Stenothyra glabra A. Adams, 1861

Qi, Lu, Kong, Lingfeng & Li, Qi, 2020, Redescription of Stenothyra glabra A. Adam, 1861 (Truncatelloidea, Stenothyridae), with the first complete mitochondrial genome in the family Stenothyridae, ZooKeys 991, pp. 69-83 : 69

Stenothyra glabra A. Adams, 1861 Figures 1 , 2 , 3

Stenothyra glabra A. Adam, 1861: 307; Yen 1939: 45, pl. 4, fig. 15; Yen 1942: 197, pl. 14, fig. 44; Zhang et al. 1964: 61; Qi et al. 1989: 32-33, fig. 30; Zhang et al. 2016: 60-61.

Material examined.

China • 4, specimens; Shandong province, Dongying, Yellow River estuary mud flat; 37°49.367'N, 119°09.035'E; 17 Sept. 2017; Lu Qi leg.; LSGB S1702; • 6, specimens; Jiangsu province, Ganyu beach; 34°51.912'N, 119°12.681'E; 16 Sept. 2018; LSGB G1801.

Original description (verbatim).

"S. testa oblonga, laevi, polita, semipellucida, aurantiaca; anfractibus 4½, convexis, supremis transversim obsolete striates; suturis marginatis; peritremate continuo; anfraetu ultimo ad aperturam concentrice striato" (A. Adams 1861).

Diagnosis.

Shell ovate, dorso-ventrally compressed, with well-inflated body whorl and narrowly constricted aperture, without dotted spiral lines. Dome-shaped, smooth protoconch ( 1¾ whorls) with some pits. Posterior foot pointed, with metapodial tentacle.

Description.

Shell minute (2.89 ± 0.14 mm in height; 1.75 ± 0.07 mm in width), ovate-conic, rather thick, dorso-ventrally compressed, with rounded to angled inflation of last whorl; up to five whorls including protoconch, convex whorls, sutures moderately deep; Surface smooth, yellowish brown, sculpture not dotted lines but continuous spiral grooves (Fig. 1A ). The aperture abruptly descending, contracted, and nearly circular; peristome continuous, showing a weak triangular area; outer lip with marked grooves (Fig. 1A ).

Operculum ovate, yellowish, translucence, with very weak angulation aligning with posterior apex of aperture; nucleus of the exterior surface is close to the inner lip, paucispiral (Fig. 1B ).

Protoconch dome-shaped; smooth, 1¾ to 2 whorls; Small pits apparently exist in a small central part of protoconch (Fig. 1C ).

Radula . Radular teeth interlocked moderately in unfolded condition (Fig. 1D ). Central tooth 1-2+1+1-2 (Fig. 1D, E ); cusp with central denticle largest, 1-2 smaller ones on each side, basal denticles diminishing outwardly. Lateral teeth 2-3+1+6-8, apical ones largest, 2-3 denticles along inner edge of cusp, 6-8 along outer edge. Marginal teeth without groove; inner marginal teeth with ~20 cusps on tip and distal half of outer edge; outer marginal teeth with ~10 cusps on distal third of inner edge.

Type locality.

Estuary of the Pei-ho River (also known as the Hai River in the current name), North China.

Geographic distribution.

From Fujian to Hebei on coast of China (A. Adams 1861; Yen 1939; Qi et al.1989; Yuan et al. 2002; Bao et al. 2007); Japan ( Kuroda 1962).

Ecology.

Inhabiting on the surface of mud flat or attaching to the under-surface of floating leaves in the freshwater estuary.

Remarks.

The type locality of Stenothyra glabra A. Adams, 1861 is "estuary of the Pei-ho, North China", which is on the coast of the Bohai Sea. One of the localities in this study, Yellow River estuary, is adjacent to the type locality. Moreover, the shells are very similar in size, shape, and microsculpture when compare with the descriptions (A. Adams 1861; Yen 1939; Yen 1942; Zhang et al. 1964), as well as with the figure of A. Adam’s type ( Yen 1942: 197, pl. 14, fig. 44). We believe that specimens collected in this study belong to a common species along the coast of the Yellow and Bohai seas in China, and is conspecific with the type material.

The radular morphology is one of the diagnostic morphological characters, but the Rachidian tooth and general radular shape of S. glabra appear similar to that of other Stenothyra species. This may be due to similarities in habit, substrate, and diet, suggesting that species delimitation in micro-caenogastropods should not rely solely on radular morphology. In fact, recent work has shown that some microgastropods exhibit morphological stasis in response to environmental stability (e.g., Weigand et al. 2011). However, there are sufficient morphological grounds for separating this species, by the shell not having dotted spiral lines and by the dome-shaped, smooth protochonch bearing some pits.

Sequence divergence.

The pairwise distance between species or non-conspecific subspecies ranged from 9.1% ( Stenothyra glabra vs. S. cf. divalis ) to 16.1% ( S. gelasinosa apiosa vs. S. monilifera ). COI sequence divergence between conspecific subspecies ranged from 3.0% ( Stenothyra paludicola timorensis vs. S. paludicola topendensis ) to 5.7% ( Stenothyra gelasinosa apiosa vs. S. gelasinosa gelasinosa ) (Table 3 ). Comparing the sequence divergences of within-taxon and between-taxon provided a sound basis for determining specific and subspecific-level differences. 3%-6% was evidence of subspecific diversity and> 9% was found between species. In this study, the divergence between S. glabra and other species fell into the latter category, having a lowest divergence of 9.1%. Notably, the divergence between Stenothyra glabra and S. cf. glabra (KC439741) is 13.2%. Stenothyra cf. glabra was collected from Mai Po, Hong Kong, China ( Golding 2014), and is likely a misidentified animal.

Figures

Figure 1. Stenothyra glabra A. Adams, 1861 A shell LSGB-G 1801 - 4 B exterior surface of operculum C protoconch D, E radula. Scale bars: 1 mm (A); 200 μm (B), 200 μm (C), 20 μm (D), 10 μm (E).

Figure 2. Map of the complete mitochondrial genome of Stenothyra glabra.

Figure 3. Summary tree from Maximum Likelihood analysis of concatenated COI, 16 S and 28 S sequences. Support indices are BI posterior probabilities (above nodes,> 0.9) and ML bootstraps (below nodes,> 70).