Sexual dimorphism in shell size of the land snail Leptopoma perlucidum (Caenogastropoda: Cyclophoridae)

Collection of specimens

We collected living L. perlucidum adults (shells with reflected lips), as snails without lips are likely to be immature from two islands, Tiga Island (N5°43′28″, E115°39′20″) and Gaya Island (N6°1′3″, E116°1′50″), both located at the west coast of Sabah, between January and August 2016. Sabah Parks approved this work (permit no. TS/PTD/5/4 Jld. 54 (112)). The snails were drowned until the animal’s soft body fully extended so that the sexes could be determined and examined under a stereomicroscope as described by Jonges (1980) (Figs. 1B and 1C). A total of 84 L. perlucidum specimens were collected: nine females and 12 males from Gaya Island, and 33 females and 30 males from Tiga Island. All specimens were deposited in the BORNEENSIS collection at the Institute for Tropical Biology and Conservation, Universiti Malaysia Sabah (BOR/MOL 6651, 6653-6654, 7873-7875, 7877, 8739-8748, 8750-8754, 8756-8784, 8786-8794, 8797-8802, 8805-8809, 8813, 8816-8817, 9444-9447, 9449-9451, and 9726-9728).

Shell morphology data

Each shell was photographed from the shell’s aperture view using a Leica Image Analyzer M205. The shell size and shape measurements were obtained using the photographs. The shell size consisted of five linear measurements, measured to the nearest 0.1 mm: aperture height (AH), aperture width (AW), shell height (SH), shell width (SW), and spire height (SpH) (Fig. 1A). The shell shape was quantified using nine landmark coordinates corresponding to the dimensions of the different parts of the shell and recorded with ImageJ (Fig. 1A). The measurements and landmarks were selected based on the traits used for taxonomic purposes (Vermeulen, 1999; Vermeulen & Liew, 2022). Furthermore, these landmarks correspond to some of the landmarks used by Larsson et al. (2020) as developmentally descriptive shape parameters to simulate the three-dimensional logarithmic spiral growth of the shell.

Data analysis

Sexual dimorphism in shell size

All five boxplots showed a difference across all shell size measurements between male and female L. perlucidum (Fig. 2). Both frequentist and Bayesian t-tests showed significant effect of sex on shell measurements, especially with respect to SH and SW (Table 1). The power of all the tests was above 0.8 for all shell measurements of both land snail populations, with the exception of AW for the Gaya Island population (power = 0.6). There were significant differences across all five measurements between female and male shells for both locations as revealed by the frequentist t-test (all with p-values <0.05) and the Bayesian t-test (BF inclusion >1). The female shell appeared to be larger than the male shell across all five measurements (Table 2, File S2).

Sexual dimorphism in shell shape

A total of 54.8% (PC1–31.8%, PC2–23%) of the shape variation in the nine-landmark coordinate dataset was explained by the first two principal components (Fig. 3). The thin-plate spline (TPS) transformation grids showed that the main source of the shape variations in the morphospace were from the aperture shape, i.e., landmarks 5, 6, 7, 8, and 9. The scatterplot showed significant overlaps between the sex and location of L. perlucidum in their shell shape. There were no significant differences between the shell shapes of male and female L. perlucidum from the two different locations (Procrustes ANOVA—Sexes: df = 1, F = 1.5247, p = 0.150; locations: df = 1, F = 1.6907, p = 0.117; sex and location interaction: df = 1, F = 0.4359, p = 0.879) (File S3). Shell images of all specimens used in this study can be found in File S4.

Sexual dimorphism in L. perlucidum shell form

This is the first investigation into sexual dimorphism in shell size and shape in the Cyclophoridae. This species exhibits sexual dimorphism in shell size but not in shell shape. Females have larger shells than males despite shell size variations. To date, only a handful of land snail studies have investigated the size variations between both sexes, and all studies showed that females are more slender and larger than males (Gofas, 2001; Reichenbach, Baur & Neubert, 2012; Páll-Gergely et al., 2020). Although females can be at least 6% (∼1mm) larger than males in the same L. perlucidum population, this may not be noticeable even when comparing the shells of both sexes side by side. Páll-Gergely et al. (2020) pointed out that these differences can only be revealed when rigorous statistical methods are used, as is the case in most other studies.

On the other hand, there is an abundance of sexual dimorphism studies on the shell form of aquatic and marine snails. These studies overwhelmingly show that females are larger than males (Jokinen, Guerette & Kortmann, 1982; Son, 1997; Estebenet & Cazzaniga, 1998; Kurata & Kikuchi, 2000; Son & Hughes, 2000; Minton & Wang, 2011; Ng et al., 2019). However, there has been a few exceptions in which males were larger than females (Kurata & Kikuchi, 2000) or where there was no sexual dimorphism in shell size (Tripoli et al., 2015; Vaux et al., 2017). In addition to the overall shell size in terms of total height and width, some studies also showed that females had a larger aperture size compared to males (Cazzaniga, 1990; Son, 1997; Minton & Wang, 2011), or that the shells of females were more globose than males’ (Jokinen, Guerette & Kortmann, 1982; Keawjam, 1987; Son & Hughes, 2000; Minton & Wang, 2011).

Currently, the biology, specifically the mating process, of the Leptopoma species remains unexplored. Therefore, we can only hypothesise that females have larger shells than males because of the different morphologies of their reproductive systems. In studies on other taxa, it has been hypothesised that sexual dimorphism in caenogastropod shells (females either have a larger shell size or a more globose shell) appears to be primarily related to female fecundity or egg mass production (Collin, 2018). Females generally have larger sized shells, especially in the last whorl, resulting in a higher shell volume for egg deposition (Gofas, 2001).

Studies on other taxa found that the driving forces for sexual dimorphism in shell form include sexual selection and the two sexes’ different growth rates (Jokinen, Guerette & Kortmann, 1982; Kurata & Kikuchi, 2000; Riascos & Guzman, 2010). The underlying driving forces of sexual dimorphism in the shell size of L. perlucidum need to be similarly investigated in future studies with more field observations and experiments to determine whether females are larger than males in mating pairs (e.g., Ng et al., 2019) and the growth rate of both sexes.

The implications to taxonomic studies on the genus Leptopoma

Future taxonomic research to resolve the status of the Leptopoma species would need to take into account the variability of shell size caused by sexual dimorphism and geographical variations (Phung, Heng & Liew, 2017). There are 125 valid names for Leptopoma species, and there are another 154 invalid names due to various nomenclature reasons (MolluscaBase, 2021). The last intensive review, conducted by Kobelt (1902) on 105 Leptopoma is outdated, and subsequent taxonomic works have mainly dealt with a handful of species in relatively small geographic areas (Gude, 1921; Vermeulen, 1999; Phung, Heng & Liew, 2017). There has been no quantitative comparison of shell shapes among species, except Phung, Heng & Liew (2017). The species diversity hotspot is in the Philippines archipelago, where many species occur sympatrically over thousands of islands (GBIF, 2021). Our study shows that in L. perlucidum at least, no shape differences (which would further complicate a taxonomic revision of the genus) are found between males and females.