THE IMPORTANCE OF MOISTURE IN THE ACTIVITY
PATTERNS OF THE ARID-DWELLING LAND SNAIL
IBERUS GUALTIERANUS
Gregorio Moreno-Rueda*
Estación Experimental de Zonas Áridas (CSIC), La Cañada de San Urbano, Ctra.
Sacramento s/n, 04120, Almería, Spain
ABSTRACT
Weather is one of the prime determinants of activity patterns in snails. Given that
snails are hydrophilic and ectothermic, they may be active only when meteorological
conditions provide a relatively warm and moist environment. Consequently, both
temperature and moisture are among the main factors governing the activity of snails and
slugs. However, the relative importance of temperature and moisture varies
geographically. For example, in arid Mediterranean environments, moisture may strongly
limit activity, while temperature may not. In this study, I examine the activity patterns of
the land snail Iberus gualtieranus in an arid environment in SE Spain. This snail showed
nocturnal activity, being rarely found during the day, and was active in autumn and
winter, but not during spring and summer. Temperature was correlated with the activity
of this snail, but this correlation disappeared on controlling for moisture (correlated with
temperature). Thus, the effect of temperature on activity was mediated by its effect on
moisture. Moisture, therefore, was the most important determinant of activity, explaining
18.4% of variance in number of active individuals. When season, daytime, temperature,
and moisture were considered in a full model, moisture explained the seasonal variation
in activity, but not the entire daytime variation. That is, after moisture was controlled for,
a portion of variance in circadian activity remained explained only by daytime (night vs.
day). These results support that, in an arid environment, moisture is the main factor
determining activity, especially seasonal activity. Moreover, the snails were primarily
nocturnal, regardless of moisture. These results have implications with respect to recent
climate warming, which presumably will decrease wetness in the study area by raising
temperatures. This in turn will narrow the seasonal activity period, extending the
aestivation period, with negative consequences for snail survival. Therefore, this
*
E-mail: [email protected]
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Gregorio Moreno-Rueda
endangered land snail may be threatened by climate warming, and measures are
necessary to avoid its extinction.
INTRODUCTION
Terrestrial gastropods have permeable skin and move by laying down moist mucus trails,
thus suffering high rates of dehydration (Prior, 1985; Luchtel and Deyrup-Olsen, 2001).
Consequently, snails and slugs have developed a number of adaptations in order to avoid or
minimize the risk of dehydration. They show morphological adaptations, such as the thick
white shell of Sphincterochila candidissima, which allows a fresh and wet environment
inside, thereby diminishing dehydration (Moreno-Rueda, 2008). Snails also show behavioral
adaptations to minimize the risk of dehydration, such as huddling in slugs, which decrease the
water loss (Cook, 1981). The main behavioral adaptation is to remain in a protective
microhabitat when weather is adverse (dry), and be active only when weather is favorable
(wet; Cook, 2001). Consequently, moisture is one of the primary determinants of snail and
slug activity patterns (review in Cook, 2001).
In addition to wetness, temperature has a role in determining snail activity, a role which
is complex. On the one hand, snails and slugs are ectothermic, and therefore they cannot be
active when temperatures are very low. Consequently they frequently show hibernation
periods in which they are inactive (e.g. Bailey, 1983). On the other hand, the risk of
dehydration increases with temperature, and thus temperature may indirectly affect activity by
affecting moisture. An optimal temperature for snail activity should be sufficiently high to
allow activity, but not so high as to increase the risk of dehydration. In any case, temperature
has also been shown to be one of the primary factors determining snail activity (review in
Cook, 2001).
Conditions of temperature and moisture greatly vary throughout the world (climatic
variation). Consequently, the relative importance of the two parameters on snail activity
should vary geographically, according to climate. For example, in relatively moist
Mediterranean environments, snails such as Theba pisana and Otala lactea are inactive in
their refuges only when temperatures are high, in order to avoid heat shock, but wetness does
not affect activity in such zones (Moreno-Rueda et al., 2009b). In contrast, in arid
Mediterranean environments, the number of snails of the species Sphincterochila
candidissima and Iberus gualtieranus found in their refuges was affected mainly by moisture
(Moreno-Rueda et al., 2009b).
With respect to Iberus g. gualtieranus, previous studies have shown that this snail is
mainly nocturnal (Moreno-Rueda, 2006a). The number of individuals found in their primary
refuges (karstic crevices in the rocky substrate) is higher in summer, when moisture is the
lowest and temperatures are the highest (Moreno-Rueda, 2007). In fact, wetness determines
the number of specimens of this species found in their refuges, with more individuals being
found sheltered when moisture is low (Moreno-Rueda et al., 2009b). By contrast, although
the number of specimens found in the refuges increases with temperature, temperature has no
effect on the use of refuges when controlled for moisture (Moreno-Rueda et al., 2009b).
According to this information, I predicted that the activity patterns of the arid-dwelling land
snail Iberus g. gualtieranus should be determined mainly by moisture, which should be the
The Importance of Moisture in the Activity Patterns…
3
main restrictive factor in its environment. I tested this prediction with data of the activity of
this snail in Sierra Elvira (SE Spain).
THE STUDY SYSTEM
Iberus gualtieranus (Linnaeus, 1758) is an endemic land snail of Spain (García San
Nicolás, 1956), the subspecies I. g. gualtieranus being endemic to south-eastern Spain
(Elejalde et al., 2005, 2008). This subspecies is an arid-dwelling snail characteristic of arid
and karstic environments (Alonso et al., 1985). Iberus gualtieranus survives in arid
environments such as Sierra Elvira thanks to the use of refuges that protect against
dehydration, karstic crevices, which conserve a fresh and moist microclimate (MorenoRueda, 2002, 2007). In fact, this subspecies has evolved a flattened shell to enter karstic
crevices for shelter (de Bartolomé, 1982; Moreno-Rueda, 2011). However, this snail must
seek food in the open, obligating it to move out of these protective microhabitats in the rocky
substrate (Moreno-Rueda, 2006a). During these movements, the snail is exposed to predators
(such as rats, Rattus rattus, Moreno-Rueda, 2009), and the risk of dehydration, which may be
high. Consequently, to survive, this snail must choose the appropriate times to be active.
There are only four populations known for this subspecies, all strongly isolated (Ruiz Ruiz et
al., 2006) and, consequently, it is considered endangered (Arrébola and Ruiz Ruiz, 2006;
Moreno-Rueda and Pizarro, 2007; Moreno-Rueda, 2011).
The study was performed in Sierra Elvira (SE Spain; 37º 14' N, 3º 47' W), a small karstic
mountain with an altitudinal range of 600-1100 m a.s.l. The climate is accentuated
mesomediterranean (Rivas Martínez, 1987). The annual precipitation is less than 500 mm,
with five months of drought (Alonso et al., 1985), making this a harsh environment for
hydrophilic animals such as gastropods. The habitat in the study zone is composed of rocky
substrates with karstic erosion and low vegetal cover, formed primarily by rosemary
(Rosmarinus officinalis), other shrubs (Stipa tenacissima, Genista sp.) and grasses, with
scattered patches of pines and Holm oaks, and some cultivation of almond and olive trees.
SAMPLING METHOD
The study was conducted from October 2000 to August 2001, in a 500-m2 site on the
Sierra Elvira with the typical habitat of the species (Moreno-Rueda, 2002, 2006b): rocky
terrain with a southern orientation and scrubby plants. The study area was divided in plots of
9 m2. This size was chosen for the plots because the maximal distance covered by a specimen
in a day is about 2 m (own data). Sampling was performed around the 15th day of the month,
with 1-7 days sampling per month. Sampling spanned all hours of the day and the night, and
hours were grouped in six intervals: 0-4, 4-8, 8-12, 12-16, 16-20 and 20-24 h (in solar hours,
12 h = midday). Data were grouped in seasons according to the Julian calendar (autumn,
winter, spring and summer). I sampled 3-5 plots per day, and no plot was sampled more than
once per month. During prospecting, I searched for individuals in bushes, under stones and
inside fissures in the rock. For each plot prospected, I recorded the number of individuals
found and whether they were active or inactive. An individual was considered inactive when
4
Gregorio Moreno-Rueda
its soft body was withdrawn inside the shell and an epiphragm was formed closing the
aperture. If the soft body was outside the shell, the animal was considered active. If the soft
body was inside the shell, but the aperture was not closed by an epiphragm, the snail was
considered active. Inactive snails usually have an epiphragm, while animals without an
epiphragm may have simply retracted themselves inside the shell. When sampling was
performed, temperature was measured 5 cm above ground using an electronic thermometer
(accuracy 0.2 ºC). Soil moisture was measured after the extraction of a cylindrical soil sample
(16 cm3). The wet weight (Sw) of the soil was recorded with a spring balance (accuracy 0.1
g), after which the sample was dried at 120ºC for 48 h and the dry weight (Sd) was recorded.
The percentage of weight lost ([Sw – Sd]  100) was calculated and used as an index of soil
moisture. Also, I recorded whether the sampling was performed during the day or the night
(after sundown). The number of active individuals was used to indicate the activity level of
the population. The fact that inactive snails are cryptic, sheltering inside crevices (MorenoRueda, 2007; Moreno-Rueda et al., 2009b), precluded the use of proportion of the active
individuals with respect to the total as an index of activity.
RESULTS
Of the Iberus g. gualtieranus found, 71.2% (n = 243) were inactive. No differences in
proportion were found between adults and immature individuals found active (adults: 56 out
181; immature individuals: 14 out 62;  = 1.57, p = 0.21), and therefore the data for both
age classes were grouped in the subsequent analyses. The average number of specimens
found active was significantly higher in autumn and winter than in spring and summer
(Kruskal-Wallis Anova, H3, 150 = 12.18, p = 0.005; Figure 1). That is, this snail is inactive
during spring and summer, when temperature is maximal and moisture is minimal (KruskalWallis, in the two cases, p < 0.001; Figures 2 and 3, respectively). According to the previous
results, I restricted the analyses of circadian activity to the activity period (autumn-winter). In
this period, the number of active individuals differed according to the time interval sampled
(H5, 109 = 25.24, p < 0.001; Figure 4). More I. g. gualtieranus were active during nighttime
(0.20  0.047 [mean ± S.E.] individuals/m2, n = 46 samples) than during daytime (0.01 
0.005 individuals/m2, n = 63; Mann-Whitney U-test, z = 3.37, p < 0.001). The number of
active individuals was minimal during midday, when insolation was the strongest.
Temperature significantly varied with the hour (H5, 90 = 50.59, p < 0.001), being minimal at
night, whereas moisture did not (H5, 97 = 2.78, p = 0.73; Figures 5 and 6, respectively).
A negative correlation was found between the number of active individuals and
temperature (rs = -0.43, p < 0.001, n = 130). Soil moisture, by contrast, was positively
correlated with the number of active individuals (rs = 0.46, p < 0.001, n = 132). Soil moisture
and temperature were negatively correlated (rs = -0.55, p < 0.001, n = 114). To determine the
independent effects of temperature and soil moisture on the activity of I. g. gualtieranus, both
variables were introduced into a multiple-regression model with the number of active
individuals as the dependent variable. The model proved significant (F2, 111 = 13.02, p <
0.001) and explained a 19% of variance. When each variable was controlled for with respect
to the other, only soil moisture, but not temperature, explained the activity of this land snail
(Effect of soil moisture: F1, 111 = 14.44, p < 0.001, β = 0.38 ± 0.10; Effect of temperature: F1,
2
The Importance of Moisture in the Activity Patterns…
5
= 0.81, p = 0.37, β = -0.09 ± 0.10). Finally, in a full model also including daytime and
season, daytime and soil moisture significantly explained a portion of variance in snail
activity, but season and temperature did not (Table 1). This full model explained 25.6% of
variance (F6, 107 = 6.15, p < 0.001).
111
Figure 1. Average number of active individuals per sampling plot found in each season. Bars indicate
standard error. Sample size (number of prospected plots) is between brackets.
Figure 2. Average temperature 5 cm aboveground in the study area, in each season, during sampling.
Bars indicate standard error. Sample size (number of prospected plots) is between brackets.
6
Gregorio Moreno-Rueda
Figure 3. Average soil moisture (percentage of wet weight) in the study area, in each season, during
sampling. Bars indicate standard error. Sample size (number of prospected plots) is between brackets.
Figure 4. Average number of active individuals per parcel, according to hour interval. Bars indicate
standard error. Sample size (number of prospected plots) is between brackets.
The Importance of Moisture in the Activity Patterns…
7
Figure 5. Average temperature (ºC) measured 5 cm above the ground during the sampling at 4-hour
intervals. Bars indicate standard error. Sample size (number of prospected plots) is between brackets.
Figure 6. Average soil moisture, measured as a percentage of wet weight of the soil at 4-hour intervals.
Bars indicate standard error. Sample size (number of prospected plots) is between brackets.
8
Gregorio Moreno-Rueda
Table 1. General linear model examining the effects of season, temperature, soil
moisture, and daytime on the number of active individuals found in the plots during
sampling
Intercept
Season
Temperature
Soil moisture
Daytime (light/night)
Error
d.f.
1
3
1
1
1
107
F
0.48
0.67
0.22
15.24
5.62
p
0.49
0.57
0.64
0.0002
0.02
CONCLUSIONS
The results in this study show that the activity pattern of the land snail Iberus g.
gualtieranus in SE Spain varies seasonally and daily. Seasonal variation was entirely
explained by weather factors (temperature and moisture). In fact, soil moisture directly
affected activity, being the primary factor determining snail activity, while the effect of the
temperature was indirect, through its relation with moisture. In fact, when the two factors
were included in a multiple-regression model, only moisture was found to affect activity,
while the effect of temperature disappeared. Consequently, this snail, in the arid environments
of SE Spain, was active when soil moisture was relatively high (therefore, when temperature
was low, as temperature reduced moisture). This implies that its activity period is limited to
autumn and winter, being inactive during spring and summer. However, circadian dynamics
were independent of weather, the snail being nocturnal regardless of weather characteristics.
As would be expected, this snail is not active when moisture is low, in order to avoid the
dehydration (Cook, 2001). However, the question arises why it is active mainly during the
night. It could be active only during the night in order to avoid high temperatures during the
day, but temperature had no effect on circadian activity when introduced into the model.
Another explanation is that this snail avoids predators during the day, such as birds, which
prey on snails, although predation on this species appears to be low (Yanes et al., 1991). In
fact, its primary predator in the study zone seems to the rat, a nocturnal animal (MorenoRueda, 2009). Additionally, it could be primarily nocturnal in order to avoid solar radiation,
especially if its skin is not resistant to radiation.
As predicted, moisture was the primary factor determining snail activity in the arid zone
studied here, while temperature, which is not limiting in the study area, has no direct effect on
activity. Similarly, the activity of the desert-dwelling land snail Sphincterochila prophetarum
is primarily mediated by moisture (Steinberger et al., 1983). In contrast, in wetter regions,
temperature is one of the primary factors determining snail activity (review in Cook, 2001).
For example, in coastal Mediterranean regions, where moisture is not limiting, snail activity is
strongly influenced by temperature, being inactive when temperatures are high, while
moisture shows no effect on activity (Moreno-Rueda et al., 2009b). These results suggest that
the environmental factors determining activity of snails vary geographically according to the
The Importance of Moisture in the Activity Patterns…
9
climatic conditions, temperature being the primary factor where moisture is unconstrained,
and moisture being the primary factor in arid environments.
For the conservation of this endangered and endemic land snail, it is vital to determine
how weather affects the activity, as temperature is increasing at this moment in its distribution
area (SE Spain, see Moreno-Rueda et al., 2009a).
As temperature increases, soil moisture is predicted to decrease, and therefore the time to
be active would diminish. This process may constraint the time for this snail to obtain
sufficient resources to survive the long aestivation period, and mortality might reach a critical
threshold. Climate change has previously been related to the extinction of other populations
of snails (Baur and Baur, 1993; Gerlach, 2007). The results here suggest that the subspecies
Iberus g. gualtieranus may be threatened by the current climate change, but more detailed
studies are needed.
In conclusion, Iberus g. gualtieranus in SE Spain has a circannual activity relegated to
autumn and winter, mediated mainly by moisture, while its circadian activity is restricted
primarily to the night. Considering the factors determining activity in this snail, it may be
predicted that climate change is threatening its survival.
ACKNOWLEDGEMENTS
I am in debt to David Díaz Fernández, Pablo Cabrera Coronas, Adela González Megías
and, especially, the late Amelia Ocaña, for their collaboration in this study. David Nesbitt
improved the English.
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