© Sociedad Española de Malacología Iberus, 15 (2): 25-34, 1997

Phenological patterns and life history tactics of (, ) snails from Northern Greece

Patrones fenológicos y estrategias de vida en Helicoidea (Gastropoda, Pulmonata) del Norte de Grecia

Maria LAZARIDOU-DIMITRIADOU and Stefanos SGARDELIS*

Recibido el 8-I-1996. Aceptado el 31-VII-1996

ABSTRACT The present study mainly concerns with the differences in the biological cycles and strategies adopted by different Helicoidea snail . In Northern Greece the climatic conditions are not very uniform. Some snails breed during the same period as in Northern Europe but most breed in autumn, as species from Southern Europe do. Breeding may take place in all sea- sons except in winter, and seems to be species-specific. Long-lived snails of big size differ from short-lived species of small size as to the time of their breeding period. The climatic con- ditions affect the time of the breeding season and their whole life cycle and phenologies. En- vironmental variables in Northern Greece are strongly seasonal and thus Helicoidea snails exhibit predictable oscillations in their activity patterns, which can be interpreted by the de- mographic response of the populations. Terrestrial snails seem to follow two different pheno- logic curve types: the semelparous and short-lived species populations show a more stable phenological pattern than the biennial and pluriennial ones, who mature after the first year of their lives, being more plastic trying to face the climatic differences from one year to another.

RESUMEN El presente estudio trata de las diferencias en los ciclos biológicos y a las estrategias adopta- das por diferentes especies de Helicoidea. En el norte de Grecia, las condiciones climáticas no son muy uniformes. Algunas especies crían durante el mismo periodo en que lo hacen en el N de Europa, pero la mayoría lo hacen en otoño, como sucede en especies del S del con- tinente. La cría puede tener lugar durante casi todas las estaciones, excepto el invierno, y el periodo parece ser específico para cada especie. Las especies longevas y de gran talla di- fieren de las de pequeño tamaño y vida más corta en la duración de su ciclo de cría. Las condiciones climáticas afectan al momento de la temporada de cría y a todo su ciclo vital y fenología. Las variables ambientales son fuertemente estacionales, así que aparecen oscila- ciones predecibles en los patrones de actividad, que pueden ser interpretadas por la res- puesta demográfica de las poblaciones. Las babosas parecen seguir dos tipos de curvas fe- nológicas distintas. Las especies semelpáricas y de corta vida muestran un patrón fenológico más estable que el de especies bianuales y prurianuales, que maduran tras el primer año de vida y son más flexibles a la hora de enfrentarse a las diferencias climáticas interanuales.

KEY WORDS: Biological cycle, phenology, Northern Greece, Helicoidea, Helix, Eobania, , , Bradybaena. PALABRAS CLAVE: ciclo biológico, fenología, N Grecia, Helicoidea, Helix, Eobania, Helicella, Monacha, Bradybaena.

* Departments of Zoology and Ecology, School of Biology, Aristotle University of Thessaloniki, 54006 Thessaloniki, Macedonia, Greece.

25 Iberus, 15 (2), 1997

INTRODUCTION 1995; LAZARIDOU-DIMITRIADOU AND KATTOULAS, 1981, 1985, 1991; STAIKOU, Greece has a Mediterranean climate LAZARIDOU-DIMITRIADOU AND FARMA- which is differentiated mainly along a KIS, 1988; HATZIIOANNOU, ELEUTHERIA- northern-southern gradient. In Northern DIS AND LAZARIDOU-DIMITRIADOU, 1989; Greece climate is transient from STAIKOU, LAZARIDOU-DIMITRIADOU AND Mediterranean (mostly coastal areas) to PANA, 1990; STAIKOU AND LAZARIDOU- temperate (inland areas). A typical cha- DIMITRIADOU, 1990, 1991). racteristic of this climate type is the coincidence of high temperatures and low precipitations during summer (las- MATERIALS AND METHODS ting from June to October). The wet sea- son is divided by a cold winter which is Data used derived from monthly milder in the coastal areas. Drought is a quantitative samplings of Helicoidea strong agent controlling population dy- snails from different parts of Northern namics and activity of most soil inverte- Greece. The following species were brates as it imposes a pause in most studied: Family Bradybaenidae, Brady- physiological activities. Low temperatu- baena fruticum (Müller, 1774); Family res during winter are also important for , Cepaea vindobonensis (Férus- population dynamics and activity as sac, 1821), (Müller, they impose hibernation in some of the 1774), Helix lucorum Linnaeus, 1758, invertebrates e. g. terrestrial gastropods. Helicella (Xerothracia) pappi (Schüt, 1962), So observed discontinuities in popula- Helix figulina (Rossmässler, 1839), Helix tion development during the transition pomatia rhodopensis Kobelt, 1906, Theba from the favourable to the unfavourable pisana (Müller, 1774); Family Hygromii- seasons and vice-versa may be attribu- dae, Cernuella virgata (Da Costa, 1778), ted to environmental thresholds. (Müller, 1774), Xero- Although the association between lenta obvia (Menke, 1828), Xeropicta climate and life history phenomena is arenosa Ziegler, 1836, Xerotricha conspur- self evident, it can vary among terres- cata (Draparnaud, 1801). Sampling trial molluscs, even between popula- lasted two or four years depending on tions of the same species. Phenology the species and their life span (LAZARI- reflects certain aspects of the demo- DOU-DIMITRIADOU, 1981, 1995; LAZARI- graphy of a population, that is the DOU-DIMITRIADOU AND KATTOULAS, timing of its life cycle characteristics in a 1981, 1985, 1991; STAIKOU ET AL., 1988; given environment. The classification of STAIKOU ET AL., 1990; STAIKOU AND phenological patterns into categories or LAZARIDOU-DIMITRIADOU, 1990, 1991). types (WOLDA, 1988) is better by using Details regarding the sites and the sam- phenological models (VAN STRAALEN, pling procedures are given in previous 1982; STAMOU, ASIKIDIS, ARGYROPOULOU studies on these species (LAZARIDOU- AND SGARDELIS, 1993). Using a phenolo- DIMITRIADOU, 1981, 1995; LAZARIDOU- gical model, complex phenograms can DIMITRIADOU AND KATTOULAS, 1985, be classified into types considering their 1991; STAIKOU ET AL., 1988). Ombrother- skewness, curtosis, phase and period. mic data for Northern Greece from 1980 The aim of the present study was to to 1990 are given in Figure 1. Data were find out whether terrestrial gastropods provided by Mahairas P., Professor of adopt a general phenological pattern if Climatology from the Aristotle Univer- they are differentiated according to their sity of Thessaloniki. origin, or their biotopes (inland and Fischer’s exact test for independence coastal areas) or life spans. The present in 2 x 2 contingency tables (ZAR, 1984) study is a part of an extensive research was used for comparisons between the done on the distribution and ecology of different categories, e. g. snails with Helicoidea gastropods in Northern autumnal and vernal-estival reproduc- Greece (LAZARIDOU-DIMITRIADOU, 1981, tive periods, long-lived (> 3 years) and

26 LAZARIDOU-DIMITRIADOU AND SGARDELIS: Phenology of Helicoidea in N Greece

40 80

30 60 °C

° C 20 40 Precipitation (mm)

10 20 Prec. (mm)

0 0 JFMAMJJASOND Months Figure 1. Ombrothermic diagram from Northern Greece. Temperature: open squares; Precipitation: solid rhombus. Figura 1. Diagrama ombrotérmico del Norte de Grecia. Temperatura: cuadrados abiertos; Precipitación: rombos sólidos.

short-lived snail species (up to 3 years), f (ET) = EXP(α+b x COS(2π x (ET-ϕ/T)) (1) large (largest shell diameter > 25 mm) and small sized snails. where the independent variable ET, The phenological pattern of Helicoi- termed Ecological time, is a function of dea species was studied by using the standard clock time (ST), ET = f (ST). In phenological model applied for the the course of standard time, ecological study of microarthropods (STAMOU ET time is going faster during periods of AL., 1993). In short, in this model when sharp changes in abundance and slower asymmetries and discontinuities are dis- during periods of abundance stability. played the scales of the time-axis were Thus, the proposed equation describing adjusted. Changing time scales results the length of the Ecological time unit in the definition of a new variable (∆ET) as a function of Standard time ST termed ecological time (ET), which is a is: function of a standard clock time (STAMOU ET AL., 1993). This technique is ∆ET= f (ST) = EXP(α1+b1 x COS(2π x based upon the following considera- (ST-ϕ1)/T1)) (2) tions: 1) the timing of a population in the field is determined by the sequence The model has eight parameters of of demographic events and/or beha- which the period T and the phase ϕ of vioural characteristics (i. e. migratory), the phenogram, as well as period T1 and and 2) the rate of the demographic the phase ϕ1 of the function relating ET events depends on the fluctuations of to ST, are the most important. The environmental variables. period T and the phase ϕ of the pheno- In this model it is assumed that the grams are expressed formally in ET phenology of any population inhabiting units. For convenience they could be a seasonal environment can be descri- expressed in ST units (as T’ and ϕ’) by bed by a symmetric periodic curve: using equation (2) for the calculation of

27 Iberus, 15 (2), 1997

the Standard time T’ or ϕ’ which corres- (ϕj in Table I). X. obvia from Karvali ponds to T or ϕ units in the Ecological peaked in July. Helix lucorum displayed time-scale (see STAMOU ET AL., 1993: fig. a similar interannual instability. In both 1). For the comparison of phenograms cases the shift in phase seems to be asso- two more parameters can be derived: a) ciated with an unexpected change of the an estimation of the sharpness (curtosis) weather, an extended dryness (STAIKOU of the phenogram C=(R2-R1)/T’, where ET AL., 1988: fig. 2) which provoked an (R2-R1) is the time interval around the overall decline of the population density phase ϕ’, during which the abundance is (Fig. 2). above overall mean, and b) an estima- The phase expressed as months after tion of the skewness of the phenogram minimum population densities (ϕm) S= (ϕ’-ϕm)/T’ where ϕm is the time (in (Table I) is a measure of the rate of ST units) when the abundance of the population increase from absolute population is at minimum. Thus, pheno- minimum to peak densities. ϕm might grams displaying a peak at ϕ’-ϕm = T’/2 have lower values when the time (half period) is symmetric (S= 0.5), phe- needed for a species to mature is short. nograms displaying a peak soon after Minimum ϕm values were estimated for the minimum (S< 0.5) are positively X. obvia in Paleokastro, the 1st and the skewed, and phenograms with S> 0.5 third year of study, indicating a rapid are negatively skewed. The model was population growth which occurs in a fitted on log-transformed census data. period of about 3 months (Fig. 2). In For ETi= f (STi) given as a time vector Karvali, where X. obvia gets mature in 2 and for a given set of ϕ and T, the para- years, ϕm value was larger (Table I), as meters a and b were estimated by least- was the case with M. cartusiana in 1985. square regression. Helicella (Xerothracia) pappi snails that need 2-3 years to mature exhibited larger ϕm values, and H. lucorum snails RESULTS which need 3-4 years exhibited the largest value of all except in 1983. The model fitted to census data for Species that mature in one year, snail populations sampled at monthly exhibit a very rapid population growth intervals from different areas are shown just after the adverse period, which is in Figure 2. The values of the most winter, and consequently they are posi- important parameters of the fitted phe- tively skewed. Species that mature in 2- nological model are given in Table I. 3 years are negatively skewed (Table II, In all but two examined cases the Fig. 3). In this case the phenological phenological pattern was strongly sea- pattern may be positively skewed or sonal (Fig. 2), with a more or less 1 year symmetrical but the population remains periodicity apart from Monacha cartu- active for longer periods and the pheno- siana which seems to display a six logical pattern is platycurtic (Table II, months periodicity at least during 1984 Fig. 3). Whereas species that mature in (Table I). Abundance of Bradybaena fruti- one year usually display a leptocurtic cum and Eobania vermiculata fluctuated phenology (Table II, Fig. 3). The only almost randomly throughout the year. species population that showed both a Population densities of the different negatively and a positively skewed phe- species do not exhibit phase synchroni- nological pattern was H. lucorum. zation. Even different populations of the The positively skewed phenograms same species do not always exhibit a are leptocurtic when they concern snail peak density at the same time of the species populations that mature in one year. Furthermore even the same popu- year whereas they may be slightly platy- lation displays a phase instability curtic when they live in regions where during successive years of study. For favourable conditions last longer as is instance obvia from Paleokas- the case of Xeropicta arenosa Ziegler in tro peaked either in January or in April Edessa. Pluriennial snail species popula-

28 LAZARIDOU-DIMITRIADOU AND SGARDELIS: Phenology of Helicoidea in N Greece

Helix lucorum Edessa Paleokastro Population density (ind./m2)

Monacha cartusiana Edessa Xerolenta obvia Karvali Population density (ind./m2)

Xeropicta arenosa Helicella (Xerothracia) pappi Edessa Philippi Population density (ind./m2)

Months Months

Figure 2. Abundance variations of Helicoidea snails from Northern Greece. Line: model estimates; asterisks: observations. Figura 2. Variaciones de la abundancia de caracoles helícidos del Norte de Grecia. Línea: estimaciones del modelo; asteriscos: observaciones. tions usually exhibit platycurtic pheno- same species or for the same population logical patterns, too, since their adults during successive years of study (Table diapause and hide in the soil or under I, MA column). plants and do not emerge massively. In Northern Greece long-lived Negatively skewed and symmetric phe- species are of big size and short-lived nograms are usually platycurtic. are of small size (χ2= 9.983, P= 0.001). It seems that there are no negatively Additionally, short-lived species breed skewed-leptocurtic species except for M. in autumn whereas long-lived species cartusiana that is negatively skewed and may breed in autumn or vernal-estival slightly leptocurtic. period (χ2= 4.261, P= 0.039) (Table III). Maximum activity duration is about In Northern Greece bigger helicids may the same for different populations of the breed during the vernal-estival or

29 Iberus, 15 (2), 1997

Table I. The estimated parameters of the fitted model in standard time units (months). Tabla I. Parámetros estimados del modelo ajustado en unidades de tiempo standard (meses).

Phase ϕj Phase ϕm Maximum Period T Years up Species Place Year (Months after (Months after activity period R2 (Months) to matutity January) minimum density) MA (Monthts) Helicella pappi Philippi 1987 12.6 2.9 5.9 7.1 0.93 2-3 1988 10.5 3.4 6.8 6.9 0.95 Xerolenta obvia Karvali 1990 12.6 6.7 4.7 4.4 0.79 2 Paleokastro 1988 14.2 0.3 2.9 4.2 0.84 1 1989 10.8 3.5 5.7 7.3 0.77 1990 10.4 0.2 2.8 2.9 0.91 Xeropicta arenosa Potidea 1979 12.7 6.1 5.5 5.1 0.87 1 1980 11.2 6.2 3.1 6.9 0.88 Edessa 1984 12.5 6.6 4.9 6.7 0.69 1 1985 12.3 7.8 5.8 6.9 0.68 Monacha cartusiana Edessa 1984 6.5 -2.4 3.7 3.2 0.53 1 1985 13.0 -1.1 6.2 4.3 0.45 2 Helix lucorum 1983 12.5 4.1 5.0 8.1 0.94 3-4 1984 12.0 8.4 7.9 8.5 0.84 1985 12.0 8.9 8.5 8.2 0.86

Table II. Skewness (negatively skewed <0.5; positively skewed >0.5) and curtosis (leptocurtic <0.5; platycurtic >0.5) from the phenograms of Helicoidea snails from Northern Greece). Abbreviations: Phil: Philippi; Pal: Paleokastro area; Kar: Karvali; Pot: Potidea; E: Edessa (the paranthesis means slightly). Tabla II. Desviación (desviado negativamente <0,5; desviado positivamente >0,5) y curtosis (leptocúrtico <0,5; platicúrico >0,5) en los fenogramas de caracoles helícidos del Norte de Grecia. Abreviaturas: Phil: Philippi; Pal: Paleokastro area; Kar: Karvali; Pot: Potidea; E: Edessa (los paréntesis significan ligeramente).

Positively Negatively Years up Species Place and Year Symmetric Leptocurtic Platycurtic Mesocurtic skewed skewed to maturity Helicella pappi Phil. 1987 (+) + 2-3 1988 + + Xerolenta obvia Paleo. 1988 + + 1 1989 (+) + 1990 + + Karv. 1990 + + (+) 2 Xeropicta arenosa Potid. 1979 + (+) 1 1980 + + Edes. 1984 + + 1 1985 + + Monacha cartusiana Edes. 1983 + (+) 2 1984 + + 1 1985 + (+) + 2 Helix lucorum Edes. 1983 + + 3-4 1984 + + 1985 + +

autumnal period and small ones breed of the reproductive period is very short mainly in autumn (χ2= 4.261, P= 0.039) whereas in the inland areas it is variable (Table III). In coastal areas the duration according to the species.

30 LAZARIDOU-DIMITRIADOU AND SGARDELIS: Phenology of Helicoidea in N Greece

Figure 3. Ordination of phenograms into skewness(S)- curtosis (C) plane (Skewness: negatively ske- wed< 0.5; positively skewed> 0.5. Curtosis: leptocurtic< 0.5; platycurtic> 0.5) of Helicoidea snails from Northern Greece. Numbers denote 1st, 2d or 3d generation. Abbreviations, xap: Xeropicta are- nosa Potidea; xae: Xeropicta arenosa Edessa; hl: Helix lucorum; hpf: Helicella pappi Philippi; mc: Monacha cartusiana; xip: Xerolenta obvia Paleokastro. Figura 3. Ordenación de fenogramas respecto al plano desviación (S)- curtosis (C) (Desviación: negati- va< 0,5; positiva> 0,5. Curtosis: leptocúrtico< 0,5; platicúrtico> 0,5) de babosas de la familia Helicoidea del Norte de Grecia. Los número denotan las primera, segunda y tercera generaciones. Abreviaturas, xap: Xeropicta arenosa Potidea; xae: Xeropicta arenosa Edessa; hl: Helix lucorum; hpf: Helicella pappi Philippi; mc: Monacha cartusiana; xip: Xerolenta obvia Paleokastro.

DISCUSSION breeding period starts with the first rainfalls and stops with low temperatu- In Northern Greece the climatic con- res (LAZARIDOU-DIMITRIADOU, 1981; ditions are not uniform (HATZIIOANNOU STAIKOU AND LAZARIDOU-DIMITRIADOU, ET AL., 1989). The ombrothermic 1991) and the vernal-estival breeding diagram for Northern Greece (Fig. 1) period which starts when the mean from 1980 to 1990 shows that the dry monthly temperature rises over 10°C season is from June to October, whilst and stops when the arid period starts the wet season is divided by a cold (LAZARIDOU-DIMITRIADOU, 1981; LAZA- winter period. Breeding may take place RIDOU-DIMITRIADOU AND KATTOULAS, almost in all seasons except during 1985; STAIKOU ET AL., 1988). Most of the winter. In Northern Greece, snails land snails though, mainly short-lived mainly breed from April to the end of and small snails, breed during the autumn. The strong seasonality of the autumnal period (Table III) as Helicoi- climate imposes a seasonal pattern of dea species from Southern Europe do breeding. Consequently, there are two (CHATFIELD, 1968; REAL AND REAL- main breeding periods: the autumnal TESTUD, 1983; HELLER, 1982; SACCHI,

31 Iberus, 15 (2), 1997

Table III. Life cycle characteristics from Helicoidea snails from Northern Greece. Abbreviations: D: largest shell diameter. Ehinos is found in Rhodope area, Edessa and Thessaloniki in North Central Macedonia, Philippi and Karvali near Kavala, and Paleokastro and Potidea in Chalkidiki. Abbreviations, Y: years up to maturity; SL: short lived < 3 years; LL: long lived > 3 years; SS: small sized D < 25 mm; LS: large sized D > 25 mm; V: vernal-estival reproductive period; A: automnal reproductive period. Tabla III. Características del ciclo de vida de los caracoles helícidos del Norte de Grecia. Abreviaturas: D: mayor diámetro de la concha. Ehinos se encuentra en el área de Rhodope, Edessa y Thessaloniki al Norte de Macedonia, Phillipi y Karvali cerca de Kavala, y Paleokastro y Potidea en Chalkidiki. Abreviaturas, Y: años hasta la madurez; SL: vida corta < 3 años; LL: vida larga > 3 años; SS: pequeña talla D < 25 mm; LS: gran talla D > 25 mm; V: periodo reproductivo estival; A: periodo reproductivo otoñal.

Species Locality Longitude Latitude Y SL LL SS LS V A Helix pomatia rhodopensis Ehinos 24° 58’ 34’’ 41° 16’ 50’’ 3 + + + Helix lucorum Edessa 22° 3’ 14’ 40° 47’ 47’’ 3-4 + + + Monacha cartusiana 2+ + + (rarely ) 1 + + + Bradybaena fruticum Edessa 2 + + + Cepaea vindobonensis Edessa 2 + + + Helix figulina Thessaloniki 22° 57’ 29’’ 41° 24’ 26’’ 2 + + + Theba pisana Thessaloniki 2 + + + Thessaloniki 1 + + + Eobania vermiculata Thessaloniki 2 + + + Helicella (Xerothracia) pappi Philippi 24° 15’ 48” 41° 2’ 26’ 2-3 + + + Xerolenta obvia Paleokastro 23° 25’ 9’’ 40° 24’ 59’’ 1 + + + Karvali 24° 30’ 11’’ 40° 59’ 44’’ 2 + + + Xeropicta arenosa Potidea 23° 19’ 24’’ 40° 11’ 24’’ 1 + + + Edessa 22° 3’ 14” 40° 47’ 47” 1 + + + Cernuella virgata Potidea 23° 19’ 24” 40° 11’ 24” 1 + + +

1971; BONAVITA AND BONAVITA, 1962; November (LAZARIDOU-DIMITRIADOU, DEBLOCK AND HOESTLANDT, 1967) and 1981, 1995; LAZARIDOU-DIMITRIADOU only some breed during the vernal AND KATTOULAS, 1985). period as land snails from Northern There is also a marked difference Europe do (POLARD, 1975; WOLDA AND between the snail species living along KREULEN, 1973). M. cartusiana, which is the sea shore and the inland ones. Cou- of Northern origin, breeds in both pling and laying of eggs is more or less seasons (STAIKOU AND LAZARIDOU-DIMI- synchronous for the populations living TRIADOU, 1990). Estival breeding period along the seashore and do not last more happens in places with a wet climate than a week each. On the contrary bree- during summer months (STAIKOU ET AL., ding lasts about a month for the inland 1988). In areas where different species species (LAZARIDOU-DIMITRIADOU, 1981; coexist (as in Logos area in Edessa) alt- STAIKOU AND LAZARIDOU-DIMITRIADOU, hough the climatic conditions are the 1991). same the species do not breed during The climatic conditions under which the same period provoking less antago- the land molluscs live do not only affect nistic intraspecific reactions to their hat- the time of the breeding season but also chings (STAIKOU ET AL., 1988). Semelpa- their whole life cycle and phenologies. rous species with an r-strategy synchro- X. obvia needs two years to mature in nize their breeding period with the coastal or semi-coastal areas and one favourable period which is October-mid year on the mountains (e. g. Paleokastro,

32 LAZARIDOU-DIMITRIADOU AND SGARDELIS: Phenology of Helicoidea in N Greece

Central Chalkidiki, unpublished data). hibernation of adult snails took place be- Similarly, M. cartusiana which is of nort- cause of good weather conditions which hern origin needs two years to mature started earlier than usually (STAIKOU ET in the south instead of one, because it AL., 1988: fig. 2). Additionally, this spe- has to face the summer aestivation, alt- cies has a low net reproductive rate (Ro hough 15% of its population in Edessa = 0.9) and a low annual turnover ratio tends to be semelparous (STAIKOU AND (P/B = 1.24), the snails live up to 12 ye- LAZARIDOU-DIMITRIADOU, 1990), as in ars and they mature after the third year Northern Europe (CHATFIELD, 1968). of their lives (STAIKOU ET AL., 1988). Species like B. fruticum and E. vermi- Negatively skewed and leptocurtic culata which are iteroparous and univol- phenograms could not be characteristic tine species, living for a few years, exhi- of a snail species since it would mean bit no seasonal trend of abundance but that this population would grow slowly fluctuate almost randomly during the and steadily during the favourable pe- year. These show rather stable patterns riod of the year and would introduce ra- despite the oscillations of environmental pidly growing immature snails just be- parameters. All the rest of the studied fore the adverse period. This would be Helicoidea species can encounter seaso- possible only if immature snails were nality by adjusting the timing of their vi- more resistant and tolerant to the stress. tal activities. They respond to the ad- Such a phenology has not also been re- verse period of the year, which is winter corded in acari or collembola (STAMOU time for Northern Greece, by displaying ET AL., 1993; SGARDELIS, SARKAR, ASIKI- asymmetric (positively or negatively DIS, CANCELA DA FONCECA AND STA- skewed phenograms) seasonal patterns MOU, 1993). However, M. cartusiana is a of abundance variation. These patterns case of negatively skewed and slightly reflect a differential response of the spe- leptocurtic phenology. In this popula- cies to tolerance against stress which se- tion 15% of juveniles mature in one year ems not to be region-specific but species- as it happens in Northern Europe. So, in specific. The annual or biennial r-strate- 1984 the dry season (summer time) was gists (X. arenosa, X. obvia) show a interrupted by a wet period (STAIKOU ET positively skewed phenological pattern AL., 1988: fig. 2) and this 15% of juveni- due to the rapid development of juveni- les, which had already matured, mana- les soon after the adverse winter period. ged to lay eggs before the majority of These juveniles have been hatched in au- the snails which were mature and ready tumn but stayed buried in the soil du- to copulate and lay eggs in autumn. ring winter. Their growth stops before This population, though, comes from summer dryness during which the geni- Northern Europe and it is found in the talia and the gonad maturation take southern limits of its distribution. place and the snails are ready to lay eggs To sum up, environmental variables before their death in autumn. On the in Northern Greece are strongly seaso- contrary, populations of pluriennial spe- nal and thus Helicoidea snails exhibit cies are characterized by low abundance predictable oscillations in their activity during the onset of the adverse period patterns, which can be interpreted by and a negatively skewed or symmetric the demographic response of the popu- phenological pattern. It seems that the lations as it has been found with soil growth rate of juveniles is much slower microarthropods (STAMOU ET AL., 1993; than that of annual species. The only SGARDELIS ET AL., 1993). The semelpa- species population that showed both a rous and short-lived snail species popu- negatively and a positively skewed phe- lations show a more stable phenological nological pattern was H. lucorum. Howe- pattern than the biennial and plurien- ver, a positively skewed pattern in 1983, nial ones, who mature after the first year that is a rapid growth of population den- of their life, and they are more plastic sity was probable just after the adverse trying to face the climatic differences period since a massive emergence from from one year to the other.

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