The genera Erhaia and Tricula (Gastropoda, Rissooidea, Amnicolidae and Pomatiopsidae) in Bhutan and elsewhere in the eastern Himalaya

Abstract Shells of the Rissooidea species that are known from Bhutan are characterized. Tricula montana is reported from that country for the first time. Two Erhaia species from Bhutan are described as new to science, viz. E. janneisp. nov., and E. pelkiaesp. nov., The holotypes of the Erhaia species that were described from Nepal are figured with photographs for the first time and compared with the congeneric taxa from Bhutan and India. Erhaia nainitalensis is considered a senior synonym of E. chandeshwariensis. An identification key is presented for the Erhaia species of the Himalayan foothills.


Introduction
The rissooidean gastropods that are widespread over the globe have a confusing history of taxonomic rearrangements that follow the increasing amount of morphological and molecular data, the ongoing methodological refinements in cladistics and the wealth of more or less conflicting speculations in the phylogeography of the taxa. The species of the Rissooidea Gray, 1847 from Bhutan, Nepal and northern India that are dealt with here, are classified in two genera that belong to different families, viz. the family Amnicolidae Tryon, 1863, with the genus Erhaia Davis & Kuo, 1985 and the family Pomatiopsidae Stimpson, 1865, with the genus Tricula Benson, 1843. Some species of these two genera are intermediate hosts for Platyhelminthes that are medically significant since they may transmit the human lung fluke Paragonimus Braun, 1899 or human Schistosoma Weinland, 1858 (Liu et al. 2014).
These species are characterized by minute shells that cannot always be recognized easily from descriptions and identified because of the limited number of diagnostic characters and the fact that many conchological character states that are used in the literature cannot be strictly quantified. The general shape of the shell and the form of the aperture may be described as ovoid, conical, subcylindrical, squat, or with another term of that kind. The convexity of the whorls and the depth of the suture are equally difficult to describe unequivocally. The surface of the shells is often heavily encrusted, so that the microsculpture of the proto-and teleoconch cannot always be recognized. Despite all this, an attempt is made here to characterize the genus conchologically.
The anatomy of these micro-snails cannot easily be investigated, so that DNA sequencing has become a promising tool to investigate the systematics of the Rissooidea. The classification of two Erhaia species from Bhutan is based now on DNA data, whereas a third species from that country is considered congeneric by reason of conchological and ecological similarity. Three nominal species of Erhaia from Nepal and one species from nearby northern India are compared with the Bhutanese taxa in more detail because of their joint occurrence in springs and brooklets of the southern foothills of the Himalaya. Photographs of Tricula montana from Bhutan, of the holotypes of the two new Bhutanese Erhaia species, and of a specimen of the third Bhutanese Erhaia species from its type locality, are provided together with photographs of the three Nepalese nominal species of Erhaia, that are published here for the first time.

Material and methods
Four species of minute snails were collected in spring areas and in a brooklet in Bhutan (Fig. 1). Tricula montana, Erhaia wangchuki and two undescribed Erhaia species could be recognized conchologically. Representative shells of these four species were photographed by Björn Stelbrink (Figs 16, 17) with a digital microscope system (KEYENCE VHX-2000; KEYENCE Corp., Itasca, IL, USA) and Mr Jeroen Goud (Figs 2-7) with a ZEISS SteREO Discovery.V20. Specimens of T. montana and of one of the two new Erhaia species were used for DNA analysis. Only two specimens of the third Bhutanese Erhaia species were available and these were kept as dry shells.
Photographs of the holotypes of the three Erhaia species that were described from Nepal and illustrated with drawings only were made with a Nikon SMZ25 stereomicroscope by Ms Sara Schnedl and provided for study by Ms Anita Eschner (both Museum of Natural History, Vienna, Austria). The only Erhaia species that is known from the Himalayan foothills in India is compared with the species from Bhutan and Nepal on the basis of its detailed description and photographs that are available in the literature. An identification key for the Erhaia species in the study area, using shell characters, is provided.
The standard CTAB protocol for molluscs was used for the DNA lab isolation (Winnepenninckx et al. 1993). In addition to the 16S rRNA fragment (c. 535 bp; using standard primers by Palumbi et al. 1991), the mitochondrial COI gene was also amplified (658 bp; standard primers by Folmer et al. 1994). The final genetic dataset mainly comprised sequences from the study by Liu et al. (2014) and additional sequences for Tricula that were obtained from Guan et al. (2008). Uncorrected genetic p-distances for 16S rRNA and COI were calculated using MEGA 7.0.20 (Kumar et al. 2016).

Systematics
Shells. The shells vary from conical to more or less ovoid, rarely with a flaring final part of the body whorl. The apex is flattened as in the European amnicolid genus Bythinella Moquin-Tandon, 1856, because of the very low spiral of the protoconch. The peristome is continuous and may be more or less protruding. The parietal and the columellar side of the aperture are about equally long and the regularly curved palatal side of the aperture gradually passes into the basal side, forming a single, regularly curved border. Bythinella cannot be distinguished from Erhaia conchologically, but in some Erhaia species from China the columella has one or two spiral lamellae, that are not known from Bythinella. In Erhaia the protoconch may have spiral striae, which have not been described for any of the Bythinella species.
Distribution. The genus Erhaia is mainly known from China, where it has been recorded with various species from the province of Yunnan in the west to the provinces of Hunan, Hubei and Fujian in the east (Davis et al. 1985;Davis and Kang 1995;Davis and Rao 1997;Wilke et al. 2000Wilke et al. , 2001Liu et al. 2014). One species was described from northern India (Davis and Rao 1997), three from Nepal (Nesemann et al. 2007) and one from Bhutan (Gittenberger et al. 2017a). Here we deal with the systematics of only the species occurring in the southern Himalayan foothills in Bhutan, India, and Nepal. Shell. Shell obliquely ovoid, with 3½-4 convex, shouldered whorls that are separated by a deep suture; clearly higher than broad; yellowish brown with fine irregular growth lines and some blackish brown periostracal ridges, one of which runs from the apertural columellar border into the umbilicus. Peristome not reflected. Parietal, columellar and a short part of the adjoining basal apertural border thickened by a whitish callus. Most specimens with a continuous peristome and a narrow umbilical chink. Protoconch encrusted in all specimens; teleoconch without spiral sculpture.
DNA data (Fig. 18). The three individuals shared the same haplotype for both 16S rRNA (GenBank acc. no. MT239078) and COI (GenBank acc. no. MT237716). The uncorrected genetic p-distances between this species and E. wangchuki were 0.84% for 16S rRNA and 4.87% for COI. The distances were considerably higher when compared to Erhaia sp. from China, Guangxi, viz. 2.74% for 16S rRNA and 10.25% for COI (GenBank acc. nos. KC832722 and KC832701, respectively).
Notes. This species was discovered in 2012, but since only a single shell was collected then, a description was considered premature.
Etymology. The epithet jannei refers to Mr Janne Clewing, the son of the last author.  Shell. Shell elongated ovoid, with 3½ convex, shouldered whorls that are separated by a deep suture; clearly higher than broad; light yellowish brown with fine growth lines and some brown periostracal ridges. Peristome reflected at the columellar and the basal side. Parietal and columellar side of the aperture thickened by a whitish callus. An irregular umbilical chink only in the paratype might be represent a malformation resulting from repair of the shell wall. Protoconch encrustated; teleoconch with fine spiral lines.
Measurements. (N = 2): holotype and paratype H 1.9 mm, B 1.1 mm. Ecology (Fig. 11). The snails are rare on the rocks in the shaded streamlet uphill the prayer wheel. See also the data for E. jannei.
Notes. The differences between the sympatric E. pelkiae and E. jannei are too large to regard as sexual dimorphism.
Etymology. The epithet pelkiae refers to Ms. Pelki Yangdon, the daughter of the fourth author. Shell. Shell conical, with 3-3½ convex, broadly shouldered whorls, that are separated by a deep suture; a little higher than broad; pale yellowish grey with fine irregular growth lines and some dark brown periostracal ridges, one of which sometimes running from a slightly angled site of the apertural columellar border into the umbilicus. Peristome not reflected. Parietal, columellar and about half the adjoining basal apertural border strongly thickened by a whitish callus. Most specimens with a continuous peristome and a broad umbilical chink. Protoconch with faint spiral lirae; teleoconch without spiral sculpture.

Erhaia wangchuki
Measurements. (N = 124): H 1.6-2.1 mm, B 1.3-1.7 mm. Ecology (Fig. 12). See Gittenberger et al. (2017a) for data about the Gangzetem brooklet and its surroundings. The snails are very common on the pebbles and rocks in the open area near the road, next to the water powered prayer wheel.
DNA data (Fig. 18). A single individual (GenBank acc. nos. KY798003 and MT237715, for 16S rRNA and COI, respectively) is genetically distinct from E. jannei (see data for that species) and showed genetic distances of 2.74% for 16S rRNA and 11.43% for COI compared to Erhaia sp. from China, Guangxi (GenBank acc. nos. KC832722 and KC832701).
Notes. Only three relatively large shells form the type series of this species. Many more specimens, none of which exceed 1.7 mm in breadth and over 2.0 mm in height, were collected recently. This necessitated some adaptations in the description of the shells. Contrary to the original description, the shell should be described as higher than broad.

Erhaia in Nepal and northern India.
In their monograph on the aquatic molluscs of the Ganga River system Nesemann et al. (2007: 64-65) published short descriptions with drawings only of three Erhaia species from Nepal. We acquired photos of the holotypes of these nominal species, which are compared with the congeneric species from Bhutan and northern India. Fig. 10 Erhaia sugurensis Nesemann, Shah & Tachamo, 2007, in Nesemann et al. 2007 ("Nepal, Central Zone, Lalitpur District, Godawari, upper reaches of Sugure Khola forrest stream, elevation of 1700 m a.s.l.").

Erhaia sugurensis Nesemann, Shah & Tachamo, 2007
Shell. According to Nesemann et al. (2007: 65) the shells are 1.6-1.9 mm high, with 3½ whorls that are "not convex", the aperture is "rounded, widened and enlarged, the inner and outer lip is thickened". The species is said to differ most conspicuously by "the separation of the last half whorl from the shell". Material examined (photo only). Holotype. (NHMW 104172).

Discussion.
A flaring final half of the body whorl, though not as extreme as in the holotype of E. sugurensis, may also occur as an individual variation in E. nainitalensis and the width of the umbilical chink may vary, as is shown by Davis and Rao (1997: 277, figs 2A-F).

Erhaia nainitalensis Davis & Rao, 1997
Material examined (photos only). Holotype and 4 paratypes of Erhaia chandeshwariensis (NHMW 103315 and 103316). Discussion. According to Davis and Rao (1997: 277, legends to figure 2) the holotype of E. nainitalensis is 2.28 mm high. However, elsewhere in the same article (Davis and Rao 1997: 279, table 1) the shell height of a single male animal is reported as 1.62 mm whereas 1.88-1.95-2.14 mm (minimum-mean-maximum) is indicated for five female snails. The difference in measurements between the single male and the five females might be indicative of sexual dimorphism. Shells of the species are also supposed to be "minute", what is defined as 2.0 mm high or smaller (Davis and Rao 1997: 275, 289, table 5). We suppose that the dimensions of the holotype of E. chandeshwariensis , i.e. H 1.94 mm, fall within the range of the measurements of E. nainitalensis. Three of the four paratypes of E. chandeshwariensis (NHMW 103316/4) are c. 1.9 mm high, whereas the fourth shell is damaged, missing the apical whorls. Davis and Rao (1997: 278, fig. 3) figure a smooth columella for E. nainitalensis, but mention (p. 289) a "discernable glassy thickening of the columella".
Judging the nominal taxa on the basis of photographs of shells and additional data in the literature, we conclude that in general shape and apertural characters, i.e. a narrow umbilical chink, a virtually smooth columella, and a thickened outer and inner lip, the holotype of E. chandeshwariensis cannot be distinguished from the shells of E. nainitalensis that are figured by Davis and Rao (1997: 277, figs 2A-F, 280, figs 4A-D). The fact that Davis and Rao (1997: 276) indicate 3¾-4 whorls for E. nainitalensis, whereas Nesemann and Sharma (2007: 64) mention 3½ whorls for E. chandeshwariensis, might be due to different counting methods.  , London no. 1964426 (design. Davis et al. 1986: 428, fig. 3A). Tricula montana ;Preston 1915: 68. Davis et al. 1986, 4 (operculum), 5-8 (anatomy), 9-10 (radula). Nesemann et al. 2007: 62, 78  Shell. Shell slender conical, with up to c. 5 shouldered, moderately convex whorls, separated by an incised suture; with obsolete growth lines and poorly discernible dense spiral lirae. Pale yellowish grey, with a light brown apertural border when fully grown. Apex not flattened, often decollate. Aperture triangular with broadly rounded edges, its parietal side about double the length of the columellar side; palatal side straight, passing into the slightly curved basal border with a more strongly curved transitional part. Parietal border of the aperture attached, at least in the middle and not or only slightly protruding. Umbilicus closed or nearly so.
Measurements. According to Davis et al. (1986: 431) the shell height of males and females combined (N = 10) is 3.32-3.72 mm. However, for the lectotype a larger shell height is indicated, i.e. 3.92 mm (Davis et al. 1986: 429, fig. 3A, 430). That shell is not even a relatively large specimen. Additional shells figured by Davis et al. (1986: 429, 430, fig. 3F, G, H, K, L) and printed at the same scale, are larger.
Distribution. (Fig. 1). According to Subba Rao (1989: 68) this species occurs in the Indian states of Himachal Pradesh ("Jhiri valley") and Uttarakhand (= Uttaranchal). Nesemann et al. (2007: 62) refers to it as widely distributed in the western and central Himalaya, in Nepal mainly at 1300-2100 m a.s.l. The records for Bhutan, at altitudes of 527-2631 m a.s.l., extend its range eastwards.
Habitat. This species was found in Bhutan without accompanying Erhaia species mostly in densely vegetated, shaded areas with more or less overgrown springs and streamlets (Figs 1, 13-15). Davis et al. (1986: 427) describe a similar habitat for the Nainital District near the type locality of T. montana. The locality in the district of Wangdue Phodrang is an overgrown, dripping wet, vertical, rocky wall along the road. The shells from Mongar, Chhuburee, and from Lhuentse, Jarkangchhu, are all decollate (Fig. 17), whereas shells from the other localities still have their apical whorls present (Fig. 16). This might be a consequence of unknown differences in water quality at those different localities.
The genetic distances between T. montana from Chhuburee and Khardungchhu were considerably higher for COI, with 8.9% and 9.3%, respectively, between snails Notes. The species was identified conchologically by using the data provided by Benson (1843), Davis et al. (1986) and Nesemann et al. (2007), taking also the distributional data (Nesemann et al. 2007: 62) into account.
Some species of Tricula may transmit schistosomes that could in principle infect humans and other mammals. No data in respect of this are known for T. montana.
See Davis et al. (1986) for a detailed account on this species, with data on shell morphology, anatomy of males and females, biogeography, and systematic relationships.