Hydrobioides nassa (Theobald, 1866)

Bithinia nassa Theobald, 1866: 275 (Myanmar, Shan State).

Hydrobioides nassa – Annandale, 1918: 118–120, pl. 13, figs 1–7, pl. 14, figs 4, 4 a.

Hydrobioides nassa distoma Annandale, 1918 †: 120, pl. 13, fig. 1 (Myanmar, Shan States, Heho plain).

Hydrobioides nassa lacustris Annandale, 1918: 119, pl. 13, figs 4, 4 a, 5, 5 a, 7 (Myanmar, Shan States, Inle Lake).

Hydrobioides nassa rivicola Annandale, 1918: 119–120, pl. 13, figs 6, 6 a (Myanmar, Shan States, small streams at Thamakan).

Diagnosis.

Shell medium, high spiral whorls, a varix near outer lip, outer lip outward extended.

Material examined.

• 1 syntype (preserved dry), Upper Salwin River, Purchase of W. Theobald, NHMUK.1888.12.4.845; 3 specimens (preserved in ethanol), Shan State, Taunggyi District, Taungpoetgyi, near Tone Lae, river near spring (Fig. 1 B. c), 20°29'48.9"N, 96°50'17.5"E, 2 Jul. 2024, Le-Jia Zhang leg., KIZ.2400054 –2400056 • 2 specimens (preserved in ethanol), Shan State, Taunggyi District, Nyaungshwe, Inle Lake, floating islands near Intha (Fig. 1 B. b), 20°36'33.4"N, 96°54'53.8"E, 3 Jul. 2024, Le-Jia Zhang leg., KIZ.2400057 –2400058 • 2 specimens (preserved in ethanol), Shan State, Taunggyi District, Kalaw, river near Nanthe’ Cafe Garden, 20°36'30.6"N, 96°33'55.3"E, 11 Jul. 2024, Le-Jia Zhang leg., KIZ.2400059 –2400060.

Description.

Shell (Fig. 3) medium for the family (Table 1), thin but solid, conical, bright yellow or olive grey, with five whorls at adulthood; teleoconch relatively high, smooth, without shoulder, a weak to strong varix structure on the body whorl, close to or directly connected to the outer lip; aperture ovate, lip thickened, nearly half of shell in height, outer lip outward extended; umbilicus covered by inner lip completely.

Operculum (Fig. 3 E, 4 A) calcium, ovate, thin, slightly smaller than aperture, transparent grey; exterior surface smooth, nucleus close to the inner opercular margin; interior surface with wide concentric veins on inner opercular region, nuclear region relatively small, with irregular veins.

Penis (Fig. 5 A) with a grey penial appendix located in the central of penis, penial appendix slightly shorter than distal part of penis.

Radula (Fig. 6) taenioglossate (2 + 1 + 1 + 1 + 2); central tooth upper margin with one broad triangle central cusp and two to three small sharp cusps on either side, lateral margin each with two to three small sharp cusps; lateral tooth with one broad tongue-shape central cusp and three to four small sharp cusps; inner marginal tooth with 14 to 15 small cusps; outer marginal tooth with eight to nine small cusps.

Remarks.

This species can be easily distinguished from other bithyniid snails based on the varix structure near the outer lip. The location of varix can be relatively far from the outer lip (Fig. 3 I – M), viz. “ Hydrobioides nassa lacustris ” (Fig. 3 N), mostly found in Inle Lake; the varix also can be directly connected to the outer lip (Fig. 3 E – H), viz. “ Hydrobioides nassa rivicola ” (Fig. 3 O), mostly found in rivers. However, there is no clear boundary between these two morphotypes. The syntype is quite similar to the “ lacustris “ type but much bigger than the newly collected specimens (Fig. 3 A – D). This species was considered widely distributed in north to central Thailand. However, “ Hydrobioides nassa ” from Thailand does not have an obvious varix and an extended outward outer lip, and the molecular analysis has confirmed that the Thailand species is distinct from Hydrobioides nassa from the type locality, Shan State of Myanmar (Fig. 2). Therefore, we consider that Hydrobioides dautzenbergi Walker, 1927, stat. rev. (type locality: Thailand, Chieng Mai) should be a valid species name for the “ Hydrobioides nassa ” from Thailand. Hydrobioides nassa is probably endemic to Shan State, Myanmar.

Habitat and distribution.

Rivers, ponds, wetlands, and lakes of Shan State, Myanmar.

Values are arithmetic means, standard deviations, maximum and minimum values (in brackets) of shell height (H), shell width (W), and aperture height (AH) of n measured specimens.