Natella Watson, 1934

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 : 49-50

Genus Natella Watson, 1934

Natella Watson, 1934: 160 . Type species (by original designation): Helix (Patula) viridescens Melvill & Ponsonby, 1891 .

Diagnosis. Shell small (adult diameter ca 5.0 mm), thin, discoidal, broadly umbilicate, glossy, pale yellowish; axial sculpture weak, spiral sculpture lacking; protoconch smooth. Radula beloglossan; labial palps not evident; penis stout, epiphallus absent, vas deferens short and not reflexed, inserting apically into penis; penis lumen not papillate, apical verge present; retractor muscle of right optic tentacle passes to left of penis; vagina broad, no atrial diverticulum or oviduct caecum present; kidney ovate to bean-shaped, primary ureter reflexed around anterior of kidney; suprapedal mucus gland not convoluted, the posterior portion lacking glandular tissue.

Remarks. Watson’s separation of ' Nata ' viridescens in to a distinct, monotypic section [=subgenus] Natella ( Watson 1934) , is strongly supported by both the morphological data presented here and by molecular data ( Moussalli & Herbert 2016). Furthermore, since the latter data show Nata s.l. to be a deeply divergent and quite possibly non-monophyletic taxon ( Natella is not sister to Nata s.s.), we consider Natella should be treated as a full genus.

In addition to the characters given in the above diagnosis, Watson also listed the presence of a rachidian tooth in the radula as a feature distinctive of Natella . While we can confirm that a rachidian tooth is present in Natella , this can no longer be considered a distinctive character of the genus since such a tooth is also present in Nata aequiplicata sp. nov., which genital morphology and molecular data show to belong to Nata . (It is also present in some unusual specimens tentatively identified with Nata tarachodes .) As stated in the Introduction, the presence of a rachidian tooth may represent an ancestral character state retained in juveniles and small-shelled species. The presence/absence of a rachidian tooth may thus not be phylogenetically informative. Watson also noted that the bursa copulatrix duct (as ‘spermatheca duct’) of Natella was longer than in species of Nata s.s., but he provided no illustration of this and we do not find this to be the case.

A further unusual and possibly distinctive feature of the buccal apparatus of Natella is its possession of a proboscis. First observed in a drowning individual as an extended oral tube with the radula near its tip ( Fig. 2A ), this structure is visible when retracted as a tube linking the mouth to the anterior end of the buccal mass, in the vicinity of the cerebral ganglia and anterior nerve ring. A similar structure has not been seen in species of Nata and observations of the feeding behaviour of Nata vernicosa suggest that there is very limited protrusion of the buccal tube beyond the mouth during feeding.

Natella is monotypic and is endemic to southern Africa. To date it has been recorded only from north-eastern South Africa, though its range may well extend into Swaziland.

Figures

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).