/. Moll Stud. (1998), 64, 297-308
© The Malacological Society of London 1998
ASPECTS OF LIFE CYCLE, POPULATION DYNAMICS,
GROWTH AND SECONDARY PRODUCTION OF THE
PULMONATE SNAIL CEPAEA
VINDOBONENSIS
(FERUSSAC, 1821) IN NORTHERN GREECE
ALEXANDRA E. STAIKOU
Department of Zoology, School of Biology, Aristotle University of Thessaloniki, 540 06 Thessaloniki, Greece
(Received 7 April 1997; accepted 2 October 1997)
ABSTRACT
The life cycle, population dynamics, growth and secondary production of the land snail C. vindobonensis
were studied in northern Greece. Demographic
analysis of the populations of C. vindobonensis
revealed that a) three cohorts were present in the
field throughout the year, b) the reproductive period
started in late April-May and the newly hatched
snails appeared in the beginning of June, and c)
increased growth rates were observed during spring
and early summer, but also during autumn for the
newly hatched snails.
According to von Bertalanffy's method C. vindobonensis needs 7 years to attain its maximum size
measured in the field. Mortality rate is very high
during the first year of life, while life expectancy is
higher during the second year of life and decreases
afterwards. Net reproductive rate (R,,) was equal to
3.1 and the finite capacity for increase (antilogy,.)
was equal to 1.
Estimated annual secondary production with
Hynes' frequency method revealed a mean standing
crop (B) of 0.99 g/m2/year and a production (P) of
1.3 ± 0.11 g/m2/year. Annual turnover ratio (P/B)
was equal to 1.31.
INTRODUCTION
The genus Cepaea comprises four living European species. Two of them C. nemoralis (L.)
and C. hortensis (MUller) which have a western
oceanic distribution, have been widely studied
as they are considered excellent material for
research into the interactions of ecology and
genetics, because of their extensive shell polymorphisms. Numerous studies exist concerning
their biology and ecology (Wolda 1967, 1970a,
b, 1972; Wolda & Kreulen 1973; Greenwood
1974; Williamson, Cameron & Carter, 1976,
1977; Williamson, 1976, 1979; Jaremovits &
Rollo, 1979; Cook & Cain, 1980; Cain & Cook,
1989) or their ecological genetics, (Cain &
Currey, 1963, 1968; Jones, 1973a; Richardson,
1974; Jones, Leith & Rawlings, 1977; Jones,
1982; Ochman, Jones & Selander, 1983;
Lamotte, 1988; Guiller & Madec, 1991).
The other two species, C, vindobonensis
and C. sylvatica (Draparnaud) have a central
to eastern, more continental distribution and
are characterised by a simpler shell polymorphism. C. vindobonensis is the only species of
the genus that occurs in Greece. Its presence
has been reported in several regions of the
northern part of the mainland of Greece, that is
in Macedonia, Thrace and Thessaly (Frank,
1988). Only a few studies exist concerning
biology, ecology or ecological genetics of this
species (Jones, 1973b, 1974; Jones & Parkin,
1977; Sacchi, 1984,1985; Gill & Cain, 1986).
In this study are reported results from a
study on ecology, population dynamics, growth
and secondary production of C. vindobonensis
from Logos region of Edessa (Greece) where
it coexists with four other pulmonate snails
namely Helix lucorum, Bradybaena fruticum,
Monacha cartusiana and Xeropicta arenosa,
whose biology and ecology have already been
studied (Staikou A, M. Lazaridou-Dimitriadou
& N. Farmakis, 1988; Staikou A, M. LazaridouDimitriadou & E. Pana, 1990; Staikou &
Lazaridou-Dimitriadou, 1990, 1991). Preliminary results on the population dynamics of this
species in the same region showed that the
population was randomly distributed and that
the population density was bout 3.12 snails/m2.
Also it was found that the reproductive period
occured at the end of spring and that growth
followed the typical pattern for snails in
Greece (Staikou, 1994).
ALEXANDRA E. STAIKOU
298
STUDY AREA
The habitat of C. vindobonensis was situated in
Logos region of Edessa which lies about 100 km.
north-west of Thessaloniki. The study area was
fenced off from 1982 until 1986 when the study on
the ecology and biology of the edible snail Helix
lucorum which was carried out in the same region,
had made it necessary to prevent local people coDecting Helix lucorum. Fencing was destroyed after 1986
when the study on H. lucorum was completed. A full
description of the study area and the main characteristics of the coexisting snail species have been given
in a previous paper (Staikou et al. 1988). The vegetation was not uniform but had patches where different
plant species dominated. The climate of the region
was of the humid mediterranean type, characterised
by prolonged rainy periods in mid-summer (Fig. 1).
Furthermore the fact that the habitat where the study
was carried out was situated under the waterfalls of
the city of Edessa, and that the area was crossed by
small streams, resulted in a microclimate characterised by increased air relative humidity and soil
moisture even during summer months.
METHODS AND MATERIALS
The study of C. vindobonensis started in June 1991
and lasted three years. Data from June 1991 to July
1994 were used for the demographic analysis of the
populations.
Samples were taken randomly every 15 days
throughout the year. The quadrat sample-size used
(50 x 50 cm) was determined by Healy's method (in
Cancela da Fonseca, 1965). Elliot's method (1971)
was used to determine the necessary total number of
sampling units for a sampling error less than 20%.
100-
Sampling was carried out during morning hours in
the absence of rain. All snails found in a quadrat
were collected, measured and then returned to their
initial places. The largest diameter of the shell (D),
the height (H) of the shell and the peristome
diameter (d) were measured. Also the number of
snails of the other three coexisting species as well as
the plants present in each sampling unit were
recorded.
Spatial distribution of the snails in the habitat was
examined by using Taylor's power law (1961). The
parameter b from Taylor's equation s2 = axb (where
a = constant, s2 = variance, x = mean number of
snails found in a sample unit) was used as an index
of dispersion. Parameter b is fairly constant and
characterizes a species (Southwood, 1966); it is independent of the total number of samples and the total
number of animals in the samples; it depends only on
the quadrat size (Elliot, 1971).
The class interval of the monthly size frequency
histograms was 3 mm and it was determined by
Goulden's method (in Cancela da Fonseca, 1965).
The largest diameter of the shell (D) was used for the
construction of the histograms since it is generally
accepted as the most reliable morphometric parameter (Lazaridou-Dimitriadou, 1978; Charrier &
Daguzan, 1978; Daguzan, 1982).
The cohorts were discriminated using probability
paper, (Harding, 1949). This method was valid
because the modes of the age classes were separated
by at least 2.5 standard deviations (Grant, 1989),
except in September 1992 and June 1993. Although
many age classes had less than 50 individuals, the
modal values are consistent from month to month
(Fig. 3), which confirms that the modes are real and
not the result of sampling variation. This method has
been used for demographic analyses of the populations of other molluscs (Hughes, 1970; Leveque,
200
9080706050403020100
Months
Figure 1. Climatogram showing mean monthly temperatures (—) and total monthly precipitation (
) in the
region of Edessa (N. Greece) for the period 1990-1994 (shaded areas represent arid periods of the year).
ASPECTS OF LIFE CYCLE OF CEPAEA VINDOBONENSIS
1972; Daguzan, 1975; Lazaridou-Dimitriadou, 1978,
1981; Lazaridou-Dimitriadou & Kattoulas, 1985;
Staikou era/., 1988).
An age-specific life table was constructed based on
the fate of a real cohort which entered the population
in 1991 (fig. 2). The methodology for the construction
of the life table is described in detail by Staikou et al.,
(1988). The total number of snails hatched in 1991
were determined knowing a) the number of adults
present in spring 1991 from demographic analysis
of the populations b) the survival rate of adults of
different ages and c) the number of eggs layed by
adults during the first and second year they lay eggs.
The number of snails of the 1991 cohort in the
following years, was extrapolated from the results of
the demographic analyses of the populations of C.
vindobonensis.
For the study of absolute growth, data from the
modal distribution of C. vindobonensis were used
(e.g. the growth of one age class was followed by the
growth of the same age class the following month
taking into consideration that time intervals always
had to be equal). For the determination of the theoretical growth curve, Von Bertalanffy's equation
(1933, 1938) was employed: D, = D ^ [l-e^'-""],
Where D, = the largest shell diameter at age t, D ^ ,
= the asymptotical maximum possible largest shell
diameter, k = growth rate coefficient, t = time in
months, and t<, = hypothetical time when D is equal
to 'zero'.
The coefficient k and D m were determined
according to Walford's (1946) method. D m is the
intersection point of the growth curve D,+, = f(D,)
and the line drawn at 45° through the zero point. The
coefficient k is equal to -loga- 2.30259 (where a =
the slope of Walford's line). For the determination of
the date of birth of an age class on the time axis, it is
possible to use a secondary origin (t' = 0) corresponding to the smallest snails measured in the field
during the study period of a species (in my case it was
D = 2.80 mm), assuming that all the small snails of
this species have been captured with the same size
and that all age classes follow the same growth laws.
Consequently, it is possible to draw the theoretical
growth curve of largest shell diameter in relation to
time from the first capture D,' = D ^ [1 -e-*"'-" 0 ']. If
the largest shell diameter at the moment of birth is
known from laboratory data (in my case it was D =
2.40 mm), the axes may be changed taking as origin
birth, and the life span of the studied species until
D m can be estimated.
Growth of the largest shell diameter (D) relative
to the peristome diameter (d), was calculated for the
whole population, and also for juveniles and adults
separately. A t-test was applied to test for differences
between the slopes of growth regression lines of
different age groups (Zar, 1984).
To determine annual production, the snails were
grouped into ten size classes. The mean number of
snals (n) in each size class was determined using data
from the population dynamics. To determine dry
body and shell weight, 61 snails representing all size
classes were used following the methodology de-
299
scribed in detail in Staikou et al. (1988). Annual
production in 1993 was calculated by the Hynes size
frequency method modified according to Benke
(1979) and Krueger & Martin (1980). This has the
advantage that single cohorts within the data need
not be identified to calculate production, although it
may produce an overestimate (Waters & Crawford,
1973). The formulae used were given by Staikou et al.
(1988).
RESULTS
Aspects of the biology o/Cepaea vindobonensis
Snails became adults after their second year
of life but they lay eggs only the third year.
The largest part of juveniles (70-90%) were
sexually mature when the largest shell diameter (D) exceeds 21 mm. Adult size though was
largely dependent on the growth of secondyear juveniles during the spring months before
maturation. In spring 1992 the climate was very
favorable for snail activity (Fig. 1) and a large
proportion of second-year juveniles (30%)
reached a diameter of 23 mm before maturation. Adults had a mean greater shell diameter
(D) of 23.859 ± 1.233 mm (range: 21.00-27.80
mm, N = 1311) and a mean shell height (H) of
19.% ± 1.271 mm (range: 16.60-23.40 mm, N =
1311). The ratio H/D.100 (mean index of shell
depression) had a value of 83.68 ± 3.682, N =
1311. Sexual maturity was indicated externally
by the thickening of the peristome edge which
covered the small umbilicus. Thickening of the
peristome edge started after the second year
of life and was completed by the third year.
Examination of the external features of the
shell and the genitalia of 15 snails (20.00 mm
< D < 24.00 mm) showed that genitalia were
fully formed only when the peristome edge
was thickened. Gonad maturation was histologically checked in 31 snails collected in late
spring, summer and autumn 1992 with 9.00 mm
< D < 22.00 mm. Examination of the genitalia
was also performed on the same snails. It was
found that differentation of oocytes started
when D reached 10 mm but grown oocytes
were present in the gonad when D exceeded
14.00 mm. Also from the examination of the
genitalia it was found that at a size of about
14-15 mm the digitiform glands appeared as a
small swelling while they started branching at a
size of about 17.50 mm.
The reproductive period started in the
middle of April when the first pairings were
noticed and lasted two months. The peak of
egglayings was noticed in mid May. Between
300
ALEXANDRA E. STAIKOU
29-67 eggs were laid with a mean of 49,23 ±
12.54 (N = 13); the mean egg diameter was
3.169 ± 0.18 mm (N = 522). Ten clutches
with a mean of 45.6 ± 14.43 eggs were transfered to the laboratory and their weight and
size was determined. Mean live weight per
clutch was 0.887 ± 0.415 g. and mean dry
weight 0.205 ± 0.069 g. Mean live weight per
egg was 18.75 ± 3.54 mg and mean dry weight
4.49 ± 0.42 mg. No correlation corelation was
found between number of eggs/clutch and egg
live weight (r = 0.697 P = 0.05). Hatching took
place about 18 days after egglaying. Newly
hatched snails in the laboratory measured 3.28
± 0.15 mm and the smallest diameter measured
was 2.40 mm.
During summer the snails did not aestivate;
they were active during the night and they were
observed at resting sites attached to leaves or
stems of tall plants during the day. Hibernation
started at the'end of October to the beginning
of November. Adult snails entered hibernation
earlier then juveniles. Exit from hibernation
took place at the beginning of March.
Shell polymorphism of Cepaea vindobonensis
Cepaea vindobonensis is characterised by a
simple shell polymorphism. There is no variation in shell color, and banding polymorphism
is restricted to minor variations in band number, in the degree of band fusion and in the
intensity of band pigmentation (Jones, 1973b).
In the most distinct morph the band pigmentation is reduced and the bands have a light
straw color (faint-banded morph). In the population studied in Edessa the faint-banded
morph was very rare. During the four years of
the present study only two snails of this morph
were detected. The dominant morph had five
dark-brown bands (12345) and comprised 83%
of the population (N = 168). The rest of the
population comprised of five-banded snails
with the second or the third band lightly pigmented (12% and 1.2% respectively) or with
the second band absent (2.4%) or finally with
the second and third bands fused (1.2%).
Population density of Cepaea vindobonensis
Population density in the field fluctuated during the study period (Table 1). The mean population density during the period 6/1991-8/1994
was 2.80 ± 0.67 snails/m2. Statistically important rises in density appeared in April 1992 and
1993 and August 1993. A decline in density
appeared in August 1992 and July 1993.
Spatial distribution of Cepaea vindobonensis
The spatial distribution of C. vindobonensis
was found to be regular, since parameter b of
Taylor's power law was equal to 0.728 (s2 =
0.0373 ™
Demographic analysis of the populations of
Cepaea vindobonensis
The analysis of size frequency histograms (Fig.
2) with probability paper showed the following
Table 1. Cepaea vindobonensis population density in the study area from June 1991 to August 1994.
Where 5<: number of snails/m 2 ; N: number of samples
Date
6/91 7/91 8/91
9/91
10/91 11/91 3/92
4/92
5/92
7/92
8/92
8/92
9/92 10/92
Std. error
3.07 2.77 3.18
0.75 0.75 0.68
2.74
0.69
2.65
0.69
2.11
0.71
2.03
0.58
3.98
0.94
3.43
0.88
3.38
0.97
4.04
1.06
2.53
0.84
2.71
0.81
2.41
1.05
Sampling
error %
73
70
71
70
70
70
78
70
72
70
70
70
70
70
13
14
11
13
14
18
15
12
13
15
14
17
16
23
Date
3/93 4/93 5/93
6/93
7/93
8/93
9/93 10/93 3/94
4/94
5/94
6/94
7/94
8/94
Std. error
2.81 4.07 3.62
1.45 0.98 0.73
3.53
0.06
2.38 3.41
0.74 1.01
2.74 2.18 1.87
0.78 0.85 0.71
2.11
0.61
2.20
0.84
2.38 2.32
0.75 0.82
1.82
0.88
N
70
70
70
70
70
70
70
60
70
71
70
70
70
70
Sampling
error %
27
13
10
16
16
16
15
20
20
15
20
17
17
25
ASPECTS OF LIFE CYCLE OF CEPAEA VINDOBONENS1S
301
Figure 2. Size-frequency histograms of the populations of Cepaea vindobonensis at the study area from June
1991 to August 1994. The last three columns contain adult snails (D > 21 mm) of different cohorts.
(Fig. 3): a) Three cohorts were present in the
field throughout the year (taking into consideration that adults of all ages belong to the same
cohort), b) The reproductive period started in
late April-May and the newly hatched snails
appeared in the beginning of June, c) Increased
growth rates were observed during spring and
early June, especially in spring 1992, but also
302
30 -
ALEXANDRA E. STAIKOU
D mm
20 -
10 -
J J A S O N D J
1991
F M A M J J A S O N D J
F M A M J
1992
J A S O N D J
1993
F M A M J
J A
1994
Months
Figure 3. Modal distributions of Cepaea vindobonensis populations at the study area from June 199 to August
1994 (arrows show the beginning of the reproductive period).
during autumn for the newly hatched snails,
d) The largest diameter of the shell reached a
mean value of 17.00 mm one year after hatching. Snails became adults two years after hatching when their largest shell diameter exceeded
21 mm.
- Mortality rate increases from the second
year of life and on.
- Life expectancy (ex) is higher during the second year of life and decreases afterwards.
- The value of net reproductive rate (R<,, is
greater than one.
- The finite capacity for increase (antilog,.rc) is
low.
Life and fertility table
From observations of marked snails in the field
and after following ten pairs of adult snails in
the laboratory for three years it was found that
(a) the mean number of eggs laid per snail in
the first year they lay eggs is 49.231 + 12.54,
and in the second year 17.33 ± 3.79; (b) 25% of
adult snails do not lay eggs the first year, (c)
there is a 60% hatching success in the field; (d)
there was 33% mortality just after hatching;
and (e) there is 72% survival of adults between
the first year and the second year of adulthood,
a 70% survival between the second and the
third year, and a 50% survival between the
third and fourth year.
The cohort studied in the life table was
followed till the maturation and subsequent
death of most of the adults.
From the life table (Table 2) the following
conclusions may be drawn:
- Mortality rate (k%) is very high during the
first year of life.
Absolute growth
Dmax, which represents the intersection point
of Walford's equationn D, + , = 0.939D, + 1.872
with the diagonal D, = D, + 1? was found equal
to 30.69 mm. The time interval between D, and
D t + i was always equal to 28-30 days. By the
slope of Walford's equation (a = 0.939) which
shows the growth rate of the snails the coefficient k was calculated and it was found equal to
0.063.
Knowing the diameter of the smallest snails
measured in the field during the study period
(2.80 mm), and the smallest diameter of newly
hatched snails in the laboratory (2.40 mm) it
was possible to calculate the theoretical growth
curve of D in relation to age for C. vindobonensis: D, = 30.69 [1-e- 0 0 6 * 1 + 1 - M25) ]. From this
curve (Fig. 4) it was calculated that C. vindobonensis may live up to 7 years to reach its
possible maximum size according to Walford's
equation.
ASPECTS OF LIFE CYCLE OF CEPAEA VINDOBONENSIS
303
Table 2. Life and fertility table of a cohort of Cepaea vindobonensis starting in 1991 [x, age in years; a,
= numbers of animals surviving at the beginning of age class x; I, = number of animals surviving at
the beginning of age class x if a thousand were originally hatched; d, = number of animals dying during age interval x; q x = dj\, mortality rate during age interval x; L« = (I, + lx + ,)/2 number of animals
alive between age x and x + 1;Tx = Lx + Lx + ,
Lw = total number of animal x age units beyond the
age x; ex = TVI, expectation of life; l' x = number of animals alive during age interval x as a fraction
of an initial population of one (in parenthesis is noted the number of animals that laid eggs); m x =
number of living animals hatched per adult animal; V, = total number of hatchings in each age
interval; Ro = net reproductive rate; rc = capacity for increase; Tc = generation time in days; antilog.r c
= the finite capacity for increase].
Age
a.
lx
dx
q«
logax
U
Tx
ex
I',
0
1
2
3
4
5
6
7
2962
503
423
304
213
107
32
3
1000
170
143
103
72
36
11
1
830
27
40
31
36
25
10
-
0.83
0.16
0.28
0.30
0.50
0.69
0.91
-
3.47
2.70
2.63
2.48
2.33
2.03
1.51
0.48
585
157
123
88
54
24
6
0.5
1037.5
452.5
295.5
172.5
84.5
30.5
6.5
0.5
1.04
2.66
2.07
1.67
1.17
0.85
0.59
0.50
1
0.27
0.21
0.15
0.09
0.04
0.01
0.0009
mx
_
(0.11)
(0.023)
(0.067)
-
_
19.8
19.8
6.9
-
_
2.18
0.46
0.46
—
Ro = 2VX = 2l' x m« = 3.1
rc = InR^/Tc = 1.1298/1095 = 0.001
antilog.r c = 1.00
D t =30.69[l-e 0 0 6 3 ( t + 1 - 2 9 2 5 >]
40-1
30-
E
£
20-
10-
0
12
24
36
48
60
72
84
Time (months)
Figure 4. Theoretical growth curve of Cepaea vindobonensis.
Relative growth
The study of relative growth of largest shell
diameter D relative to peristome diameter d
for the whole population of C. vindobonensis
(Fig. 5) showed that there was a positive
correlation between D and d (logd =
0.8751ogD - 0.085 r2 = 0.976 N = 2111).
Regression equations were calculated for juveniles and adults separately. For juveniles
regression equation was found to be: logd =
0.979 logD - 0.173 r2 = 0.994 N = 800 while
304
ALEXANDRA E. STAIKOU
1.4n
1.2
1
1^
oec .6
O logD<21
D logD>21
.4
.6
.8
I
1.2
1.4
1.6
logD(mm)
Figure 5. Relative growth of the peristome diameter (d mm) in relation to the largest shell diameter (D mm) in
Cepaea vindobonensis.
for adults regression equation was logd = 0.785
logD + 0.023 r2 = 0.636 N = 1311. The slopes
of the two lines were compared with t-test and
they were found to differ significantly (t = 9.49
P < 0.001). The two lines were found to intersect at D = 9.77 mm which was the diameter at
which oocytes start differentiating according to
histological examination of gonads. Knowing
that the digitiform glands appear when the
largest shell diameter reached a size of about
14-15 mm, it was decided to examine whether
relative growth was the same in the two size
groups, that is those with D < 14 mm and those
with D > 14 mm. A statistical difference was
found between the slopes of the two regression
lines derived (t = 45.86 P < 0.01). The two
lines were found to intersect at D = 18.62 mm
that is at a size after the digitiform gland
started branching.
Secondary production
The computed estimates of annual production
according to the Hynes' size frequency method
are listed in Table 3. The mean biomass of each
size class is expressed in dry weight on the basis
of the following relationship between dry body
weight (Wb:) and D and dry shell weight (Ws:)
andD:
Log Wb = 3.051 Log D - 4.669 (r2 = 0.971;
N =61)
Log Ws = 3.414Log D - 4.895 (r2 = 0.967; N
= 61)
The shell organic matter of adult snails (D =
23.50 ± 1.88 mm) was 1.92% (N = 31 snails);
that of immature snails (D = 13.80 ± 1.39 mm)
was 3.23% (N = 30 snails). These values do not
differ statistically, therefore a common mean
value of 2.35 ± 0.901% (N = 61) was used to
determine the organic matter of the shell of all
the snails used.
Applying Benke's correction, values of n
(mean annual density), B (mean annual crop)
and P (annual production) for 1993 were calculated to be 4.12 snails/m2, 0.994 g/m2/year and
1.3 ± 0.11 g/m2/year respectively. The annual
turnover ratio P/B was 1.31.
DISCUSSION
The life cycle of C. vindobonensis in the locality studied in Northern Greece showed similarities with previously reported life-cycle
ASPECTS OF LIFE CYCLE OF CEPAEA VINDOBONENSIS
305
Table 3. Calculation of production of Cepaea vindobonensis by the size-frequency method. Annual
production based on 12 sets of data from March 1993 to February 1994 (where Fl| = mean number
of snails at the size class j ; U = variance of n^ W = mean individual body dry weight + mean dry
shell organic matter; G = geometric mean; B = mean standing crop or population biomass; P =
annual production; P/B = annual turnover ratio; a = number of size classes; CPI = cohort production
interval).
Size
class
0-3
3-6
6-9
9-12
12-15
15-18
18-21
21-24
24-27
27-30
rym
0.02
0.35
0.43
0.29
0.16
0.15
0.34
1.18
1.14
0.06
2
Un,
0.00010
0.00240
0.00850
0.00700
0.00200
0.00100
0.00270
0.00530
0.00680
0.00004
n-,-n-j + 1 /m
-0.33
-0.07
0.13
0.13
0.01
-0.20
-0.83
0.04
1.08
0.06
2
W, (mg)
0.075
2.159
10.309
28.884
62.370
115.347
192.472
298.465
438.102
616.207
4.12
G,
( w r w i + ,)0.5
0.40
4.72
17.26
42.44
84.82
149.00
239.68
361.60
519.58
616.21
B
[nym2-W,(mg)]
(mg/m2)
P'
(n",-nl+1) (Gj)
(mg/m 2 )
0.0015
0.7622
4.3829
8.4523
9.8327
16.7317
66.3781
351.9914
499.1280
36.3583
-0.1341
-0.3403
2.2868
5.7290
1.0684
-29.7726
-200.0045
14.4797
561.2966
36.3583
994.0193
390.9673
P = a • P' • 365/CPI = 10 • 390.9673 • 365/1095 = 1303.2243 mg/m2 = 1.3 g/m2/year
U(P) = Un, (Gi-G,.,) 2 • (365/CPI)2 • a2 = 28688.6230 • (365/1095)2 = 3187.6248
Conf. limits of P= P ± 2[U(P)°-6] = P ± 2(56.46) = P ± 112.918 mg/m2 = 1.3 ± 0.112 g/m2/year
P/B = 1.3/0.99 = 1.31
characteristics of the same species in Italy
(Sacchi, 1985) but also with the life-cycle characteristics of C. nemoralis and C. hortensis
from European populations.
Mean adult size (D and H), as well as mean
adult shell depression index (H/D.100), were
similar to the ones reported by Sacchi (1985)
for one population of C. vindobonensis living
in a black Pine forest in north-east Italy. Furthermore variation in adult size and shell
depression index was remarkable among
neighbouring Italian populations and this
variation was explained as caused by habitat
differences or sympatry with populations of
C. nemoralis. In Italian populations, largest
diameter of the shell in adult snails ranged
from 20.5 mm to 26.30 mm and shell height
from 16.50 mm to 22.30 mm. In the Greek
population studied, the upper limits of the
range of both shell diameter and height were
greater, denoting that the snails reached a
larger adult size, probably due to favourable
conditions for the growth of subadult snails
which prevail in the region of Edessa.
A time interval of two years between hatching and lip formation, found for the Edessa
population has previously been reported for
populations of C. nemoralis in Britain (Cain &
Currey, 1968) and Netherlands (Wolda, 1970a,
b). On the other hand for other British popu-
lations of C. nemoralis and for C. hortensis a
three-year interval between hatching and lip
formation has been reported (Williamson,
1976).
The duration of the reproductive period of
the population of C. vindobonensis was limited
in relation to reproductive period of C.
nemoralis in Netherlands (Wolda, 1972) which
lasted from May until the beginning of August.
This difference can be attributed to difference
in latitude and climate between North and
South Europe.
Also the fact that no correlation was found
between clutch size and egg size is in accordance with Wolda's (1970b) results for C.
nemoralis.
The rise in population density observed in
April 1993 and 1994 may be due to snails
emigrating from nearby farms when disturbed,
because during March and April fanners are
preparing their fields for cultivation. The
decrease in density observed during August 92
and June-July 93 may be due to unfavorable
dry conditions that prevailed by that time of
the year (Fig. 1).
Regular distribution of snails has never
before been reported for any snail species in
Greece. For the same species during preliminary elaboration of the first year's data a
random distribution was found but this was
306
ALEXANDRA E. STAIKOU
probably due to very few data being used
(Staikou, 1994). In the same habitat spatial distribution of the other coexisting snail species
was either random (Helix lucorum, Staikou et
al., 1988) or contagious (Bradybaena fruticum,
Monacha cartusiana, Staikou et al., 1990;
Staikou & Lazaridou-Dimitriadou, 1990). It
seems that coexisting snail species in Logos
region show different patterns of spatial distribution either because of competition among
them or because their distributions reflect each
species' - preferences in a non-homogenous
habitat. No significant aggregation has also
been reported for adult C. nemoralis in Britain
while for juveniles of the same species a contagious distribution was found (Williamson et al.,
1977).
Increased growth during spring and early
summer (March-June) when the weather is
most favorable for snails' activity (Fig. 1), has
also been reported for the other coexisting snail
species in the Logos region of Edessa (Staikou
et al., 1988; Staikou et al., 1990; Staikou &
Lazaridou-Dimitriadou, 1990) as well as for
other snail species in Greece, namely Xeropicta
arenosa, Cernuella virgata and Eobania
vermiculata (Lazaridou-Dimitriadou, 1981;
Lazaridou-Dimitriadou & Kattoulas, 1985).
Also variation of growth between years as
shown for second year juveniles among spring
92 and spring 93, 94 confirms the high dependance of snails' growth on weather conditions.
The fact that internal Changes in genitalia
and maturation of the gonad of this species
correspond to external morphometric changes
of the shell is in agreement with the results
reported for othr helicids such as E. vermiculata, H. aspersa (Lazaridou-Dimitriadou &
Kattoulas, 1981), Cernuella virgata and Xeropicta arenosa (Lazaridou-Dimitriadou, 1986)
and H. lucorum (Staikou et al., 1988) in Greece
and elsewhere as in H. aspersa (Charrier &
Daguzan, 1978) and other helicidae (YomTov, 1971; Bonavita, 1972; Williamson, 1976).
The life span of C. vindobonensis as it was
calculated from Von Bertalanffy's equation is
similar to the life span of C. nemoralis reported
by Lamotte (1951).
The value of the annual turnover ratio P/B,
seems to be related to the life span of the
species (Russell-Hunter & Buckley, 1983).
According to Lamotte & Stern (1987), the
shorter the life span of the species, the higher
the turnover ratio must be. In the Logos region
of Edessa among the snail species studied, H.
lucorum (Staikou et al., 1988) and C. vindobonensis showed the lowest values of P/B as well
as the longest life span (14 and 7 years respectively). B. fruticum and M. cartusiana (Staikou
et al., 1990; Staikou & Lazaridou-Dimitriadou,
1990) with intermediate life span (5 and 3 years
respectively) showed higher values of P/B (2.37
and 2.11 respectively), while X. arenosa
(Staikou & Lazaridou-Dimitriadou, 1991) with
an annual life span had the highest value of P/B
(3.51).
ACKNOWLEDGEMENTS
Thanks are due to Dr. Michaelidis and to A. Gogas
for their help during field work, to Prof. M. Lazaridou-Dimitriadou for reading the manuscript and
providing helpful comments, and to the staff of the
Hydroelectric station of Agras for providing climatic
data.
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