J. Moll. Stud. (1999), 65, 241–250
© The Malacological Society of London 1999
A QUA NTITATIVE STU DY OF ENVIR ONMENTAL F ACTOR S
INFLUEN CIN G THE SEASONAL ONSE T O F REPRO DUCTIV E
BE HAV IOU R IN T HE SOUTH AM ER ICAN APPLE-SNAIL
POMACEA CANAL IC UL ATA ( G A S T R O P O D A :
AMPULLARIIDAE)
~
E.A. AL BREC HT, N .B. CAR RENO and A . CASTRO -VA ZQUEZ
Laboratorio de Reproducción y Lactancia (LARLAC), Consejo Nacional de Investigaciones Cientificas y
Técnicas (CONICET), and Cátedra de Fisiología Normal, Universidad Nacional de Cuyo, Casilla de Correo
855, RA-5500 Mendoza, Argentina
(Received 26 January 1998; accepted 23 July 1998)
ABSTRACT
INTRODUCTION
Two laboratory experiments were carried out on sets
of winter (inactive) pairs of Pomacea canaliculata
(Lamarck), one to study the influence of water
temperature (either 18°C or 25°C, corresponding to
late winter and summer temperatures in Mendoza,
respectively) and day length (either 10 h light/day or
14 h light/day, corresponding to the shortest and the
longest day of the year in Mendoza, respectively),
and the other to determine the effects of food availability (ad libitum feeding vs. restriction to 25% of
the ad libitum requirements) on the frequency of
copulation and spawning, on spawn measurements
(number and volume of eggs, and spawn volume),
and on an index of the reproductive effort. Animals
in all experimental groups were observed during 4
weeks, after seven days of acclimation to the experimental aquaria (under low water temperature and
short photoperiod). Low water temperatures induced
a lesser and later activation of copulation and spawning than that induced by warm temperatures. Day
length was without effect on both behaviours. Both
the frequency of spawning and the reproductive
effort index decreased under restricted feeding, without altering the spawn measurements. The latency
periods for both copulatory and spawning behaviours
were similar in both ad libitum and restrictedly fed
groups. We conclude that water temperature may be
the critical factor that causes the seasonal onset of
copulatory and spawning behaviour in these applesnails. However, food availability may also be a determining factor, mainly to trigger spawning activity.
These results suggest that the factors that control the
seasonal onset of the sexual activities in Pomacea
canaliculata are similar to those that control the
seasonal arrest, thereby establishing the annual pattern of reproduction in this species.
The
apple-snail
Pomacea
canaliculata
(Lamarck, 1822) is widely distributed in lentic
habitats throughout the Amazon Inferior Basin
and the Plata Basin (Ihering, 1919; Hylton
Scott, 1957; Pain, 1972; Castellanos & Fernandez, 1976). The threat to rice crops posed by
the anthropogenic spread of this Neotropical
ampullariid snail to Asia (Hirai, 1988; Halwart,
1994; Cowie, 1998), as well as its role as a
potential control agent of aquatic weeds
(Cazzaniga, 1981, 1983) and disease-bearing
snails (Milward de Andrade & Souza, 1979;
Cazzaniga, 1990b), have aroused an increasing
interest in its biology.
Reproduction of P. canaliculata occurs during
spring and summer (October to April, in southern South America) and the animals remain
inactive during the rest of the year (Bachmann,
1960; Andrews, 1964). In a recent paper
(Albrecht, Carreño & Castro-Vazquez, submitted) it was found that the decrease in water
temperature that occurs in autumn seems to be
the main factor for causing the seasonal arrest
of copulatory and spawning behaviours in this
species, while the seasonal shortening of the
photoperiod may be of little or no importance.
Shortage of food may also be a determining
factor although it may just reflect the decrease
in ambient temperature.
Also, it has been suggested that water temperature may be an important environmental
cue to stimulate emergence from winter dormancy in this species (Estebenet & Cazzaniga,
1992, 1993; Albrecht et al., submitted). The
present study was aimed to test this possibility,
and to determine whether other environmental
Send correspondence to: Eduardo A. Albrecht, LARLACCONICET,Casilla de Correo 855, RA-5500 Mendoza, ARGENTINA
FAX: 54 61 380232
Email: [email protected]
242
E.A. ALBRECHT, N.B. CARREÑO & A. CASTRO-VAZQUEZ
factors, such as food availability and daylength,
may also have some influence.
MATERIALS AND METHODS
General methods
Sexually mature individuals of P. canaliculata, of
both sexes, were collected in Palermo Park (Buenos
Aires, Argentina) during the spring and summer
seasons of 1993–1996 and maintained in outdoor
ponds until they were used for studies. Voucher
specimens of this population were deposited at the
collection of the Instituto Argentino de Investigaciones de Zonas Aridas, Mendoza, Argentina. The
observations were conducted in four 56 41 20 cm
aquaria, each of which was divided with a plastic
mesh into ten 11 20 20 cm compartments; water
level was kept 10 cm deep. Continuous water renewal
(about 9 times/day) was ensured by a constant input
flow of tap water. The aquaria were artificially illuminated with 15 W white fluorescent tubes located
22 cm above water.
Individuals used were inactive, obtained from the
external ponds during winter, and their shell lengths
ranged from 30 to 60 mm (sexual maturity is reached
at about 25 mm shell length in this species: Martín,
1986; Estebenet & Cazzaniga, 1992). A pair compris ing a male and a female was placed in each compartment (sex was determined externally by the shape of
the operculum, according to Cazzaniga, 1990a, and
confirmed at autopsy at the end of the experiments);
shell length of each individual was measured with a
Vernier caliper to the nearest 0.1 mm, at the beginning and at the end of the experiments.
Two experiments were carried out. One, to evaluate the effects of water temperature and day length,
and the other to evaluate the influence of either
ad libitum or restricted feeding. Replicate observations were run for both experiments, starting on July
2, 1994; July 25, 1995; August 27, 1996: since no differences were apparent between these sets of observations, they were pooled for presentation. Each
experiment was preceded by an acclimation period of
one week to the experimental aquaria, during which
the animals were maintained on a photoperiod of
10 hs of light-day (approximating that of the shortest
day of the year in Mendoza) and water temperature
was kept at 18 1.9°C (mean SD), i.e., a temperature in the upper limits of winter temperature. Fresh
lettuce was provided ad libitum during the acclimation period, although the animals ate very little under
these conditions. We also attempted to test a lower
temperature (16.4
1.1°C, mean
SD); however,
most animals (75% males and 100% females) died
when kept at this temperature in the experimental
aquaria.
After this initial period, the pairs were observed
for copulation and spawning during an experimental
period of four weeks. Observations were made at
7–9:30 AM, 1:30–2:30 PM and 6:30–9 PM (the middle
of the illuminated period of the day was fixed at
noon). A single episode of copulation was recorded if
copulatory activity was observed on either one or
more than one consecutive observations. An episode
of spawning was recorded whether a female was seen
during egg-laying or whether an egg mass was seen
already deposited (P. canaliculata females lay their
eggs above water level).
The frequency of active pairs (i.e., those that copulated at least once during the 4-week experimental
period), and the frequency of active females (i.e.,
those that produced at least one spawn during the
same period) were calculated for each group. The
latency of either copulation or spawning was defined
as the time elapsed since the beginning of the experimental period until an episode of either copulation
and spawning occurred. The frequency of either copulation or spawning was recorded as the cumulative
number of episodes of either behaviour during the
experimental period.
The egg masses were collected as soon as they were
observed and were dispersed in 2% sodium hydroxide, washed and air-dried thoroughly, and the eggs
were counted (Albrecht, Carreño & Castro-Vazquez,
1996). The mean egg volume (mm3) calculated on the
basis of the mean diameter of a ten eggs sample of
each spawn, was measured with a Vernier caliper to
the nearest 0.1 mm. The spawn volume was calculated as the product of the mean egg volume and the
number of eggs in the spawn. An index of reproductive effort (similar to that used by Calow, 1979) was
calculated as the quotient of the egg volume laid during the experimental period (number of eggs laid
times the mean egg volume in mm3) and the live mass
of the female (g). Only females who spawned at least
once were included in these calculations. Results
were expressed as the ‘weighted’ mean SEM (i.e.,
the mean value,
SEM, of the individual means of
females).
Experimental procedures
Experiment I: Effect of water temperature and day
length on copulation and spawning
High and low water temperatures, and long and short
day lengths, were combined to give several treatment
combinations, which were applied during 28 days
after the acclimation period. In all cases, the snails
were allotted at random to the different experimental groups. Water temperatures for the experimental
period were chosen in the upper limits of summer
(January) and winter (August) temperatures that
were recorded during 1993 in outdoor ponds in Mendoza. Day lengths were chosen to approximate either
the longest or the shortest day of the year in Mendoza.
A set of four groups was defined as follows: (1)
WL, warm water (24.9
1.2°C, mean
SD) and
long photoperiod (14 h light/10 h dark per day); (2)
WS, warm water and short photoperiod (10 h light/14
h dark); (3) CL, cold water (18.3
1.4°C, mean
SD) and long photoperiod; and (4) CS, cold water
and short photoperiod. The snails were fed with fresh
lettuce ad libitum throughout the experiment.
SEASONAL ONSET OF REPRODUCTION IN POMACEA
Ten males and ten females died during the experimental period. All of them had been exposed to the
lower water temperature (18°C); 5 males (5/18,
27.8%) and 4 females (4/18, 22.2%) pertained to the
group exposed to the short photoperiod, while 5
males (5/14, 35.7%), and 6 females (6/14, 42.8%) were
exposed to the long photoperiod. The recordings
of copulation and spawning obtained from these
animals and their partners were not included in the
Results.
The differences between frequencies of active and
inactive pairs/females were evaluated with the Fisher
Exact Test (P 0.05). The period of latency, and the
number of episodes of either copulation and spawning were submitted to Kruskal–Wallis Analysis of
Variance by Rank; the Mann–Whitney U Test was
used for subsequent individual comparisons. In this
and the following experiment, significance level was
fixed at P 0.05 (Zar, 1984).
Differences in growth (increase in shell length)
during the experimental period, in both males and
females exposed to either warm or cold water temperatures, were evaluated with a one-tailed Mann–
Whitney U Test (Siegel, 1956). A correlation coefficient of the increase in shell length during the
experimental period, and the frequencies of either
copulation or spawning of each individual, was calculated after a square root transformation of the data
(Zar, 1984).
Experiment II: Effect of food availability on
copulation and spawning
Fresh lettuce was used as food. After the acclimation
period, two experimental treatments were applied
during the following 28 days, during which warm
water temperature (24.2
0.8°C, mean
SD), and
long photoperiod (14 h light/10 h dark) were maintained: (1) Control—the snails were fed ad libitum
with fresh lettuce; (2) Restricted—they were offered
daily 42.5 g of fresh lettuce per kg live mass, i.e., 25%
of the amount eaten by active snails under summer
conditions (Albrecht et al., submitted).
The differences between frequencies of active
and inactive pairs/females were evaluated with the
Fisher Exact Test (P
0.05). Differences in the
period of latency, and in the number of episodes of
either copulation and spawning, as well as in the
spawn measurements (number and mean volume of
eggs, volume of the spawn) and the reproductive
effort index, were evaluated with the two-tailed
Student’s t Test for independent samples (Zar, 1984).
Differences in growth (increase in shell length)
during the experimental period, in both males and
females feeding on an ad libitum vs. a restricted diet,
were evaluated with the one-tailed Mann–Whitney U
Test (Siegel, 1956). A correlation coefficient of the
increase in shell length during the experimental
period, and the frequencies of either copulation or
spawning of each individual, was calculated after a
square root transformation of the data (Zar, 1984).
243
RESULTS
Experiment I: Effect of water temperature and
day length on copulation and spawning
Most pairs kept in warm water copulated at
least once during the experimental period (WL
group: 14/16; WS group: 16/16) while copulation was recorded just for 1 out of 9 pairs in the
CL group and for 7 out of 13 pairs in the CS
group. The frequency of copulating pairs in the
CL group differed significantly from that in the
WL and the WS groups, while the frequency in
the CS group differed significantly only from
that in the WS group (Fisher Exact Test; P
0.05).
The differences in the frequency of sexually
active females showed a pattern similar to that
of copulating pairs: 11 out of 16 females in the
WL group, and 13 out of 16 females in the WS
group, spawned at least once during the experimental period, while only a few females kept in
cold water did spawn (CL group: 1/8; CS group:
2/14). Both groups maintained in cold water
(CL and CS groups) differed significantly from
those in warm water (WL and WS groups;
Fisher Exact Test; P 0.05).
Figure 1 depicts the changes in other measurements of reproductive activity (i.e., the
latencies to first copulation and to first spawning, and the total number of episodes of either
copulation or spawning) as influenced by the
experimental conditions. The mean latency to
first copulation was about 13 days in the groups
exposed to warm temperature, while the groups
maintained in cold water showed mean latencies of about 21 and 27 days; the difference was
statistically significant (both WL and WS
groups vs. both CL and CS groups; Kruskal–
Wallis Test, H (3)
20.68, P
0.0001, and
Mann–Whitney U Test, P 0.05). The onset of
spawning activity occurred later than that of
copulation in all but one couple (this was a
single active pair in the CL group). The mean
latency to first spawning was about 17 days in
both groups maintained in warm water (WL
and WS groups), while it was about 25 and 27
days in those maintained in cold water (CL and
CS groups). The difference was statistically
significant (Kruskal–Wallis Test, H (3) 15.46,
P
0.0015, and Mann–Whitney U Test, P
0.05) between warm water (WL and WS) and
cold water groups (CL and CS).
Significantly fewer episodes of either copulation or spawning were recorded in the groups
exposed to cold water (as compared to those
exposed to warm water; Kruskal–Wallis Test,
244
E.A. ALBRECHT, N.B. CARREÑO & A. CASTRO-VAZQUEZ
Figure 1. Experiment I: Effects of combinations of either warm (W) or cold (C) water, and of either long (L) or
short (S) photoperiod. Speckled bars indicate the mean latency ( SEM, indicated by a T-shaped line sticking
up each bar) to first copulation (upper panel), and the mean latency to first spawning (lower panel). Empty
bars indicate the mean ( SEM) number of episodes for both behaviours. Number of cases is shown in brackets. Asterisks indicate statistically significant differences (P
0.05) from both warm water groups (WL and
WS); other differences were not significant.
and Mann–Whitney U Test, P
0.05), both
under a long and a short photoperiod. However, animals that were active in cold water
showed activity levels (for both copulation and
spawning) that were within the range observed
in the groups maintained in warm water. All
females that showed spawning activity also
showed copulatory activity; however, some
females that copulated during the experimental period (5 on CS; and 3 each on WS and
WL) did not spawn.
Both males and females exposed to warm
SEASONAL ONSET OF REPRODUCTION IN POMACEA
water temperature increased their shell length
during the experimental period (males: 3.0
0.5 mm, n 32, and females: 3.8 0.5 mm, n
30; mean
SEM), while those maintained in
cold water showed either low or no growth
(males: 0 mm, n
14, and females: 0.3
0.2
mm, n 12; mean SEM). These differences
(warm temperature vs. cold temperature) were
statistically significant (Mann–Whitney U Test,
P 0.001). No correlation was found between
the number of copulatory episodes observed
and the growth of the animals exposed to warm
water (under both short or long photoperiod),
but a significantly negative correlation was observed between the number of spawning episodes and growth in the group maintained in
conditions of warm water (both with long and
short photoperiod; r
0.6, P
0.0005,
n 30; Fig. 2).
Since only a few females exposed to cold
water produced eggs during the experimental
period, it was not possible to evaluate statisti-
245
cally the effect of cold vs. warm temperature on
the spawn measurements; however, the spawn
measurements obtained from cold water animals were within the range found in warm
water individuals (Table 1).
Besides that, two pairs of females that were
mistakenly put together in the same compartment (one of them exposed to WS, and the
other to CL), laid three and two spawns, respectively. The measurements of those spawns were
within the normal range.
Experiment II: Effect of food availability on
copulation and spawning
No significant differences (Fisher Exact Test;
P
0.05) between ad libitum and restricted
feeding were found in the frequencies of copulating pairs (control: 14/16 vs. restricted: 10/17)
and of spawning females (control: 11/16 vs.
restricted: 9/17), nor in the latencies of copulatory and spawning activities (Student’s t Test,
Figure 2. Experiment I: Correlation between the number of spawns per female and the increase of shell length
(mm) in individuals maintained in warm water (and either short or long photoperiods). The correlation coefficient was calculated after a square root transformation of f(x), x Õ (x 0.5)1/2.
246
E.A. ALBRECHT, N.B. CARREÑO & A. CASTRO-VAZQUEZ
Table 1. Spawn measurements and reproductive effort index in females maintained under combinations of either warm (W) or cold (C) water, and of either long (L) or short (S) photoperiod (Experiment I)
WL
11 females
61 spawns
Number of eggs
per spawn
Mean egg
volume (mm3)
164.7
(104.6
8.4
(6.7
14.3
239.3)
0.2
9.7)
WS
13 females
55 spawns
167.0
(71.0
8.9
(7.5
CL
1 female
3 spawns
CS
2 females
4 spawns
19.2
248.3)
285.7
139.5
(129.0
0.3
10.6)
10.5
10.7
(9.1
Spawn volume
(mm3)
1408.2
(671.9
138.4
2269.3)
1490.8
(573.3
171.1
2507.1)
2993.2
1517.7
(1182.7
Reproductive effort
index (mm3/g)*
478.0
(144.5
64.5
889.3)
386.2
(135.5
51.6
917.7)
483.5
136.6
(67.0
7.4
150.0)
1.1
12.4)
236.9
1852.8)
49.3
206.3)
When two or more females were active, results were expressed as the ‘weighted’ mean ( SEM) and
the range of the mean values of individual females (between brackets).
*The reproductive effort index was calculated as the volume (mm3) of eggs laid during the
experimental period, per unit (g) of live mass of the female.
Table 2. Spawn measurements and reproductive effort index in females fed
with either a control (ad libitum) or a restricted diet (Experiment II). All
animals were maintained in warm water and under a long photoperiod
ad libitum feeding
11 females
61 spawns
Number of eggs
per spawn
Mean egg volume
(mm3)
164.7
(104.6
8.4
(6.7
14.3
239.3)
0.2
9.7)
Spawn volume
(mm3)
1408.2
(671.9
138.4
2269.3)
Reproductive effort
index (mm3/g)*
478.0
(144.5
64.5
889.3)
restricted feeding
9 females
26 spawns
193.7
(90.0
8.2
(6.1
1590.5
(1075.8
195.1
(70.8
20.3
281.0)
0.6
12.5)
167.2
2527.0)
30.7
337.8)
When two or more females were active, results were expressed as the
‘weighted’ mean ( SEM) and the range of the mean values of individual
females (between brackets).
*Reproductive effort was calculated as the volume (mm3) of eggs laid
during the experimental period, per unit (g) of live mass of the female.
P 0.05; Fig. 3). The mean number of spawns
per female was significantly smaller under
restricted feeding (Student’s t Test, P 0.05),
but the mean number of copulatory episodes
did not differ significantly (same test; Fig. 3).
Also, no significant differences between
treatments (Table 2; Student’s t Test, P 0.05)
were found in the spawn measurements (number of eggs per spawn, mean egg volume and
spawn volume). However, the reproductive
effort index was significantly lower (same test,
P 0.002) in the females exposed to restricted
feeding, than in those maintained under ad libi
tum feeding.
As expected, males and females fed ad libi tum grew significantly more (males: 3.1
0.7
mm, n 16, and females: 4.6 0.7 mm, n 14;
mean
SEM) than those receiving the
restricted diet (males: 0.6 0.2 mm, n 8, and
females: 1.0
0.3 mm, n
8; mean
SEM;
Mann–Whitney U Test, P 0.05). Again, there
was a significantly negative correlation between
the number of spawning episodes and growth
in the group maintained in ad libitum condi-
SEASONAL ONSET OF REPRODUCTION IN POMACEA
247
Figure 3. Experiment II: Effects of food availability on animals maintained under conditions of warm temperature and long photoperiod. Speckled bars indicate the mean latency ( SEM) to first copulation (upper
panel), and the mean latency to first spawning (lower panel). Empty bars indicate the mean ( SEM) number
of episodes for both behaviours. Number of cases is shown in brackets. Asterisks indicate statistically significant differences (P 0.05) from control group.
tions (r
0.75 P
0.002, n
14), but the
correlation coefficient was found to be non significant between the number of copulatory
episodes and the growth in the animals also
maintained in ad libitum conditions. However,
the rather unfrequent episodes of either copulation or spawning, and the almost null growth
observed in animals feeding on a restricted diet
precluded any correlation analysis of these
variables.
A female under restricted feeding that
showed no copulatory activity during the experimental period, did lay a large spawn (281
eggs, a few of which were fertile). Conversely,
248
E.A. ALBRECHT, N.B. CARREÑO & A. CASTRO-VAZQUEZ
some females that were observed copulating
(1–7 times; 3 on ad libitum feeding, and 2 on
restricted feeding), did not show any spawning
activity during the experimental period.
DISCUSSION
In tropical and subtropical zones, apple-snails
are active throughout the year (Hylton Scott,
1957; Fausto Filho, 1962, 1965; Milward de
Andrade, Carvalho & Guimaraes, 1978), while
in temperate zones these species show reproductive activity during the spring and summer
months, and reproduction ceases during autumn and winter (Bachmann, 1960; Andrews,
1964; Aboul-Ela & Beddiny, 1970; Demian &
Ibrahim, 1971; Hurdle, 1973; Martín, 1993).
This laboratory study has shown that water
temperatures observed in late winter/early
spring in Mendoza (around 18°C) may be a
threshold to trigger sexual activity (copulation
and spawning) in P. canaliculata, since some
animals exposed to such temperature became
active during the experimental period. Also,
the first spawns occurring in outdoor ponds are
observed at the end of September (early spring)
in Mendoza. The rise of water temperature
occurring from that time may act as a proximate factor (Baker, 1938) to induce the snails’
emergence from winter dormancy, and the
frequency of both copulation and spawning
would increase gradually with increasing
temperatures, and build up to a maximum
when the summer mean maximum is reached
(approximately 25°C). Our results are in agreement with previous, occasional observations
in other gastropod species, including P. canalic ulata (Agersborg, 1932; Andrews, 1964; Van
der Steen, 1967; Demian & Ibrahim, 1971;
Estebenet & Cazzaniga, 1992).
The snails exposed to warm water temperature (25°C) were reproductively active earlier
(after approximately 13 days), and the number
of behavioural episodes recorded (copulation
and spawning) was higher than observed in
those exposed to cold water temperature
(18°C). The frequencies obtained for both
reproductive behaviours at 25°C were similar to
those observed in summer animals (Albrecht
et al., 1996; Albrecht et al., submitted). Copulation was activated before spawning started,
although mating does not seem to be an essential prerequisite for the onset of the oviposition
in a particular animal: indeed, some females can
deposit eggs before any copulatory episode is
recorded (Estebenet & Cazzaniga, 1993;
Albrecht et al., 1996; and this paper). Fertile
sperms can be stored by a female throughout
winter dormancy, enabling her to deposit fertilized spawns, without intervening copulation,
once exposed to a critical water temperature.
The low water temperature used in our
experiments (18°C) was associated with the
death of about one third of animals. Mortality
was even higher when the animals were
exposed to a lower temperature (16°C). Although there are reports that ampullariid snails
may endure a wide range of temperatures
(Demian & Ibrahim, 1972; Cazzaniga, 1981),
only 5–20% of animals were found to survive
the entire winter in a temperate zone (Oya,
Hirai & Miyahara, 1987). In our experiments,
however, mortality occurred mainly (9 out of 20
cases) in the first week of observations, which
may indicate that the stress caused by the collection, measurement and transfer of the animals from the external ponds to the aquaria,
may have enhanced the effect of low temperature.
The growth of P. canaliculata is not continuous but seasonal, and also seems to be regulated by temperature (Estebenet & Cazzaniga,
1992), with active growth phases in the spring
and summer, and with slow or null growth
in the cold season (fall and winter). In the present study, even if some animals exposed to
cold water temperature (18°C) started reproductive activity, they did not resume growing.
Both the onset of copulation and spawning,
and of increase in shell length, were observed
in individuals exposed to warm water temperature (25°C). However, the negative correlation
between the number of spawns and growth
recorded in the groups exposed to warm conditions (WL and WS), suggests a competition in
the distribution of resources between growth
and reproduction (Calow, 1979).
Bodies of water warm up slowly and steadily
in the spring, in contrast to the greater fluctuations of air temperature; the water temperature
that has been reached by a certain date is
probably even better than daylength as a predictor of the future availability of nutrients,
whose growth depends partly on temperature.
Accordingly, aquatic animals may rely upon
temperature as a proximate cue controlling
reproductive physiology (Olive, 1980) to a much
greater extent than terrestrial animals.
Also, the effects of water temperature seem
to be independent of daylength, which suggests
that photoperiodic changes may be of little
or no importance to facilitate copulation and
spawning behaviours. This observation may
SEASONAL ONSET OF REPRODUCTION IN POMACEA
not be unexpected, since in their natural habitats, most individuals remain partially or completely buried in the mud during winter, and so
they would be unable to receive light signals.
The availability of food may also act as a
proximate factor to activate reproductive behaviours. Low temperatures may also act through
arresting metabolism and thereby affecting digestion and assimilation of nutrients (Aldridge,
1983). Our results indicate that restricted
feeding, when imposed simultaneously to an
increase in temperature, slows the onset of
spawning; however, restricted feeding of snails
maintained in warm water does not affect the
frequency of copulation to a similar extent.
Also, the latency periods of both copulatory
and spawning behaviours were similar both in
animals maintained under ad libitum and
restricted feeding.
We conclude that water temperature is the
major environmental factor responsible for the
seasonal onset of copulation and spawning in
Pomacea canaliculata. Also, food availability
may affect the onset of spawning activity.
These results suggest that the factors that control the seasonal onset of sexual activities in
this snail are similar to those controlling the
seasonal arrest of these activities (Albrecht et
al., submitted), thereby establishing the annual
pattern of reproduction in this species.
ACKNOWLEDGEMENTS
The authors are indebted to Nancy G. Mantován for
invaluable assistance throughout this study. This
research was supported by grants from the Research
Councils of the National University of Cuyo and
from the Province of Mendoza, and from the
National Research Council of Argentina, from which
N.B.C. and A.C.V. are established members, and
E.A.A. is a predoctoral fellow.
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