Notes on a population of Columella edentula (Gastropoda: Pulmonata: Truncatellinidae) at a sedge meadow in Western Poland

Columella edentula (Draparnaud, 1805) is a species of a minute land snail inhabiting moist habitats. Its biology and behavioral patterns are unrecognized. This study focuses on population dynamics of C. edentula and investigates the tendency of the species to climb up plants. The field researches were carried out at a moist sedge meadow in western Poland during two growing seasons (spring and summer of 2008 and 2009). The data analysis revealed differences in patterns of climbing behavior and population dynamics in the studied period.

The biology and behaviour of Columella edentula are poorly recognized. Based on the development of copulatory organs, three types of individuals may be distinguished in populations of the species: euphallic, aphallic and hemiphallic (Pokryszko 1987). Although both, self-fertilization and crossfertilization may occur, self-fertilization is probably more common for this truncatellid (see Pokryszko 1987). A study on the reproduction of C. edentula was done by Myzyk (2011), who recorded that in the laboratory the species laid eggs in April while hatching in the wild (at a site in NW Poland) occurred in May and at the beginning of June. Pokryszko (1990a) reported that in summer the species was found on plants such as Asarium europaeum L., Glechoma hederacea L., Cirsium oleraceum (L.) but later it occupied leaf and plant litters.
This study is focused on population dynamics of C. edentula and the tendency of the species to climb up plants in relation to the time of the year (spring and summer) was investigated. Field researches were carried out at a moist sedge meadow in western Poland during two growing seasons in 2008 and 2009.

Study site and data collection
The study site is located in western Poland in a valley of a small lowland river (52°20′09.7 N; 15°03′07.3 E; ca. 0.15 ha in area). The area was fairly open and densely covered by Carex acutiformis and Urtica dioica, the moss cover was poorly developed. Mean moisture level of the site in 2008 was 78.1% (σ = 19.4) and in 2009 it was 77.3% (σ = 15.8). Groundwater level varied from −20 cm to −5 cm during the study period, pH ≈ 7.6 (for a detailed description of the site, see Książkiewicz et al. 2013;Książkiewicz-Parulska and Ablett 2017). The meteorological conditions (mean temperature and total precipitation) in the study periods are given in Table 1.
The field study was carried out during the spring and summer seasons of 2008 and 2009. Samples were collected six times each year (three times per season). In 2008 samples were collected on 23 May, 5 June, 30 June (spring) and 9 August, 30 August, 23 September (summer); the dates of material collection in 2009 were the following: 12 May, 2 June, 26 June (spring) and 3 August, 25 August, 13 September (summer). Samples were collected along four parallel transects at 10 m intervals using a 0.25 × 0.25 m frame (0.0625 m 2 in the area). Along all transects, four samples were collected each time. All loose litter and dead vegetation cut from the soil and root-mat was harvested to a depth of 5 cm and all vegetation to ground level was collected (Cameron 2003). In the laboratory, litter and soil samples were dried and divided into two fractions using a 0.5 mm sieve. Particles passing through the sieve were checked manually for the presence of snails under a stereo microscope (for detailed methodology see Książkiewicz et al. 2013). The retained fraction and cut plants were carefully examined for snails using a magnifying glass. For further analyses, only data on the living individuals were used (snails extracted from samples were identified and classified either as alive or dead at the time of sampling based on shell wear and the presence or absence of a dried body, Cameron 1982).
To illustrate the population dynamics, snails were divided into juveniles and adults based on shell development. Individuals with a shell containing five whorls or less were classified as juveniles, and those with a greater number of whorls were classified as adults (Pokryszko 1987). Calculations of species' densities on particular days were based on all samples collected from the studied site in the given years.  Table 2). All statistical analyses were performed with the Past software.

Results and discussion
Different patterns of population dynamics were recorded in the two-year study period (Fig. 1). The maximum density of juveniles in 2008 was recorded in spring, on 5 June, what corresponds with observations presented by Myzyk (2011). A year later, in 2009, the maximum quantity of juveniles was recorded in the summer period, in August.
In both years the peak in the number of the truncatellid was preceded by extensive precipitation what may suggest the role of the weather in shaping patterns of population dynamics (see also Pokryszko 1990b). The stimulating effect of the rainfall in land snails' reproduction was previously described in other species (e.g. Pollard 1975;Tompa 1984;Silva and Omenta 2014). Comparing the two studied years, the lower mean number of C. edentula was recorded in 2009, when maximum oviposition probably occurred later in the growing season than in 2008. It could have resulted from generally less suitable environmental conditions for the species (i.e. affected by the weather) in the second year of the study. Similarly to the results presented by Cowie (1984) for Theba pisana (Müller, 1774) (the late season clutches were smaller than the early ones), I also speculate that C. edentula may have a tendency to deposit a smaller number of eggs later in a season. In simpler termsunder suitable conditions, a single individual lays more eggs earlier in a season than later in a season. The costly reproduction (in the case of minute land snails investment in egg production is high; see Baur 1989) in the period preceding winter may enhance mortality rate during hibernation (see e.g. in Nicolai et al. 2010). Additionally, these efforts may go to waste if juveniles do not achieve a critical shell size before hibernation and die in the winter time, which seems to be more likely for juveniles that hatched in late summer than for those that hatched in spring. It results from the high sensitivity of the young individuals of Vertigo to unfavourable environmental conditions (Pokryszko 1992).
The pattern of climbing behaviour at the studied site also varied between the years 2008 and 2009 (Fig. 2). The Wilcoxon signed-rank test showed that in spring and summer of 2008 more snails occurred within the litter than on plants (spring: z = 5.113, p < 0.001; summer: z = 4.251, p < 0.001) while results of Chi 2 test were statistically insignificant (Chi 2 = 1.048, p = 0.497). In the spring of 2009 significantly more snails were found within the litter (z = 3.356, p < 0.001) while in the summer the differences in the number of individuals found within the litter and on plants were statistically insignificant (z = 1.745, p = 0.089). The Chi 2 test showed that the percentage of snails on plants and within the litter was statistically different between the spring and summer of 2009 (Chi 2 = 15.022, p < 0.001). Thus individuals of C. edentula were usually more abundant within the litter than on plants during the studied period and only in the summer of 2009 the tendency for climbing was more pronounced. This way, the information provided by Pokryszko (1990a) on summer climbing of C. edentula was partially confirmed, as in some years, individuals of the species may be abundantly found on plants. The reason for such behaviour is unknown. It may be speculated that climbing up plants is related to the presence of predators (e.g. Saeki et al. 2017). The shell of C. edentula is devoid of apertural barriers which could enhance their protection against predators (Pokryszko 1997).
Also, microclimatic conditions near the ground may trigger the upwards migrations in search of more suitable conditions as it was shown for arid dwelling snail species (e.g. Cowie 1985). In such habitats, on sunny days, the surface of the ground and the adjacent air reach a higher temperature than the air above them (see in Cowie 1985). Comparing the weather in the summer of 2008 and 2009, the mean temperature was higher while the total precipitation was lower in 2009 (see Table 1). It might have contributed to the deterioration of microclimatic conditions near the ground and forced snails to climb up the plants. Such a situation most likely concerned the least moist microhabitats at margins of the studied site where vegetation was less compacted and individuals of 588 Biologia (2020) 75:585-590