Natella viridescens ( Melvill & Ponsonby, 1891 )

David G. Herbert & Adnan Mousalli, 2016, Revision of the dwarf cannibal snails (Nata s. l.) of southern Africa — Nata s. s. and Natella (Mollusca: Gastropoda: Rhytididae), with description of three new species, Zootaxa 4094 (1), pp. 1-67 : 50-52

Natella viridescens ( Melvill & Ponsonby, 1891)

Figures 2A , 3 D, 4H, 5C, 6F, 9C, 37–40

Helix (Patula) viridescens Melvill & Ponsonby, 1891: 238 . Melvill & Ponsonby, 1892a, pl. 4, fig. 1. Type loc.: Pretoria.

Patula viridescens —Pilsbry, 1892 in 1892–1893: 78, pl. 30, fig. 32.

Natalina viridescens — Melvill & Ponsonby, 1898b: 171. Connolly, 1912: 98.

Rhytida (Macrocycloides) viridescens — Möllendorff & Kobelt, 1903, in 1903 –1905: 61, pl. 10, fig. 26. Kobelt, 1909: 53.

Nata (Natella) viridescens — Watson, 1934: 160. Connolly, 1939: 103, text fig. 7 (radula). Zilch, 1959 –60: 554, fig. 1937. Schileyko, 2000: 748, fig. 974.

Nata viridescens — Bruggen, 1967: 28, 29. Richardson, 1989: 46. Herbert & Kilburn, 2004: 219.

Etymology. From viridescens (L.)–greenish; presumably in reference to the ‘olivaceous epidermis’ (periostracum) noted by the original authors.

Type material. Holotype of Helix (Patula) viridescens Melvill & Ponsonby, 1891 in NHMUK (1905.1.26.24), diameter 4.5 mm (in poor condition).

Additional material examined and literature records: See Appendix 6.

Identification. Shell characterised by its small size, low-discoidal profile, weak sculpture and wide umbilicus. Distinctive anatomical features are given in generic diagnosis.

Shell ( Fig. 37 ): Discoidal, spire low to almost flat; largest specimens with approx. 2.0 teleoconch whorls; suture above periphery, not noticeably descendant prior to aperture; periphery usually slightly below mid-whorl; surface glossy throughout. Protoconch very small, diameter 0.7–1.0 mm, comprising ±1.0 whorl, smooth and glossy; junction with teleoconch usually weakly defined. Teleoconch initially sculptured only by faint growthlines, but last adult whorl with traces of weak pliculae below suture and with occasional stronger, irregularly spaced growth-lines; sculpture weaker on base. Aperture roundly ovate; peristome interrupted by bulging parietal region; outer lip thin with membranous periostracal fringe when undamaged; base and umbilical margin evenly rounded; umbilicus broad, patent to apex; columella lip not reflected.

Translucent, periostracum more or less uniformly pale golden-yellow to pale olive-yellow; little or no darker axial banding.

Dimensions: Largest specimen, diameter 5.3 mm, height 2.4 mm (Ithala Game Res., NMSA V2530); H:D of adults 0.42–0.47 (N=21).

Living animal ( Fig. 4 H): Few specimens examined alive; head and neck grey to dark grey, with scattered microscopic orange pigment granules, tentacles and retractor muscles often somewhat darker; foot paler ventrally, particularly the tail and beneath shell; ventro-lateral margins of foot with more numerous orange pigment granules; skin texture relatively smooth; mantle edge pale, often densely speckled with orange pigment granules; lining of mantle cavity lacking dark makings, whitish kidney sometimes conspicuous.

Radula ( Figs 6 F, 38): Formula 1+7 +(3–4); length up to 1.9 mm, with 27–32 broadly V-shaped rows of teeth; rachidian present, narrow with a short, curved cusp; no clear distinction between lateral and marginal series, largest tooth taken to be outermost lateral; inner lateral teeth slender, the cusp down-curved; teeth progressively increasing in length from first to sixth; seventh tooth considerably larger (fang tooth), and much broader and blade-like; 3–4 teeth (marginals) beyond largest lateral, the first also flattened, but somewhat smaller and rather more narrowly acuminate, remaining marginals decreasing rapidly in size, a minute fourth marginal sometimes evident.

Distal genitalia ( Fig. 39 ): Genital pore below pneumostome. Penis short and stout (length ± 0.7 mm), not obviously narrowing basally, bluntly rounded at its apex, from which the vas deferens and penial retractor muscle arise. Interior of penis with a broad verge extending for half its length from apex, basal section lined with low longitudinal folds, lacking papillation. No evidence of a swollen epiphallic region prior to insertion of vas deferens; the latter not reflexed and adnate to side of penis, but runs directly backward to its origin at base of spermoviduct. Genital atrium simple and vagina short with thickened wall. Bursa copulatrix and its duct about one-third length of spermoviduct, slender basally, but apical portion slightly swollen; the whole adnate to free oviduct and lower spermoviduct. A concentration of muscle fibres arises close to base of bursa copulatrix duct, connecting with adjacent body wall.

Distribution ( Fig. 40 ): Endemic to north-eastern South Africa; from the Soutpansberg (Limpopo), south through N.W. Province, Gauteng and Mpumalanga, to the KwaZulu-Natal Midlands (Howick–Dargle area); mostly at altitudes over 500 m and commonly over 1000 m.

Habitat. Evidently catholic in habitat preferences. Most records from the northern part of the Great Escarpment are from forested habitats, but some of the localities in Gauteng, N.W. Province and KwaZulu-Natal indicate that the species also inhabits somewhat drier, thicket-type vegetation. Primarily in leaf-litter and under logs, but will climb walls and probably tree trunks.

Remarks. Natella viridescens is the smallest southern African rhytidid. Nata erugata sp. nov. and Nata vernicosa , with which it is known to co-occur, are substantially larger species and lack such a wide umbilicus. Juvenile specimens of both have deeper, fewer whorled shells with a much narrower umbilicus. The smaller Nata species, N. aequiplicata sp. nov. and N. tarachodes have stronger sculpture and occur only in the southern and western Cape respectively. Specimens of Natella viridescens from northern KwaZulu-Natal are often somewhat larger than those from other localities.

Conservation. With its wide distribution and broad habitat tolerance, Natella viridescens is not currently a species of conservation concern.

Figures

FIGURE 3. Protoconchs of Nata and Natella. A, Nata dumeticola, Kommetjie, W. Cape (NMSA 2767). B, Nata tarachodes, Spes Bona bush, Cape Peninsula (NMSA V 9848). C, Nata vernicosa, Pietermaritzburg, KwaZulu-Natal (NMSA A 7114). D, Natella viridescens, Ithala Game Res., KwaZulu-Natal (NMSA V 2530). Scale bars = 0.5 mm.

FIGURE 4. Living animals of Nata and Natella. A, Nata aequiplicata sp. nov., holotype, Stinkhoutkloof, E. Cape, shell diameter 6.5 mm (NMSA W 9248 / T 3066). B, Nata dumeticola, West Coast National Park, W. Cape, shell diameter 10.8 mm (NMSA W 3224). C, Nata erugata sp. nov., Tullach-Mohr, Nelspruit area, Mpumalanga, shell diameter 20 mm (MV ex NMSA W 3301). D, Nata tarachodes, Grootbos Private Nat. Res., Gansbaai, W. Cape, shell diameter 7.7 mm (NMSA W 5658). E, Nata watsoni sp. nov., Hankey, E. Cape, shell diameter 7.5 mm (paratype NMSA W 6524 / T 3921). F, Nata watsoni sp. nov. Camdeboo National Park, Graaff-Reinet, E. Cape, shell diameter 13.0 mm (paratype, NMSA W 6091 / T 2303). G, Nata vernicosa, Vernon Crookes Nat. Res., KwaZulu-Natal, shell diameter 14.0 mm (NMSA W 8020). H, Natella viridescens, Ithala Game Res., KwaZulu-Natal, shell diameter 3.7 mm (NMSA W 7263).

FIGURE 6. Radulae of Nata and Natella, automontage light micrographs of half-row portions of stained radula mounts. A, Nata aequiplicata sp. nov., type locality (paratype, NMSA W 9249 / T 3067). B, Nata dumeticola, Jacobsbaai, W. Cape (NMSA V 7760). C, Nata erugata sp. nov., Golden Gate Highlands National Park, Free State (paratype, NMSA W 4243 / T 3913). D, Nata tarachodes, Oubos, W. Cape (NMSA W 5671). E, Nata watsoni sp. nov., Graaff-Reinet, E. Cape (paratype, NMSA W 6091 / T 2303). F, Natella viridescens, Ithala Game Res., KwaZulu-Natal (NMSA V 2530). Scale bars: A = 100 µm; B = 200 µm; C – E = 250 µm; F = 50 µm.

FIGURE 37. Natella viridescens (Melvill & Ponsonby, 1891). A – C, Weenen, KwaZulu-Natal, diameter 4.7 mm (NMSA A 7126). D – F, Ithala Game Reserve, KwaZulu-Natal, diameter 5.0 mm (NMSA V 3177). G – I, Pretoria, Gauteng, diameter 3.6 mm (NMSA A 7122).

FIGURE 39. Natella viridescens (Melvill & Ponsonby, 1891), genitalia (NMSA V 3177). A, middle and lower portion of genital tract. B, internal structure of penis. Abbreviations: ag, albumen gland; bc, bursa copulatrix; bd, bursa copulatrix duct; fod, free oviduct; ga, genital atrium; m, muscle fibres; p, penis; pf, folds inside penis base; pr, penial retractor muscle; pv, penial verge; sod, spermoviduct; v, vagina; vd, vas deferens.

FIGURE 40. Distribution of Natella viridescens (Melvill & Ponsonby, 1891). Triangles with a white centre indicate localities for specimens used in DNA analyses (Moussalli & Herbert 2016); contour at 1000 m.

FIGURE 2. A, Natella viridescens (Melvill & Ponsonby, 1891), drowning individual with proboscis everted, Ithala Game Res., KwaZulu-Natal, shell diameter 3.7 mm (NMSA W 7263). B, Nata vernicosa (Krauss, 1848), mating pair, Qora River mouth, E. Cape, respective shell diameter 20.5 mm and 14 mm (NMSA W 4015).