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New data on the rare snail Soosia diodonta (A. Ferussac, 1821) (Gastropoda: Helicodontidae) in Bulgaria

In the present article, we summarised all known information on the species Soosia diodonta (A. Ferussac, 1821) from Bulgaria.Anew locality in urban environment, photos of live animals and the reproductive system of the species are given. New information on its ecology and biology is provided.


Introduction
The snail Soosia diodonta (A. Ferussac, 1821) was described by Férussac in 1821 from Banat, Romania as Helix diodonta (Férussac A. & Deshayes, 1819-1851. Based on the anatomical features Hesse (1918) placed the species in a separate genus, Soosia Hesse, 1918. Presently, S. diodonta is the only member of the genus (Schileyko, 2006;Bank & Neubert, 2017). Soosia diodonta is reported for the first time for Bulgaria, after only one live specimen, by Urbanski (1964) from the Vitosha Mountains, Boyana Waterfall area. Despite the collecting efforts during the last 25 years in the area of the waterfall, the species has not been confirmed again. Georgiev & Stoycheva (2007) reported S. diodonta from a new locality in the Eastern Balkan Mountains, western of Kotel Town.
Outside Bulgaria, the species is found in the southern Carpathians, in the Western Balkan Mountains (eastern Serbia) and in the lowlands of the Lower Danube River. There are only a few known locations, summarised by Fehér (2020) In the present article, new information is given about the distribution and biology of the Bulgarian population of the species, as well as pictures of the reproductive system and live animals in situ.

Material and methods
The snail was collected from two localities in the region of the Balkan Mountains: 1) western of Kotel, road to Zelenich area (Fig. 1); 2) Tryavna, in the town, ruins (Fig. 2). The type of habitats is determined according the Council Directive 92/43/92 and the EUNIS habitat classification. Both literature and new distribution data of the species S. diodonta in Bulgaria are summarised in Table 1. The snails were hand collected and photographed in situ. Some species were killed, fixed and stored in 75% ethanol for further analyses. Abbreviation used: Coll. ID = identification number in the   Urbanski (1964). molluscs collection of the Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences, Sofia.

Distribution in Bulgaria
In Bulgaria, the species is considered to be rare. Presently, the species is known from three localities: one in the Vitosha Mountains ( Fig. 3) and two in the region of the Balkan Mountains (Table 1) (Urbanski, 1964;Georgiev & Stoycheva, 2007;present article). Despite the collecting efforts, the species was not found again in the area of the Boyana Waterfall, Vitosha Mountains. The new locality is in the town of Tryavna, in the region of the Central Balkan Mts.

Biology
The biology of S. diodonta was studied keeping specimens in terrarium (for more details about the conditions in the terrarium and the used food see Table 2). Similarly to many other gastropod species, S. diodonta is active at night. The pre-copulation habits and copulation activity were not observed. The first eggs were laid about 20 days (D. Georgiev) and 3.5 months (F. K. Glogger) after the collecting of the adult specimens from Tryavna Town (most probably, the specimens grown by D. Georgiev were taken from nature fertil-ised, while the specimens of F. K. Glogger copulated in the terrarium). The eggs were laid in clutches (3-5 eggs in groups, mostly 3-4 eggs together), adhered to the substrate. The eggs were round-elliptic (about 2.5x1.5 mm), milky-whitish, looking dotted. The period from the eggs-laying to hatching was 15-20 days (F. K. Glogger) and 22 days (D. Georgiev). The freshly hatched offspring were with 2 whorls and a size of about 2 mm. It took about 5 months for the offspring to turn into adults (Fig. 4).
Anatomy ♂♀ The atrium is relatively short and wide; ♂ penis is cylindric-fusiform, vas-deferens enters the penis apically, penial retractor relatively wide and inserts to vas deference at small distance from penis; ♀ vagina rather short, carrefour well visible. Length of spermatheca a little more than half of sperm oviduct. The investigated reproductive systems from the Balkan Mountains, after the town of Kotel, is typical for the species and match the descriptions of Schileyko (2006) (Fig. 5).

Discussion
According to Grossu (1983), S. diodonta occurs in the moist foliage of forests, on the bark of trees in rainy weather, and under the bark of rotten logs. Rarely even New data on the rare snail Soosia diodonta (A. Ferussac, 1821) (Urbanski, 1964;Grossu, 1983;Georgiev & Stoycheva, 2007). The abundant population in the town of Tryavna shows good adaptation to the new type of habitat -open terrain with numerous shelters, keeping good shadow and moisture. For Romania, Grossu (1983) considers S. diodonta a rare species with limited distribution. According to Fehér (2020), the range of this species is far from being well studied. The new finding in an urban environment may challenge the assumption about the species narrow ecological tolerance. On the other hand, S. diodonta is rare in its entire narrow range, which is bounded to well-preserved forests in general. Such type of habitats are diminishing within its potential range (Fehér 2020). Therefore, the species is assessed as Near Threatened by the IUCN. This newly discovered population is very important because it can be a good model to investigate the ecological requirements and the survival potential of this species. A possible reason for the species rarity could be its low reproduction capacity (the small number of eggs), rather than its ecological preferences (the species survives in inappropriate urban environment). In the sketch given by Grossu (1983), the vagina of the S. diodonta is too long and this drawing mistake is corrected in Schileyko (2006). The reproductive system of the Bulgarian specimens of S. diodonta is typical for the species -the mucus gland is missing and the vagina is short (Schileyko, 2006).