A review of the microgastropod genus Systenostoma Bavay & Dautzenberg, 1908 and a new subterranean species from China (Gastropoda, Pulmonata, Hypselostomatidae)

Abstract A review of the microgastropod genus Systenostoma is provided. Thai and Malaysian species are transferred to a new genus, Angustopila (type species: Systenostoma tamlod Panha & Burch, 1999). A new subterranean Angustopila species is described here. Conchologically, the new species is most similar to the cave-dwelling, Thai A. tamlod (Panha & Burch, 1999). One Thai species (Systenostoma edentata) is transferred to the genus Hypselostoma. Vietnamese members remain in the genus Tonkinospira (nomen novum) for Systenostoma Bavay & Dautzenberg, 1908 (non Systenostoma Marsson, 1887). A comprehensive map of former Systenostoma species is presented. SEM and NanoCT images, including a video of A. huoyani sp. n. internal shell morphology, provide novel perspectives of the shells of Angustopila and of the scarcely known Vietnamese Tonkinospira species. The biology of these snails is not yet known but collection localities suggest a troglophilic ecology.


Introduction
Microgastropods are less than 5 mm in size and represent the majority of worldwide tropical land snail diversity. They are restricted to specific microhabitats such as limestone rock surfaces or caves, have limited active dispersal ability and thus, frequently demonstrate high local endemism. Due to their small size and high degree of endemicity, our knowledge of the taxonomy and ecology of microgastropod taxa such as the hypselstomatid genus Systenostoma is limited. Consequently, very little is known about tiny species and thus, the complex systematics of most tropical microgastropod groups is based on conchological characters only.
The microgastropod genus Systenostoma was established as a subgenus of Helix by Bavay and Dautzenberg (1908) for two Vietnamese species described in the same paper, namely Helix (Systenostoma) pulverea and H. (S.) pauperrima. The diagnosis of Systenostoma however, was given in another paper published a year later (Bavay and Dautzenberg 1909). The third Vietnamese species was described as Systenostoma defixa Bavay & Dautzenberg (1912). After these descriptions, Systenostoma remained in the dark for almost four decades. Jaeckel (1950) described Angustopila depressa from the debris of an unknown Tonkinese (Northern Vietnam) tropical river. Therefore, knowledge of the distribution and ecology of this species is lacking. More recently, four species of Systenostoma were described from Thailand, namely S. concava and S. elevata by Thompson and Upatham (1997) and S. edentata and S. tamlod by Panha and Burch (1999). The genus was also reported from Malaysia, but the systematic position of the Malaysian samples is not yet clarified. Although Benthem-Jutting's (1949) figure of "Hypselostoma laidlawi" (Fig. 9) and her Paraboysidia neglecta (Benthem-Jutting, 1961) are similar and represent the same species according to her explanation, these illustrations likely show two different species (see Panha and Burch 1999).
The classification of Systenostoma is problematic. Together with the suspected, closely related genera (e.g. Acinolaemus Thompson & Upatham, 1997, Anauchen Pilsbry, 1917, Boysidia Ancey, 1881, Hypselostoma Benson, 1856, Krobylos Panha & Burch, 1999, Gyliotrachela Tomlin, 1930, Systenostoma is sometimes classified within Pupillidae (e. g. Panha and Burch 1999) or Vertiginidae (e.g. Thompson and Upatham 1997). These related genera are classified within Hypselostomatidae by Schileyko (1998). After examining the type species of the genus, Schileyko (1998) concluded that unlike these other genera, Systenostoma probably does not belong to Hypselostomatidae, but rather, likely belongs to Helicodiscidae because of the characteristic spiral sculpture. Later, Schileyko postulated that the genus is possibly related to Aulacospira as considered by Pilsbry (1917) or to Pupisoma (Valloniidae) (Schileyko 2011). Before the description of the Thai species, previous diagnoses (Bavay and Dautzenberg 1909, Pilsbry 1917, Zilch 1959 described the genus as a taxon lacking apertural dentition. Thompson and Upatham (1997) claimed that Systenostoma species bear no teeth and that a low fold may be present on the parietal wall. Moreover, they described the sculpture as "dense mesh of very fine granular reticulation superimposed upon which are fine spiral threads. Spiral sculpture may be present or absent on the protoconch". Thompson and Upatham (1997) hypothesized a close relationship between Systenostoma and Acinolaemus based on the likely, homologous parietal lamella and similar protoconch sculpture.
All Systenostoma have a relatively simple shell compared to the other members of the family Hypselostomatidae, whose shells usually have oddly coiled shapes and multiple apertural denticles. Still, Systenostoma species show a high diversity in general shell shape, aperture shape and dentition and shell sculpture. Systenostoma seems to represent a "collection bin" taxon for species possessing few or no denticles. The reduction of the apertural teeth however, could have evolved independently in different evolutionary lineages. In this case, congeners may well have been classified/lumped within one genus but systematically belong to at least three genera. In the following, we describe a new, subterranean species from China, present an overview of all former Systenostoma species, describe a new genus for Thai, Malaysian and the new Chinese species, transfer S. edentata to the genus Hypselostoma and assign a new name (Tonkinospira) to the Vietnamese species because the name Systenostoma (non Systenostoma Marsson 1887) is preoccupied. We present SEM and Nano-CT images of shells of the new species and the scarcely known Vietnamese members of the genus.

RBINS
Royal Belgian Institute of Natural Sciences (Brussels, Belgium) SMF Senckenberg Forschungsinstitut und Naturmuseum (Frankfurt am Main, Germany) MCSMNH Malacological collection of the Slovenian Museum of Natural History (Ljubljana, Slovenia) SMNS Staatliches Museum für Naturkunde Stuttgart (Stuttgart, Germany) Image acquisition SEM: One paratype of Angustopila huoyani sp. n. was mounted on an aluminium stub, gold-palladium sputtered using the Edwards Kniese Sputter Coater S150B (Marburg, Germany) and subsequently scanned on the CamScan CS 24 scanning electron microscope (Dortmund, Germany). Specimens of Tonkinospira nom. n. were non-coated and imaged with the Zeiss EVO LS15 scanning electron microscope (Jena, Germany) using the Variable Pressure (VP) mode. Micro-CT: Tonkinospira nom. n. species were imaged using a nano-computed tomography system (nano-CT), manufactured and developed by Bruker-Micro-CT/SkyScan (SkyScan 2011, Kontich, Belgium) at the Department of Experimental Radiology, Justus-Liebig University Biomedical Research Center Seltersberg (BFS), Giessen, Germany. The scanner is based on a nanofocus tube generating X-rays in cone-beam geometry. Briefly, the system contains an open pumped type X-ray source, a LaB6 cathode and a transmission anode consisting of a tungsten-coated beryllium window. Enhanced edge sharpness is gained by a high-focussed X-ray spot of 300 nm side length (see Langheinrich et al. (2010) for more details). Specimens of Tonkinospira nom. n. were mounted on a computer-controlled stage. They were then scanned 180° around their vertical axis in rotation steps of 0.2° at 80 kV tube voltage and 120 µA tube current. Reconstruction of cross sectional images was performed using a modified Feldkamp cone-beam reconstruction algorithm. Image resolution of the cross sectional images was 1 µm isotropic voxel side length with a grey scale resolution of 8 bit. The video of Angustopila huoyani sp. n. was created using a SkyScan 1172 scanner at RJL Micro & Analytic GmbH, Karlsdorf-Neuthard, Germany. The scanner is equipped with a sealed micro focus x-ray source and a 11 Mpx CCD detector. The specimen was scanned with 4 µm voxel size in rotation steps of 0.6° at 59 kV tube voltage and 167 µA tube current. Reconstruction with cross sectional images followed the same aforementioned, cone-beam reconstruction algorithm. Image resolution of the cross sectional images was 4 µm isotropic voxel side length with a grey scale resolution of 8 bit. The animated video was generated using a direct volume rendering method implemented in the software CTvox. Digital images: Angustopila huoyani sp. n. was photographed using a Kontron-Electronik-ProgRes-3012 microscope camera (Jena, Germany) and a Leitz MZ12 stereomicroscope.
Etymology. The name derives from the combination of the Latin angustus (= narrow) and pila (= pillar, column). Gender: feminine.
Remarks. Angustopila gen. n. differs from Tonkinospira nom. n. (former Vietnamese Systenostoma) by smaller shell size, more elevated spire, slightly reflexed apertural rim and general dentition present within the aperture. Acinolaemus usually has more teeth and a turban-like shell. Krobylos species have angulated whorls, lack spiral lines on the shell and possess a relatively large, toothless, adnate aperture.
Distribution. The genus is known from Thailand and Malaysia. The Chinese Angustopila huoyani sp n. is located very distant, almost 1500 km from the northern Thai localities.

Diagnosis.
A tiny, thin-shelled conical snail with very deep and narrow umbilicus, 5 shouldered whorls and two apertural denticles (parietal and palatal).
Description. Shell thin, greyish white, semi opaque; conical, widest at its base, with a homogeneous powdery superficial texture and regularly increasing, shouldered whorls separated by deep suture; smooth with no notable spiral or radial sculpture. It is characterized by very fine irregular axial lamellae and reticulating microgranules producing the powdery superficial texture; protoconch shows reticulating granules and recognisable radial lines only at the upper part of the first whorl; aperture semi-circular, slightly oblique from ventral view; peristome very slightly thickened and reflexed; parietal callus adnate (attached to the penultimate whorl); aperture with two well-developed but short denticles, one on the parietal and the other on the palatal side; umbilicus very deep and narrow.
Measurements. See Table 1. Differential diagnosis. Tonkinospira defixa, T. pulverea, T. pauperrima and T. depressa are much larger than the new species, have reticulated sculpture and lack denticles in the aperture. Moreover, T. defixa has a more depressed spire, fewer whorls, wider umbilicus and slightly keeled body whorl; T. pulverea has fewer whorls, a comparatively larger aperture, somewhat keeled, wider body whorl and its umbilicus is partly closed by the apertural margin; T. pauperrima shows increased bulging in whorl configuration; T. depressa has a lower spire, slightly keeled body whorl and a large aperture without denticles. Hypselostoma (?) edentata also lacks denticles in the aperture and possesses a very wide, laterally compressed body whorl. A. tamlod, the most similar species, is slightly smaller, has fewer whorls, wider umbilicus and obvious spiral Video 1. Micro-CT Video of Angustopila huoyani sp. n. Video: Markus Heneka. Video available for download in full resolution from http://www.pensoft.net/J_FILES/1/articles/7488/ export.php_files/Jochum_Video_1.avi striation on the teleoconch. A. concava has a much wider body whorl than that of A. huoyani, has weaker apertural denticles and prominent spiral sculpture. A. elevata has no denticles in the aperture and possesses a wider umbilicus and spirally striated shell. A. neglecta (see also notes under that species) has a wider umbilicus and more rapidly growing whorls, resulting in a comparatively wider body whorl than in the new species. A. neglecta also has spiral lines on the shell and its sinulus is wider. Shell characters and ecological information of all Angustopila species are presented in Table 3.
Etymology. The new species is named after the Gorges of Huoyan, where the type locality is located.
Distribution. The new species is known from the type locality only. Ecology. The new species is known only from the Feihu Dong ("Cave of the Wind Tiger"). A. huoyani were culled from samples of rocky-loamy substrate collected in the entrance corridor of the cave. It is highly likely that the distribution of A. huoyani sp. n. is restricted to this cave only.
Conservational status. Our knowledge of the biogeography of the genus is very limited. However, we assume that most Angustopila, especially the cave-dwelling species, are narrow-range endemics. Since extreme endemism always makes species vulnerable to human encroachment, this species warrants conservation priority. Currently, no direct threats are known. Table 1. Shell measurements (mm) for Angustopila huoyani sp. n. from the type locality. SH -shell height, SW -shell width, AH -aperture height, AW -aperture width, SW/SH×100 -shell width shared with shell height and multiplied 100, AW/AH×100 -aperture width shared with aperture height and multiplied 100).  Remarks. Although the specimen on Benthem Jutting's (1949) figure is similar, it probably is not conspecific to her other figure (Benthem Jutting 1961). See detailed explanation in Panha and Burch (1999).
Diagnosis. A genus of small, conical or depressed-conical species with regularly growing, rounded or angulated whorls. The sculpture is characterized by spiral lines on both the protoconch and the teleoconch, decussated by irregular radial lines resulting in a reticulated surface structure. The aperture is toothless, adnate or slightly adnate and shows a sharp peristome.
Etymology. The new name is established by the fusion of Tonkin (northern Vietnam, the area of distribution) and the Latin spira (a coil, twist). Gender: feminine.
Remarks. Tonkinospira differs from Krobylos by the increased degree of angulation of the whorls and the lack of spiral lines on the teleoconch. For differences with Angustopila gen. n., see above.
The systematic position of the genus is questionable. It most probably belongs to the family Hypselostomatidae, but its relationship with other families such as Helicodiscidae (see Schileyko 1998) or Valloniidae (see Schileyko 2011) cannot be excluded.
Distribution. So far, the genus is reported from Northern Vietnam only.