INTRODUCTION
Terrestrial gastropods (Mollusca: Gastropoda) are an important group among the invertebrates, constituting the second largest invertebrate taxa with about 24,000 species (Lydeard et al. 2004) with extensive variations in their shape, size, body colouration and habitat utilisation patterns (Raheem & Naggs 2006, Ramakrishna et al. 2010). Kaliella barrakporensis (Reeve, 1852) (Gastropoda: Chronidae) is one among many “micro-snails”, with shells less than 5 mm maximum dimension (Panha & Burch 2005), native to India. It is also found in other countries around the Indian Ocean, and in hothouses elsewhere (Blanford & Godwin-Austen 1908, Herbert & Kilburn 2004, Verdcourt 2006, Preece & Naggs 2014, Sow-Yan & Lau 2020, Nurhayati et al. 2024). It is typically a species of damp, shady microhabitats such as moist spaces in gardens, detritus, under logs, stones, or other debris. While its distribution and plant preferences are well known (Barman & Aditya 2021), there is little or no information on biological characteristics, including morphology, life-history and reproductive traits of this micro land snail.
While it is not a threatened species, its vulnerability to microhabitat changes, coupled with its ability to spread, passively, beyond its natural range (Barman & Aditya 2021), make it a target for understanding its life history, reproductive capacity and population structure. In this study, we aimed to describe the life history traits and life table parameters based on laboratory observations on K. barrakporensis. As factors such as habitat destruction, climate change, and human activities can significantly impact these traits and the survival of micro snail populations, understanding life history traits is crucial for developing effective conservation strategies and ensuring the sustainability of ecosystems where these snails play a substantial role. In addition, this study addresses a critical knowledge gap by providing baseline information on the biology of K. barrakporensis, which has remained poorly studied to date.
MATERIAL AND METHODS
SPECIMEN COLLECTION AND REARING
The micro snail, K. barrakporensis, was collected from the gardens in Ballygunge campus, University of Calcutta, India (22°31'37"N, 88°21'46"E), during the rainy season. The specimens were collected by handpicking and with a wet paintbrush early in the morning and late in the evening. The collected specimens were placed in sample containers (100 ml volume, Tarsons®, India) with moist tissue paper to maintain the higher humidity. The specimens were identified by their external morphological characters (Mitra et al. 2004). The adult snails had thin, pyramidally trochiform, brownish, translucent shells, which were narrowly perforate with six whorls, the last of which has a peripheral keel that is not descending (Mitra et al. 2004). The field collected snails (n = 50) were kept in 5 rearing plastic containers each with 10 snails (6 × 6 × 8 cm, Tarsons®, India) in the laboratory, which were prepared by placing humid soil with a sufficient amount of coco peat at the base. Throughout the rearing period, the snails were fed ad libitum with cucumber slices, and the food and faeces were removed from the container at 24-hour intervals. The rearing containers were kept at ambient temperature (25–27 °C), and the containers were kept moist by spraying the requisite water each day. The field-collected K. barrakporensis (F0) began producing hatchlings after a few days of acclimatisation in the laboratory. The hatchlings were collected and transferred to new rearing containers for the subsequent generations (generation F1 and F2 respectively). The progeny of F2 (F3) were used in the study.
EXPERIMENTAL DESIGN
The < 1-day-old hatchlings of K. barrakporensis (F3) were considered to estimate the life history traits and life table parameters of K. barrakporensis (F3 hatchlings were used to ensure that all individuals originated from laboratory-acclimatised lineages, thus minimising any possible bias due to field-collected parental stress). Ten cohorts with a fixed number of hatchlings (n = 10) were set up in the conditions mentioned earlier. The snails were fed with sliced cucumber, and the containers were cleaned regularly to remove the faecal material and dead individuals.
The shell height and body mass of randomly selected 30 individuals (n = 30; 3 from each of the 10 replicates) were measured using a stereo microscope using an ocular micrometre (Erma®, Japan) and an electronic balance (Afcoset®, India) to the nearest 0.1 mg, respectively, after every two weeks, since intitation of the experiment. The data on the life history traits, such as the age at sexual maturity (in days), fecundity and mortality, were recorded till the death of all individuals in all cohorts.
DATA ANALYSIS
The mean (±SE) shell height (mm) and body mass (mg) of K. barrakporensis at every two-week intervals were assessed by descriptive statistics, and the growth pattern was estimated by fitting the shell height and body mass to the age of K. barrakporensis.
The age-specific survivorship (lx) and life expectancy (ex) were estimated using the formulas:
and
respectively, where lx = proportion of the individuals that survived the age x, nx = number of individuals at age x, n0 = total number of individuals at the start of the experiment, ex = life expectancy at age x and LX = (nx + nx−1) / 2) (Stiling 1996, Krebs 1999, Smith & Smith 2001).
The fecundity (mx) was measured using the observed number of hatchlings by snails in laboratory conditions each week, and the age-specific fecundity table was calculated by estimating the following life history characteristics. The net reproductive rate (R0), i.e., the average number of offspring produced by individuals during their life was estimated using the following formula:
where mx is the total number of hatchlings produced by K. barrakporensis. The cohort generation time (Tc) was calculated as:
and the intrinsic rate of increase (rm) was calculated as:
(Smith & Smith 2001). Additionally, the age-specific reproductive values (Vx) were calculated as:
where y is the age of all individual snails passing through from age x (Smith & Smith 2001), and the finite rate of increase (λ) was estimated as λ = er.
RESULTS
The growth pattern of K. barrakporensis (in terms of shell height and body mass) was found to be asymptotic, as growth rate was rapid initially (till 7–9th week), and then became slower, becoming negligible in the last weeks of life (Figs 1–2). The mean (± SE) shell height and body mass of K. barrakporensis were 1.56 ± 0.03 mm and 1.34 ± 0.07 mg at the age of 2 weeks and 2.71 ± 0.1 mm and 9.75 ± 0.89 mg at the age of 20 weeks.
Figs 1–2
Graphical representation of changes in growth of the snail Kaliella barrakporensis in terms of shell length (mm) (1) and body mass (mg) (2)
The maximum life span of K. barrakporensis was observed to be 22 weeks under laboratory conditions. The mortality rate was higher at the younger stage, while the mortality rate slowly decreased at the older stage. The survivorship (lx) and life expectancy (ex) of K. barrakporensis is depicted in Fig. 3. The age of K. barrakporensis at attaining sexual maturity was 44 ± 0.88 days. During the laboratory rearing, no eggs were seen, but hatchlings were present in all rearing containers, which indicated that this species is either viviparous or ovoviviparous. During their life cycle, K. barrakporensis had a 6 week pre-reproductive period, 10 ± 0.66 weeks reproductive period, and 1 week post-reproductive period. The age-specific fecundity (mx) varied with age, ranging from 0 to 2.3 hatchlings per individual, and the maximum number of hatchlings were laid in the 8th week (Fig. 4), which were 2.39 ± 4.74 and the minimum number were laid in the 18th week, which were 0.3 ± 0.3. The total number of hatchlings laid by all the individuals was 802. The reproductive values (vx) were maximum in the 7th and 8th weeks and declined until their death (Fig. 4). The net reproductive rate (R0), generation time (Tc in weeks), finite rate of increase (λ) and intrinsic rate of increase (rm) of K. barrakporensis were 8.54 ± 1.46, 10.89 ± 0.3, 0.11 ± 0.03, and 1.1 ± 0.02, respectively (Table 1).
Fig 3
The survivorship (lx – the proportion of surviving individuals) and life expectancy (ex) of Kaliella barrakporensis, reared under laboratory conditions
DISCUSSION
The life history strategies of snails exhibit significant variations in terms of growth, survival, and reproduction due to the changing environments and different modes of reproductive strategy (Dillen et al. 2009, D’ávila et al. 2018). Due to their limited mobility and specific habitat requirements, micro snails are effective bioindicators of environmental changes and anthropogenic impacts (Douglas et al. 2013, Gheoca et al. 2021). Therefore, evaluating the life history traits of micro land snails is crucial for understanding the ecosystem health (Hodges & McKinney 2018), as well as biodiversity and conservation strategies (Sen et al. 2012). In the Indian context, life history patterns of various land snails and slugs, including Allopeas gracile (Subba Rao et al. 1981, Nandy & Aditya 2022a, 2022b), Cryptaustenia ovata (Chakraborty et al. 2024), Achatina fulica (Raut & Barker 2002, Sarma et al. 2015, Kumar et al. 2021), Glessula gemma (Subba Rao et al. 1985, Raut & Mitra 1985), Succinea daucina (Raut et al. 1997), Semperula birmanica (Panigrahi 1995, 1998), Laevicaulis alte (Nagabhushanam & Kulkarni 1971), Mariella dussumieri (Barman et al. 2022), and Deroceras laeve (Gupta et al. 2024) were observed. In the global context, several studies were conducted on the observations of life history traits of micro land snails, including Lauria cylindracea (Heller et al. 1997), Habroconus semenlini (Silva et al. 2009) and several vertiginid micro snail species (Myzyk 2011). However, apart from the taxonomy and distribution of micro land snails in India (Aravind et al. 2008, Barman & Aditya 2021, Barman et al. 2023), little or no information is available regarding their life history traits.
In the present study, K. barrakporensis showed an indeterminate growth pattern, which is similar to other land snails such as Subulina octona (D’ávila et al. 2018), H. semenlini (Silva et al. 2009), A. gracile (Nandy & Aditya 2022a, 2022b), and C. ovata (Chakraborty et al. 2024). Indeterminate growth in snails refers to a growth pattern where individuals continue to grow after sexual maturity (Heller 2001). The growth of K. barrakporensis was rapid till the 8th week and then gradually slowed to a steady state until all individuals died. Compared to another micro land snail, H. semenlini (Silva et al. 2009), which had an average shell height of 2.51 ± 3.69 mm, K. barrakporensis has 3.05 ± 0.09 mm of shell height at the adult stage.
This study shows K. barrakporensis has a short life span of around 22 weeks, while the life span of other Indian land snails, such as A. gracile (Nandy & Aditya 2022a, 2022b) and C. ovata (Chakraborty et al. 2024), was observed to be 27–53 weeks and 29 weeks, respectively. Despite the absence of egg-laying in K. barrakporensis, hatchlings were observed in every rearing container, suggesting that the species is either viviparous or ovoviviparous. However, histological examinations of the species are required to confirm the reproductive strategy (Heller 2001). In the case of K. barrakporensis, sexual maturity was observed around the 7th week, similar to H. semenlini, A. gracile and S. octona, which showed their first oviposition on about the 8th, 4th and 8th weeks, respectively (D’ávila et al. 2018, Nandy & Aditya 2022a, 2022b). In contrast, a few terrestrial snails took a much longer time to attain sexual maturity, such as Bradybaena similaris (Carvalho et al. 2001), Archachatina marginata (Egonmwan 2004) and Limicolaria flamea (Egonmwan 2004), which need 160 days, 19 months, and 5 months to attain sexual maturity, respectively.
The results of the present study indicate that K. barrakporensis has only one reproductive event, has higher mortality at the juvenile stage, and produces live young rather than laying eggs. Considering all observed traits, the present study indicates that K. barrakporensis is an r-strategist species. Similarly, the life cycle of K. barrakporensis in terms of longevity, survivorship and reproductive period indicates that it is a semelparous species. Several studies showed the semelparous nature of land snails, such as Arion vulgaris (Rabitsch & Essl 2006), Habroconus semenlini (Silva et al. 2009), B. similaris (Carvalho et al. 2008), in contrast with the other iteroparous land snails, such as Cornu aspersum (Nicolai et al. 2010) and Allopeas gracile (Nandy & Aditya 2022a, 2022b). These life history traits of K. barrakporensis, when combined with its adaptability to modified habitats, reinforce its invasive potential and justify the need for closer ecological monitoring in its non-native regions.
Considering the potential effects of rapid environmental changes on the adaptive capacities and survival of micro land snails, which may lead to population declines or local extinctions, understanding their life history traits may help to protect their native habitat and develop targeted management strategies that enhance the survival of vulnerable species. Further studies can be carried out to observe the changes in the life history traits of K. barrakporensis under various food, density, and temperature conditions.
