum may gain a few more days of water input from soil moisture under stones. Another factor which may prolong the number of days of water intake by S. prophetarum is the phenomenon of surface water runoff in rocky
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desert ecosystems. It was found by Evenari et al. (1971) that runoff water may increase water input to an area equivalent to an addition of 100-200 mm of rainfall. If this runoff water flows under stones it increases the water available to the snails under them. Thus S. prophetarum may compensate for its physiologically lower rates of water input capability by prolonging the water input periods. It may therefore be concluded from this study that the proximate factor in the geographical distribution of the two species of Sphincterochila is habitat selection, and not physiological characteristics. S. prophetarum is distributed in the more arid areas in spite of its physiological limitations, because it is able to utilize runoff water and moisture under stones as water resources. My thanks and appreciation to Dr Vera Fretter for reviewing the manuscript and for her valuable suggestions.
References Bar, Z. (1975). Distribution and habitat of the genus Sphincterochila in Israel and Sinai. Argamon, Israel Journal of Malacology, 5: 1-19. Evenari, M., Shanan, L. & Tadmor, N. (1971). The Negev-The Chailenge of a Desert. Harvard University Press. 124 pp. . Machin, J. (1965). Cutaneous regulation of evaporative water loss in the common garden snail Helix aspersa. Naturwissenschaften 52: 18. Schmidt-Nielsen, K, Taylor, C. R. & Shkolnik, A. (1971). Desert snails: problems of heat, water and food. Journal of Experimental Biology, 55: 385-398. Schmidt-Nielsen, K., Taylor, C. R. & Shkolnik, A. (1972). Desert Snails: problems of survival. In Maloiy G.G.O. (Ed.). Comparative Physiology of Desert Animals, pp. 1-14. Academic Press. 413 pp. Shachak, M., Orr, Y. & Steinberger, Y. (1975). Field observation on the natural history of Sphincterochila zonata, Argamon, Israel Journal of Malacology, 5: (1-4): 20-46. Shachak, M., Chapman, E. A. & Orr, Y. (1976). Some aspects of the ecology of the desert snail Sphincterochila boissieri (= zonata) in relation to water and energy flow. Israel Journal of Medical Sciences, 12: 887-891. Shachak, M. & Steinberger, Y. (1980). Desert snail food chain: energy flow and soil turnover. Oecologia (in press). Steinberger, Y. (1980). Some Aspects of the Ecology of the Desert Snail Sphincterochila prophetarum. Unpublished Ph.D. thesis, Bar Han University, Ramat Gan, Israel. 135 pp. Yom Tov, Y. (1970). Investigations in the ecology and survival of two snails in the Negev Desert. Unpublished Ph.D. thesis, University of Tel Aviv, Israel. 127 pp.