Phenotypic plasticity in reproductive traits: importance in the life history of Helix aspersa (Mollusca: Helicidae) in a recently colonized habitat

Biological Journal of the Linnean Society (2000), 69: 25–39. With 2 figures doi: 10.1006/bijl.1999.0324, available online at http://www.idealibrary.com on

Phenotypic plasticity in reproductive traits: importance in the life history of Helix aspersa (Mollusca: Helicidae) in a recently colonized habitat LUC MADEC∗, CHRISTOPHE DESBUQUOIS AND MARIE-AGNES COUTELLEC-VRETO UMR CNRS 6553, Service de Zoologie et d’Ecophysiologie (L. A. INRA), Universite´ de Rennes I, Campus de Beaulieu, Av. du Ge´ne´ral Leclerc, 35042 Rennes, France Received 23 October 1998; accepted for publication 11 January 1999

Reproductive traits of the land snail Helix aspersa Mu¨ller were investigated under artificial conditions from two samples, one collected from a population exposed to unpredictable human pressures in its natural environment, i.e. a recently created polders area with intensive agriculture, and the other from a snail farm in which animals were reared under constant conditions defined as ‘optimal’ for growth and reproduction. Results were compared with data collected from natural populations of the same region (Brittany) and from habitats spanning the environmental heterogeneity of the range of the species. A large part of the variation among populations could be explained by different phenotypic covariances between shell size, clutch size and egg size, but not by the number of clutches per snail. Thus, the higher egg production of snails from the polders was related to (i) a strong correlation between clutch size and shell size, shell size being in the upper limit of the overall range for the region concerned; (ii) an uncommonly low egg weight in comparison with the ‘norm’ of Helix aspersa, this trait seeming to be involved in a trade-off with clutch size. Second clutches were smaller than the first ones, but their eggs were significantly heavier. This difference may be linked to a size-dependent mortality of juveniles during winter which arises in all populations in which hibernation occurs as an adaptation to low temperatures. In addition to the selective regime usually involved for populations of helicid snails from Western Europe, several unpredictable mortality factors occurred in the polders area: herbicide and pesticide treatments (lethal for young snails), human predation (lethal for adults) and burning (lethal for all snails). Life-history patterns of Helix aspersa are discussed in relation to its ability to successfully colonize a large range of habitats modified by humans, to such an extent that it can become a pest.  2000 The Linnean Society of London

ADDITIONAL KEY WORDS:—reproductive traits – trade-off – colonization success – agricultural habitat – helicid land snail. CONTENTS

Introduction . . . . . . . . . . . . . . . . . . . . . . .

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∗ Corresponding author. E-mail: [email protected] 0024–4066/00/010025+15 $35.00/0

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 2000 The Linnean Society of London

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Material and methods . . . . . . . . . Study organism . . . . . . . . . . Sampling sites . . . . . . . . . . Laboratory experiments . . . . . . . Statistical methodology . . . . . . . . Results . . . . . . . . . . . . . . Variation in number of matings and clutches Variation in egg weight and clutch size . . Discussion . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . References . . . . . . . . . . . . .

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INTRODUCTION

Since the Holocene, the common form of the garden snail Helix aspersa Mu¨ller has successfully colonized a large range of man-disturbed habitats in Western Europe (domestic gardens, agricultural and suburban areas) and is considered an important pest in lands where it has been recently introduced, especially in North America (Godan, 1983; Fisher & Orth, 1985) and Australia (Rudman, 1999). With the notable exception of life-history studies on Theba pisana (Baker & Vogelzang, 1988; Baker & Hawke, 1990), previous works on reproductive components of helicid snails have always been restricted to non-agricultural environments. They have documented intra- and inter-population variation in reproductive patterns in response to various proximate factors such as climatic pressures or population density (Sacchi, 1971; Wolda & Kreulen, 1973; Oosterhoff, 1977; Cowie, 1984; Baur & Raboud, 1988). Spatial and temporal heterogeneities in the range of Helix aspersa are also associated with a large phenotypic variation in combinations of life-history traits, especially reflecting a high degree of plasticity (Madec & Daguzan, 1993). As a result, the temporal variation between successive generations or the microspatial variation among demes in the same population are sometimes greater than the variation among different populations of the same region, and both express environmentallyinduced phenotypic lability (Potts, 1975; Crook, 1980; Madec & Daguzan, 1987, 1991; Lazaridou-Dimitriadou & Bailey, 1991). However, these works often failed to consider either the correlations between egg and clutch sizes (despite their important role in the population response to its environment (Calow, 1983; Baur, 1990, 1994)), or the adaptive value of the observed plasticity (e.g. Gotthard & Nylin, 1995). Nevertheless, this plasticity, especially in trade-offs involving age-size at maturity and reproductive traits, may be a key to understanding the specific mechanisms which account for the persistence of Helix aspersa in habitats involving frequent extinction-recolonization processes (intensive agricultural zones), when closely related species like Cepaea nemoralis are unable to make such colonizations, despite a frequent sympatry in other environments and closely related dispersal strategies (Oosterhoff, 1977; Fearnley, 1993). The present study reports on variability in reproductive traits of Helix aspersa obtained from two contrasting environments, i.e. the constant ‘optimal’ conditions of a snail farm and the unpredictable ‘harmful’ conditions of an agricultural system. The aims are: (i) to present an empirical explanation for the persistence of the species in sites where stochastic factors drastically affect the demography of local populations; (ii) to complete our investigations into the phenotypic plasticity of sizefecundity relationships and allocation trade-offs among components of reproduction.

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MATERIAL AND METHODS

Study organism Various aspects of the ecology and life history of Helix aspersa have already been investigated (Crook, 1980; Madec & Daguzan, 1993; Fearnley, 1993). The relevant sexual biology has been fully described by Giusti & Lepri (1978), Tompa (1984), Chung (1987) and Adamo & Chase (1988). Moreover, numerous data on the reproductive outputs of snails from different French regions are available because this edible species is the principal subject for heliciculture. However, few studies deal with the life history of Helix aspersa in natural environments (Potts, 1975; Crook, 1980; Fearnley, 1993; Iglesias, Santos & Castillejo, 1996). For this experiment, the first sample of snails were taken as adults and immature sub-adults (without reflected lip on the peristome) in their natural habitat in April 1994, just after hibernation ended. They represented the two age classes which would breed during the following late spring and summer, sometimes autumn in this region. A second sample of snails was randomly collected from a population produced in a snail farm near Rennes (Brittany), soon after an artificial hibernation of 3 months. All these snails had a recently formed shell lip, indicating their maturity. They were born in late spring 1993, like the majority of sub-adults of the first sample. Because snail farmers kill their snails after the first reproduction, data concerning two year old snails reared under farm conditions were taken from Le Calve´ (1995).

Sampling sites The first sample site was a polders area managed only for the last 60 years in the bay of Mont Saint Michel. Detailed information on the abiotic environment, history of management and agricultural practices in this intensive cropland are available in Acx (1991). This environment could be considered as stressful for the snails because: (1) in summer, occasional burning along bank lines in which snails aestivate (trees, tall weeds for the young) causes mortalities of all age categories; (2) twice a year (May, September), when the snails are active, chemical treatments (herbicides, pesticides) are administered, of which several are known to be lethal, at least for young snails (Godan, 1983); (3) predation occurs, especially human harvesting of adults for consumption. Harvesting is controlled and strictly forbidden during the breeding season. The second site was a snail farm ( Janze´, near Rennes, Brittany) in which all snails were reared in artificial conditions defined as ‘optimal’ for their growth and reproduction (Daguzan, 1981). They are subject to imprecise artificial selection: from the start of rearing, the farmers visually sort the future breeders (around 2000 per generation) according to their body size. Thus, mean shell breadth of these snails (D=33.2 mm) was higher than most of those from this region (Madec, 1989a).

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Laboratory experiments Breeding After collection, snails were reared in controlled temperature and relative humidity rooms maintained at 20±1°C, 80±5% R.H. and a 16:8 light:dark cycle. They were housed in polythene containers (29.0×18.0×7.0 cm) with a density of 15 individuals per container. These values are optimal for breeding activity of Helix aspersa in Western France (Daguzan, 1981). Depending on the number of snails available, from two to four replicate cages were used per sample to take possible ‘cage effects’ into account. Furthermore, the location of boxes in the rearing room was changed each day. All individuals were fed ad libitum with composite snail-food (produced by the Arrive´ company) renewed at least twice a week. Water was available from a water dish and from the synthetic foam covering the cage bottom which was kept moist and washed three times per week. Four laying jars containing moist, light soil (sterilized compost + sand) were placed in each cage. Jars were replaced as soon as snails laid in them and jars with egg clutches were transferred to incubation conditions (T =20±1°C; R.H. =100%; 12L:12D). Adult measurements and monitoring Adult shell maximum breadth was measured to the nearest 0.1 mm using a vernier calliper; each animal was numbered with an adhesive label. Mating and egg-laying in Helix aspersa last about 8 hours and 20 hours respectively, so two daily observations (08:00 h; 18:00 h) permitted monitoring of all layings and matings. Dates when individuals resumed activity after hibernation and dates of death were also recorded. The length of the reproductive period was based on the end of layings (not of matings). This generally coincided with the start of a higher mortality, roughly 14 weeks after the first matings. Egg collection and measures Each clutch was identified by its parentage and whether it was the 1st, 2nd or 3rd clutch of the snail which laid it, its date of laying, its number of eggs and hatching date. In each clutch, 30 eggs chosen at random were weighed individually (±0.001 g) because there is a strong linear correlation between wet weight and diameter of the egg (Madec, 1989b). All the eggs were then replaced in a soil cavity and the laying-jar was covered by a plexiglass plate before being placed in incubation conditions. Newly-hatched juveniles emerging from the soil were counted, removed and the durations of incubation and hatching noted. However, during this experiment, many clutches were infected to different degrees by various parasites, mainly nematodes, present in the laying substrate. According to the relations already known between clutch size, egg size, juvenile size and their survival (Daguzan, 1982; Madec, 1989b; Le Calve´, 1995), we have inferred some consequences for corresponding juveniles. Statistical methodology The first stage of the study was to explore, for each sample: (i) the phenotypic relationships between reproductive traits, i.e. numbers of matings and clutches,

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numbers of eggs per clutch (NE) and per season, egg wet weights (WE) and related coefficients of variation (CVWE), and (ii) the effects of snail size (represented by maximum shell breadth) on these traits. No transformations of variables were required to conform to the assumptions of parametric analyses (see Results). In addition to the significance tests for regression lines, residuals were visually examined by plotting them (i) as a function of the corresponding X values (homoscedasticity) and (ii), after standardization, into a standard deviation scale. Homogeneity among correlation coefficients was tested after Fisher transformations. In the second stage, the effects of origin, age, date of oviposition and their eventual interactions were tested with ANOVA on residuals of regressions of the trait (clutch size, egg weight) on the significant covariate(s). Prior these analyses, we performed separate hierarchical analyses of variance for each trait with cage nested within each factor to detect possible cage effects. All data analyses were performed using BIOMECO programs (Lebreton et al., 1990), except interaction models for which we used type III sums of square provided by the GLM procedure of SAS (SAS, 1985).

RESULTS

Variation in number of matings and clutches The total reproductive activity of the snails differed among the three groups (Fig. 1), because of a tendency of older snails from St Michel to mate more (v2 =12.12; P =0.06; number of matings divided into four classes) and to lay significantly more often (v2 test and STP procedure; P=0.03). However, all three samples showed a percentage of mating snails close to 100% (Table 1): only one snail from St Michel did not mate during the experiment and four from Janze´. Within each sample, the numbers of matings per individual were highly variable and it seemed that larger snails were inclined to mate more, but the global test (n=135) did not detect a significant association between shell size (three classes) and number of matings (v2= 11.01; P=0.09). Snails of different sizes also had similar oviposition activity (v2= 7.52; P=0.11 for three ‘laying’ classes), which was expressed, after nearly 12 weeks of reproduction, by about 1.3 clutches per older snail and only 1.0 for the others. Furthermore, there was no relation between the timing of egg laying (oviposition date) and the snail size (v2 tests; P >0.50). However, younger adults from St Michel started to reproduce later, perhaps because many snails became sexually mature only after the resumption of activity after hibernation (Fig. 2). The most important relationship detected was the highly significant association between numbers of matings and layings in each sample (v2 tests; P <0.001); snails which mated often were also those which laid more clutches. In addition, it did not seem that intensive reproductive activity (at least five matings and/or two clutches) had a direct effect on the breeder’s survival (v2 tests; P >0.70). However, there was a significant difference between older and younger snails of St Michel in total number of deaths during the breeding period (Table 1). Thus, the majority of the older snails survived (24.4% mortality), whereas 53.3% of the younger ones died (v2 test; P <0.001).

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Percent snails

70 60 50 40 30 20 10 0 70 60 50 40 30 20 10 0 70 60 50 40 30 20 10 0

A

B

C

0

1 2 3 4 5 6 Number of matings and clutches per snail

Figure 1. Distributions of the snails (as %) according to their total numbers of matings (Ε) and clutches (C) in the three samples of Helix aspersa aspersa studied. A, Janze´; B, older snails from St Michel; C, younger snails from St Michel. T 1. Reproductive traits, shell size and mortality of Helix aspersa aspersa from two age classes and two origins in uniform laboratory conditions (mean ± S.D.). In parentheses: sample size Origin Age class Sample size Shell breadth (mm) Rate of mating (%) Ovipositing snails (%) 1st clutch size (NE1) 2nd clutch size (NE2) Number of eggs per breeder C1 egg weight (mg) C2 egg weight (mg) Mortality (%)

young

St Michel old

Janze´ young

30 33.7±2.3 96.7 70.0 179±28 (21) 130±22 (8)

45 32.8±1.9 100.0 82.2 157±30 (34) 138±32 (17)

60 33.2±2.5 93.3 78.3 150±42 (43) 112±42 (10)

224±76 28.7±3.4 30.6±4.5 53.3

234±.95 28.4±4.5 29.7±3.9 24.4

169±56 28.9±4.6 30.1±5.2 11.7

F=2.54; NS v2c=3.22; NS v2=1.73; NS F=2.97; NS (0.06) F=3.58; P=0.04 F=7.99; P <0.001 F=0.20; NS F=0.81; NS v2=18.45; P <0.001

Variation in egg weight and clutch size Phenotypic correlations among reproductive traits and snail size Examination of matrices of partial and total correlations showed the same significant influence of body size in the ‘wild’ (St Michel) and ‘farm’ ( Janze´)

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100 80

A

60 40 20 0

100 B Percent snails

80 60 40 20 0

100 C 80 60 40 20 0

1

2

3

4

5

6

7 8 Weeks

9

10 11 12 13

Figure 2. Weekly variations of mating (Ε) and oviposition (C) frequencies in Helix aspersa, according to the age class and origin (as % of the total number of surviving snails per sample). A, older snails from St Michel; B, younger snails from St Michel; C, Janze´.

samples, i.e. a positive correlation between maximum diameter D and reproductive components, and a null or negative one between egg number (NE) and egg weight (WE) (Table 2A, C). However, the correlation of number of eggs in first clutches (NE1) against D was significantly higher in ‘wild’ snails (Zdr=2.31; P=0.01). For all comparisons, data concerning first clutches (C1) of old and young snails from St Michel were combined because there was no significant difference between correlation and regression lines. Regarding snails from St Michel which had laid at least two clutches (n=25), the mean number of eggs of the first clutch (NE1) was significantly higher than that in the later clutches (tp-test; P <0.001). However, eggs of the second clutch were significantly heavier (tp-test; P=0.017), and there was a strong correlation between WE1 (mean egg weight of the first clutch) and WE2 (mean egg weight of the second clutch) (Table 2B). There was no difference in egg number or egg weight between first clutches of snails which had laid more than one clutch and ‘single’ clutches. In addition, comparisons of regression lines on shell size of NE (1, 2) and WE (1, 2)

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T 2. Matrices of product-moment correlation coefficients (upper diagonal: partial correlations) between reproductive traits and shell breadth in Helix aspersa aspersa. Level of significance of r: ∗ P <0.05; ∗∗ P <0.01; ∗∗∗ P <0.001. (A) first and single clutches of snails from St Michel (n=55); (B) first and second clutches of snails from St Michel (n=25); (C) first and single clutches of snails from Janze´ (n=43). A D NE1 PE1 CVPE1

D — 0.629∗∗∗ 0.306∗ 0.229∗

NE1 0.631∗∗∗ — 0.099 0.095

PE1 0.336∗∗ −0.136 — −0.046

CVPE1 0.250∗ −0.083 −0.135 —

B D NE1 NE2 PE1 CVPE1 PE2 CVPE2

D — 0.708∗∗∗ 0.500∗∗ 0.579∗∗∗ 0.164 0.261 0.325

NE1 0.625∗∗∗ — 0.363∗ 0.497∗∗ 0.064 0.272 0.015

NE2 0.144 0.074 — 0.466∗∗ 0.410∗ 0.012 0.325

PE1 0.253 0.061 0.369∗ — 0.169 0.577∗∗∗ 0.098

CVPE1 −0.035 −0.045 0.326 0.022 — −0.019 0.248

PE2 −0.008 0.035 −0.303 0.587∗∗∗ −0.001 — −0.076

CVPE2 0.387∗ −0.304 0.160 −0.085 0.126 −0.045 —

C D NE1 PE1 CVPE1

D — 0.385∗∗ 0.287∗ 0.201

NE1 0.462∗∗∗ — −0.176 0.062

PE1 0.390∗∗ −0.324∗ — 0.098

CVPE1 0.172 −0.034 0.108 —

only indicated a significant difference in intercepts for regressions with NE1 (single or first clutches, n =55) and NE2 (ANCOVA; F=24.1; df=1.77; P <0.001). Effects of origin and age of snails Comparisons of the mean values for residuals of reproductive trait-shell size regressions showed only one significant difference among sizes of the second clutch, with more eggs in second clutches of snails from St Michel (ANCOVA; Table 1; P =0.04). Consequently, the ‘age’ factor seemed to influence neither the clutch size nor the egg weight. The comparison of mean numbers of eggs deposited per snail during the reproductive season (cumulative fecundity) showed that the differences between samples were in accordance with the previous comparison of clutch sizes because of the lower mean clutch size and the lower mean number of clutches laid per ‘farm’ snail (Table 1). Evolution during the breeding season To test the effect of oviposition date, the egg-laying period was divided into three intervals, each of 3 weeks. A first set of analyses performed on data from St Michel (D; NE1, WE1 residuals) detected an interaction between age and oviposition date. Oviposition date had a significant effect on NE1, which increased during the season

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for the two age categories (F=4.01; P=0.02). Egg weight was modified by the significant ‘age x oviposition date’ interaction (F=3.18; P =0.05): eggs laid by old individuals were heavier later in the season, whereas those from young had a fluctuating weight but a higher value at the beginning of the season. This result has to be interpreted with caution because there were only 20 pieces of data for the ‘young’ group. Two other analyses, performed on residuals of the multiple regression on body size and origin (98 pieces of data), also showed the significant increase of egg number of the first and single clutches (F =4.38; P =0.02) and the relative stability of WE1 (F=1.45; NS), despite a tendency to increase during the season.

DISCUSSION

The artificial environment used in the present study was initially selected by INRA programs [Institut National de la Recherche Agronomique, Domaine expe´rimental du Magneraud, Surge`res] to enable snails collected in western France to reach a maximum reproductive output. It matched conditions prevailing during the period of natural reproduction in populations of Brittany but, obviously, did not replicate the daily and seasonal variation of climatic factors and food quality available in natural habitats. Moreover, variation in egg production of Helix aspersa cannot be dissociated from shell size, itself dependent on several proximate factors which act, via changes in growth, on the trade-off between age and size at maturity. The natural constraints on land snail growth also include population density, as shown by laboratory studies on Helix aspersa (Dan & Bailey, 1982; Lucarz & Gomot, 1985; Lazaridou-Dimitriadou et al., 1998) and field studies on related species (Williamson, Cameron & Carter, 1976; Oosterhoff, 1977; Cameron & Carter, 1979; Perry & Arthur, 1991; Foster & Stiven, 1994). Nevertheless, the results obtained from laboratory experiments, while they may not be used to predict the mean phenotypes expressed in the field, can be related to other comparative works on Helix aspersa carried out in the same environment (Madec & Daguzan, 1991, 1993; Le Calve´, 1995; Dupont-Nivet et al., 1998) and related to both theoretical predictions (Calow, 1983; Parker & Begon, 1987; Stearns, 1992) and field observations. This approach, which we have used in the present study, is intended to provide ad hoc explanations to the following questions. (i) Can the exceptionally high fecundity of snails from St Michel be related to the selective regime of the polder? (ii) Can the variation of the intra-individual trade-off egg weight/egg number be adaptive and if so, what ultimate factors can explain it? As in other terrestrial gastropods (Bengtsson & Baur, 1993), the invasive ability of Helix aspersa does not depend on a life-history response related to r-K selection, but seems to be based on the following tactics: (i) A fluctuating sexual asymmetry associated with a polymorphism for dispersal (Fearnley, 1993, 1996). Fearnley demonstrated empirically that a variable but low proportion of snails was involved in exchanges between local demes and in colonization of new areas. The dispersal tendency would be promoted by an increase of population density in Helix aspersa, as in Cepaea nemoralis (Oosterhoff, 1977), and would involve large snails which then avoid crowding effects especially during reproduction. Some parts of the female genitalia (albumin gland) would mature later in these larger snails so hermaphroditism of the

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species would show a real protandry only in one part of the populations. However, such a dispersal tendency has to be set against the cost of locomotion which is excessively high in terrestrial pulmonates (Denny, 1980) and also to the higher mortality during the dispersal stage. Thus, snails generally live in aggregated populations and exhibit a marked homing behaviour (Bailey, 1989; Lorvelec, 1990), in relation with a low amplitude in their trivial movements. (ii) A considerable life-history plasticity which is largely retained within each population of the habitat generalist H. a. aspersa (Madec & Daguzan, 1993). Thus, the sample of St Michel is characterized by a mean clutch size almost never observed in H. a. aspersa, but close to that of H. a. maxima. Among the artificial populations reared for at least two generations, the one from Janze´ exhibits increased shell and clutch sizes which are comparable to the increase observed in H. a. maxima reared and sorted with the same methods (Madec et al., 1998). However, because there is no precise breeding control in snail farms, the importance of the genetic component in variation (polymorphism and/or plasticity) of these traits in H. a. aspersa is unknown. Nevertheless, other studies show, through three successive generations of snails reared under artificial conditions identical to those of the farm, (i) that heritability of shell size is high (>0.40) (Dupont-Nivet, Guiller & Bonnet, 1997), and (ii) that there is an increase of the mean genetic level for adult weight and egg number, for which positive genetic correlations were observed (Dupont-Nivet et al., 1998). Thus, we have to dissociate the influence of selective pressures or ecological constraints observed at the regional scale from their specific effects in a patchy environment like St Michel. In addition, temporal variation between and/or within generations of the same population has also to be considered. Previous work has shown that clutch size and total egg production of snails from a Breton sample reared in artificial conditions were not different from those living in the field. However, the length of the breeding period and the timings of mating and oviposition were notably labile, according to unpredictable seasonal (and annual) fluctuations in climatic factors (Madec & Daguzan, 1987). Until the present work, such a result was compounded with the variation in reproductive activity of spatially separated populations of the same region (Madec & Daguzan, 1991), and it was only at the scale of the subspecies range that spatial variation significantly affected quantitative traits (e.g. clutch size) with no fixation of the mean phenotypes observed. This plasticity was related to a differential mortality in eggs and juveniles by desiccation, predation or frost (Potts, 1975; Daguzan, 1982), winter survival being considered as a key factor in population dynamics of other Helicidae in Europe (Wolda & Kreulen, 1973; Pollard, 1975; Peake, 1978; Cain, 1983; Cowie, 1984). In the location of St Michel, winter survival could also explain some results which are not special features of the population studied. Thus, many works have shown a decrease of the clutch size during a breeding season (e.g. Baur, 1990) and one of them has partially attributed this decrease to the smaller size of the second clutches in Cepaea nemoralis (Oosterhoff, 1977). In Brittany, second clutches of Helix aspersa are often laid in late summer and autumn, just some weeks before hibernation. The larger eggs collected in laboratory conditions are produced to the detriment of clutch size but, as shown in Arianta arbustorum (Baur, 1994), their higher nutrient content leads to larger juveniles which, in natural conditions, require only a short period of growth to reach a shell diameter higher than 20 mm. Below this size, snails are unable to hibernate

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T 3. Main results on reproductive activity during 10 weeks in 2 generations (‘wild’ snails (F0) and their offspring born and reared in ‘farm’ conditions without selection (F1) and in 2 age classes (F (‘young’) and F+1 year (‘old’) with an artificial hibernation of 3 months between the 2 breeding seasons) of H. a. aspersa from Western France (raw data from Le Calve´, 1995) ‘wild’ population Young (F0) Old (F0+1 year) Sample size Shell breadth D (mm) Ovipositing snails (%) Mean clutch size (NE) Mean egg weight (mg) Mean clutch weight (mg) Mortality (%)

200 34.5±1.9 29.0 80±22 36.4±6.2 2879±852 19.0

126 32.7±1.7 29.4 101±27 30.5±4.8 3111±684 59.5

‘experimental’ population Young (F1) Old (F1+1 year) 160 33.3±1.5 57.5 96±31 30.1±4.5 2890±1052 35.6

12 33.0±1.3 66.6 99±21 32.9±3.5 3252±699 58.3

and, consequently, have a lower overwinter survival (Biannic, 1995). Thus, the ultimate cause of this trade-off would be related not only to energy reserves carried over winter but rather to the acquisition of behavioural and physiological traits required for hibernation. The demonstration will be given in the field if eggs of autumnal clutches are significantly heavier. For the clutches laid in spring or summer, heavy eggs and a correlated larger size for hatchlings are probably less advantageous than numerous offspring. Thus, the only significant increase detected in this work for ‘first’ and ‘single’ clutches concerns the egg number. In other respects, Dupont-Nivet et al. (1998) observed, for snails with comparable past and present conditions of life, positive phenotypic and genetic correlations between egg number and (i) time from emergence from hibernation to laying and (ii) time from mating to laying, but no relation of (i) and (ii) with egg weight. In fact, high temperatures and correlated water loss of eggs and juveniles (Riddle, 1983; Klein-Rollais, 1993) are not decisive mortality factors in such a rainy region and still less in the polder, because of frequent irrigation. Temperature conditions and food availability during this period are also optimal for the juvenile growth and, consequently, would allow them a higher prospective survival whatever their initial size. A key factor in the life cycle of this population would be then an unpredictable size-independent mortality by burning and poisoning, burning also affecting adult snails. In this station, Helix aspersa is confronted with a fluctuating environment with cyclic (i.e. seasonal) periods of severe stress, and a random component of mortality, especially for juveniles but also for adults which are periodically harvested. Consequently, in addition to a flexible form of diapause as a response to predictable seasonal constraints (Bailey, 1981), the life-history plasticity finds here its expression in a large size at maturity (related to a growth without crowding constraint) and a surprisingly large egg production, in old as well as in young adults, such an output not being previously observed in any studied population of H. a. aspersa, including the farm sample studied here. The strong correlation between shell breadth and first clutch size also reflects energy investment in the egg number rather than in egg size, the latter being not very variable (Table 2 vs Table 3). Therefore, maximization of egg number per snail results in first clutches with the lowest mean and variance of egg weight ever recorded, and could be determined by a phylogenetic constraint. An increase of egg weight of the first clutches during the breeding period was observed for older adults but, as shown in

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other species (Baur, 1990), it could be caused insufficient energy reserves of the mother for clutches laid a few weeks after the end of hibernation. Selected snails from the farm are not characterized by a significantly higher total egg production when compared with natural populations of Brittany (Madec & Daguzan, 1991). On the other hand, higher egg number per snail from the polder can be explained by the proximate factors involved above, which affect the two age classes considered. Thus, older adults produced a first clutch with a fewer eggs but laid more clutches. As shown in other works on iteroparous land snails (Wolda, 1963; Foster & Stiven, 1994; Le Calve´, 1995), a balance is found, for one breeding season and one population, in an investment in reproduction not different from one age class to the other (e.g. Table 3). However, such a high production could contribute to the reduced life span recorded in this experiment in younger adults of St Michel. Even if the snails of the polder seem to have retained, in laboratory conditions, a combination of life history traits which expresses an appropriate response of the population to the specific constraints of its habitat, the demonstration of a local adaptive plasticity requires further laboratory experiments combined with reciprocal transplantations in which phenotypes produced by a population would have theoretically greater performances in their site of origin (Via, 1994; Gotthard & Nylin, 1995). Thus, future directions of this work include designed and field experiments (i) to evaluate the genetic basis of reaction norms of life history traits and related trade-offs (e.g. Stearns, 1989; Via et al., 1995) and (ii) to assess the connections between dispersal strategies as described by Fearnley (1993) and the variation in life history traits in subpopulations.

ACKNOWLEDGEMENTS

Many thanks to Dr S.E.R. Bailey (Manchester, England) and Professor R.D. Simpson (Armidale, Australia) for their comments which greatly improved earlier versions of the manuscript.

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