GROWTH RATE OF CHILOSTOMA FAUSTINUM (ROSSMÄSSLER, 1835) (GASTROPODA: PULMONATA: HELICIDAE) UNDER NATURAL CONDITIONS

Growth of Chilostoma faustinum (Rossmässler, 1835) was recorded under natural conditions in Romincka Forest (NE. Poland), one of the northernmost localities of the species. During the growing season (April–October) the average growth rate was 0.22 whorl/month. In the local conditions Ch. faustinum needs three growing seasons to complete its shell growth. Shells of Ch. faustinum from Romincka Forest are slightly smaller than in the other studied populations: their average width is 16.5 mm (range 13–19 mm); and the average number of whorls is 4.5 (range 4–5).


RESEARCH AREA
The research was conducted in Romincka Forest (NE.Poland), which is a compact forest of ca.360 km 2 situated on both sides of the Polish-Russian border north-west of Suwa³ki.It is a part of the East European Lowland and belongs to the Lithuanian Lakeland macroregion (KONDRACKI 2000).In the geobotanical division of Poland Romincka Forest belongs to the North Section -Augustowski-Suwalski land (MATUSZ-KIEWICZ 2007).This area is characterised by the most severe climate in the Polish lowlands, with the lowest mean annual temperature (6.2°C), the shortest growing season: number of days in the year with the mean temperature over 5°C (194 days), the highest number of days in the year with the maximum temperature under 0°C (66 days), the highest number of days in the year with snow-cover (100 days) (MATUSZKIEWICZ 2007).
The topography of the area has been shaped by the Pleistocene glaciations, leaving an undulating surface between 150 and 300 m a.s.l., which gives rise to a great variety of soil types dependent both on underlying deposits and drainage.Typical dry-ground forest (Tilio-Carpinetum) is abundant on better drained land, and swampy forest is found in badly drained areas.Most of the forest is affected by human activity, and managed conifer forests now dominate.The Polish part of the forest is protected within Natura 2000 (Special Area of Conservation PLH 280005) and as the Romincka Forest Landscape Park.
So far, three localities of Ch. faustinum have been found in Romincka Forest, in valleys of small streams in riparian forest.The research plot covered a part of an ash-alder forest Fraxino-Alnetum in the Du¿y Budier stream valley.The stand consists mainly of alder trees ca.60 years old, with single birch, spruce and oak trees.The understory is formed by hazel.The ground cover layer is abundant and dense, with dominant nettle Urtica dioica.Detailed information on mollusc communities of Romincka Forest is presented elsewhere (MARZEC 2010).

MATERIAL AND METHODS
About 50 individuals of Ch. faustinum were caught at the beginning of each month from August 2005 to October 2007.The snails were collected from plants and soil surface, in a plot of ca.400 m 2 .Previous studies showed no effect of shell painting on growth rate (BAUR 1984), thus all the individuals were marked with nail varnish: a transverse stripe was painted on the shell near its aperture so that it marked the current size of the shell and the increment could be read on recapture.Each month a different colour of varnish was used.The whorls of each snail were counted to the nearest 0.1 whorl (WIKTOR 2004), and the shell width was measured with a ruler to the nearest 1 mm.After marking, the snails were always released in the same place in the centre of the research plot.No observations were conducted in winter, when the plot was snow-covered.Live Ch. faustinum were found in the plot from April or May to October.In March and in November, even during warm and snowless years, no live snails were found.
In total, 692 individuals of Ch. faustinum were marked and 71 were re-captured (10.3%) (Table 1).Among these, 62 individuals (87.3%) were re-captured once, five (7%) -twice and four (5.6%) -three times.The material for morphometric studies included the shells of live snails (n = 692) and fresh empty shells found in the plot (n = 122).Individuals with completed lip were regarded as adult.

GROWTH RATE
Under natural conditions the average growth rate of Ch. faustinum during the growing season was 0.22 whorl/month (Table 2).Individuals of different size classes did not differ in growth rate (One-Way ANOVA, F = 0.31; df = 2, 49; p = 0.73).During winter the growth was limited (Fig. 1).The overall mean annual growth rate, including winter, was 0.12 whorl/month (growth rate of one individual observed precisely for one year).
Under the local conditions Ch. faustinum needs three growing seasons to complete the shell growth.Depending on the year, the snails here are active for six to seven months, and with the observed mean growth rate (0.22 whorl/month) the shell can be completed within 14 months of activity (Fig. 2).The three year period of snail's growth is also confirmed by the analysis of growth of individuals observed for a year or longer.For example, an individual observed for the longest time (14 months) was marked in April 2006 (2.7 whorls) and re-ceptured in June 2007 (4.4 whorls, shell growth still not completed).This individual could not hatch in the spring 2006, because before it was caught in April, it may have been active for at most one month, which is not enough to achieve the observed size even assuming the maximum observed growth rate.Thus, that individual hatched in 2005 and probably completed its growth in the autumn 2007, in the third year of its life.
No clearly distinguishable cohorts were observed in the studied population of Ch. faustinum; individuals of all size classes were found during all the observation months (Fig. 3).

SHELL SIZE DISTRIBUTION
The shells of Ch. faustinum from Romincka Forest reach 13 to 19 mm in width (mean 16.5, SE ± 0.091, n = 157), with the number of whorls ranging from 4 to

DISCUSSION
Snails of the subfamily Ariantinae from lowland populations reach maturity usually within one or two years (GASSIES 1849, GERMAIN 1930, FRÖMMING 1954).Under natural conditions the growth rate of land snails often depends on climate conditions.In localities characterised by a shorter growing season and lower temperature more time is needed to complete shell growth (UMIÑSKI 1975, TERHIVUO 1978, SULIKOWSKA-DROZD 2010, 2011).In the case of Alpine populations of Arianta arbustorum, it can take as long as five years (BAUR & RABOUD 1988).In the Mediterranean region analogous slowing down of growth resulting in an extended growth period is caused by heat and drought (LAZARIDOU & CHATZIIOANNOU 2005, KISS et al. 2005).
The average growth rate, together with the analysis of individual growth histories, indicates that under the conditions of Romincka Forest growth of Ch. faustinum takes three years.Although in theory, at the average growth rate, it is possible for the snail to complete shell building within two years (14 months of growth with two growing seasons of seven months each), yet under the actual conditions it is unlikely.The time necessary to incubate eggs should also be considered, and in most helicid snails it is two to three weeks (FRÖMMING 1954, TERHIVUO 1978, HELLER & ITTIEL 1990, MALTZ 2003, KISS et al. 2005).Development rate depends on the temperature: the lower it is, the slower the embryo develops and hatching occurs later (TOMPA 1984).Under particularly favourable conditions and with the maximum growth rate it might be possible for some individuals to complete shell building within two growing seasons, however, there are no indications that Ch. faustinum can complete its growth within one season.
The growth model presented in The growth rate in some members of Helicoidea is irregular, faster during the early development stages, when the shell is still small, and slower at the later development stages (TERHIVUO 1978, MALTZ 2003, KUZNIK-KOWALSKA & ROKSELA 2009).A similar tendency was observed for Ch.faustinum (Table 2), however, possibly due to too small sample sizes, the differences were not statistically significant.
The shells of Ch. faustinum from Romincka Forest are relatively small, compared to the previously studied populations (Fig. 5).
Shells smaller than average were found for example in high-altitude populations of Arianta arbustorum (BAUR 1984), Vestia turgida (SULIKOWSKA-DROZD 2001), and Vestia gulo (SULIKOWSKA-DROZD 2011), as well as in a population of Chilostoma banaticum (DOMOKOS 2001) living in unfavourable conditions; the authors explain this by harsher climate conditions in these localities: mainly shorter growing season and lower temperature.In Romincka Forest, characterised by the most severe climate in the Polish lowlands (MATUSZKIEWICZ 2007), the effect of climate conditions on the ultimate shell size of Ch. faustinum is analogous.The shell size may also be influenced by other factors.Shells of Arianta arbustorum from populations of high density were by 1.5 mm smaller than those from low-density populations (BAUR 1988).
Chilostoma faustinum is considered to be a mountain species (KERNEY et al. 1983, RIEDEL 1988, WIKTOR 2004, WELTER-SCHULTES 2012).Until the end of the 20th century an isolated site of Ch.  2002).Therefore, its presence in the lowlands was regarded as a result of accidental introduction (SOÓS 1948, WIKTOR 2004).At present also other lowland sites of Ch. faustinum are known (MARZEC 2005, SVERLOVA & GURA¼ 2005).It indicates that in the past the species' range may have been much larger and the locality in Romincka Forest may have a relic character.It is highly probable that Ch. faustinum is a species of primary forests from periods of colder climate.This can be concluded from its current localities, which are mainly well-preserved fragments of moun-tain forests (SZYBIAK 2000, DELI 2002, HLAVAÈ 2002, SZYBIAK et al. 2005, JUØIÈKOVÁ et al. 2005, 2006, KAPPES et al. 2006, SULIKOWSKA-DROZD & HORSÁK 2007).Unfortunately, so far the species has not been found in Bia³owie¿a Forest (CAMERON & POKRYSZKO 2004) to further support this hypothesis.

Fig. 3 .
Fig. 3. Chilostoma faustinum: shell size distribution among individuals caught in consecutive months of the study

Fig. 4 .
Fig. 4. Chilostoma faustinum: width of shells with various numbers of whorls (mean and range).Black bars show the proportion of adults in each size group

Table 1 .
Chilostoma faustinum: number of whorls of re-captured individuals with dates of sampling

Table 2 .
Chilostoma faustinum: growth rate of individuals of different size (April to October) Fig. 1.Chilostoma faustinum: growth of selected individuals, for which the observation time included winter Fig. 2. Chilostoma faustinum: model of growth under natural conditions; black line: during growing season mean growth of 0.22 whorl/month, outside growing season without growth; grey line: growth rate of 0.12 whorl/month throughout the year.Average adult size (4.5 whorls) marked