Afrorhytida trimeni ( Melvill & Ponsonby, 1892 )

Herbert, D. G. & Moussalli, A., 2010, Revision of the larger cannibal snails (Natalina s. l.) of southern Africa - Natalina s. s., Afrorhytida and Capitina (Mollusca: Gastropoda: Rhytididae), African Invertebrates 51 (1), pp. 1-1 : 103-108

publication ID	https://doi.org/ 10.5733/afin.051.0101
DOI	https://doi.org/10.5281/zenodo.7911552
persistent identifier	https://treatment.plazi.org/id/110B87C2-FFE2-FFBF-D72F-FB12FCA1FD24
treatment provided by	Felipe
scientific name	Afrorhytida trimeni ( Melvill & Ponsonby, 1892 )
status

Treatment

Afrorhytida trimeni ( Melvill & Ponsonby, 1892) View in CoL View at ENA

Figs 4B View Fig , 54C View Fig , 55H View Fig , 70–72 View Fig View Fig View Fig

Helix (Aerope) trimeni: Melvill & Ponsonby 1892: 237 , pl. 13, fig. 1. Type loc.: given as ‘ S. Africa’, but label states ‘ Cape Colony’ [Trimen].

Natalina tremeni [sic]: Pilsbry 1893 in 1892–93: 135.

Natalina trimeni: Pace 1895: 232 View in CoL ; Melvill & Ponsonby 1898: 170; Sturany 1898: 31; Connolly 1912: 97; 1939: 114; Barnard 1951: 142, pl. xxi, fig. 7.

Rhytida (Afrorhytida) trimeni: Möllendorff 1903: 62 , pl. 11, fig. 3.

Natalina arguta Melvill & Ponsonby, 1907: 98 View in CoL , pl. 6, fig. 8; Connolly 1912: 90; 1939: 115; Watson 1934: 156, pl. 19, fig. 10 [radula]. Type loc.: East London , E. Cape [Burnup]. Syn. n.

Etymology: Named for Roland Trimen (1840–1916), an entomologist and curator at the South African Museum (1872–95), who sent the original specimens to Melvill and Ponsonby.

Identification: The shell of Afrorhytida trimeni is characterised by its brownish rather than greenish colour and the fact that the axial sculpture remains distinct even on the last adult whorl. A. kraussi kraussi has a similarly brown shell, but the whorls are less deep and the upper part of its outer lip is usually distinctly flattened; its radula also has more marginal teeth and lacks lateral flanges on the inner laterals. In terms of its radula A. trimeni is most similar to A. burseyae in that there are relatively few marginals (<10), but in the latter the innermost laterals lack lateral flanges and distinctly uncinate tips. Its shell is also considerably more globose and of a more yellowish green hue. A. knysnaensis is likewise generally more globose (except those from the north-west of its range) and more yellowish, and its radula has many more marginal teeth of intermediate size.

Description ( Fig. 70 View Fig ): Shell globose-lenticular, spire height generally low but rather variable; comprising up to 4.75 whorls, last adult whorl descending slightly prior to aperture; apical surface lustreless, base more glossy. Protoconch diameter 3.5–4.1 mm, initially somewhat smooth, but with distinct axial riblets developing along adapical suture during first whorl, these extending to abapical suture on second whorl ( Fig. 4B View Fig ); protoconch/teleoconch junction usually poorly defined. Apical surface of teleoconch sculptured throughout by close-set axial riblets, these not becoming noticeably weaker on last adult whorl; riblets weaker below periphery and on base, but becoming stronger again around and into umbilicus; base with traces of weak, close-set, spiral lirae; aperture roundly-ovate; outer lip weakly, but distinctly thickened in adult, white; upper part of outer lip not or only minimally flattened; umbilicus deep and relatively wide, scarcely, if at all obscured by upper part of columella lip.

Fresh specimens more or less uniformly mid brown; little difference between apical and basal surfaces except that the former is dull and the latter glossy; some specimens with faint traces of slightly darker radial banding. Brown colour fading after death and becoming more yellowish brown; old specimens frequently yellow-ochre.

Dimensions: Largest specimen (NMSA W5254, Hamburg), diameter 25.5 mm; H:D of adults 0.56–0.70 (N=18).

Living animal ( Fig. 55H View Fig ): Information available from photographs of only two specimens. Head-foot grey-brown dorsally, with a paler longitudinal stripe extending backward from each optic tentacle, bordered ventrally by a conspicuous dark, grey-black line, boldest beneath shell; paler dorsal areas slightly tinged with apricot; sides of foot pale greyish white with darker skin grooves, slightly darker just above pedal margin; pedal margin itself whitish, slightly tinged with apricot; tentacles darker grey or grey-brown; mantle edge pale greyish white; lung wall often extensively marked with fine, anastomosing black blotches, not obviously following pattern of lung venation.

Radula ( Fig. 71 View Fig ): Formula 1+(13–14)+(6–9) (N=4); length up to 19.5 mm, with 56– 76, broadly V-shaped, transverse rows of teeth, 3.9–4.4 rows/mm in adult. Rachidian about two-thirds length of innermost lateral; cusp broadly acuminate, slightly longer than its base-plate. Inner lateral teeth (1–10), relatively short and broad, increasing only slightly in size, their cusps with a lateral flange on outer margin and a bluntly rounded apex bearing a small but distinct uncinate tip ( Fig. 71C View Fig ); outermost 3 or 4 lateral teeth increasing rapidly in size and developing a stout quadrate base-plate, with a strong, gently curved, sharply pointed cusp; penultimate lateral is the largest. Innermost marginal considerably smaller than outer lateral, with a much reduced base-plate, but retaining a distinct cusp; remaining laterals progressively smaller, the fourth and subsequent vestigial.

Distal genitalia: See generic description. Wall of penis lumen coarsely papillate; epiphallus approx. 0.75 length of penis; its internal structure ( Fig. 54C View Fig ) similar to that of A. knysnaensis in possessing longitudinal ridges with digit-like tips, except that there are only four such ridges additional to the pair of folds running along the inner wall, and the ridges are longer than in A. knysnaensis , extending forwards for approx. 0.75 of epiphallus length from its junction with vas deferens. Anterior to this the lumen wall is smoother, but bears indistinct longitudinal rows of pits running forward from the intervals between the swollen tips of the longitudinal ridges.

Spermatophore: No spermatophores have been found, but the internal morphology of the epiphallus suggests that the mid region and tail will be largely smooth, perhaps only with weak longitudinal furrows, but the anterior portion will bear approx. five rows of small spines on its outer, convex surface.

Type material: Lectotype of Helix (Aerope) trimeni Melvill & Ponsonby, 1892 (designated Connolly 1912: 97) (= figured syntype) in BMNH (1911.8.8.3), diameter 23.8 mm (24.2 mm fide Connolly 1939) ( Figs 70A–C View Fig ). Three paralectotypes in NMW 1955.158.736. Holotype of Natalina arguta Melvill & Ponsonby, 1907 in BMNH (1911.8.8.6), diameter 20.5 mm ( Figs 70D–F View Fig ); one paratype in NMSA (1495/T593).

Additional material examined (all NMSA unless otherwise indicated): SOUTH AFRICA: E. Cape: Grahamstown, C.J. Swierstra, vi/1925 (V5278); ditto, ex Farquhar Coll’n (BMNH 1937.12.30.1345); ditto, bottom of Fernkloof, 475 m, under stones, i/1912 (E7908); Kap River Nat. Res. (33.48541°S: 27.08474°E), 65 m, indigenous forest, in leaf-litter, A. Moussalli, D. Stuart-Fox & M. Bursey, 9/xii/2005 (W4206); Port Alfred area, nr Kleinemonde, Tharfield farm. Miss E. L Barber (W5986); Begha R., Peddie District, J. Hewitt, xii/1935 (V6849); Hamburg, Keiskamma R. mouth (33.27555°S: 27.48639°E), 3 m, riverine forest on east bank near river mouth, in leaf-litter, M. Bursey, 14/ii/2007 (W5254, ELM W03187); East London, H. Burnup Coll’n (B0032); ditto, Umtiza Nat. Res. (33.016°S: 27.809°E), 160 m, coastal forest, in leaf-litter, eating Chondrocyclus sp. , D. Herbert & M. Bursey, 4/iii/2000 (V7904); ditto (33.016°S: 27.809°E), coastal indigenous forest, under log, A. Moussalli & D. Stuart-Fox, 26/xi/2005 (W4182, W5466); ditto, (33.01444°S: 27.80806°E), forest, in leaf-litter, R. Botha, 07/ii/2007, don. M. Bursey (W5253, ELM W03208).

Additional literature records (material not seen): SOUTH AFRICA: E. Cape: Coega ( Pace 1895); Port Alfred (fide Crawford, Connolly 1939).

Distribution ( Fig. 72 View Fig ): Endemic to E. Cape Province. Recorded from the Grahamstown– Port Alfred region east to East London, with one additional literature record from Coega, near Port Elizabeth ( Pace 1895); from sea level to approx. 500 m in the Grahamstown area.

Records from the Somerset East–Cradock area given by Connolly (1912) were almost certainly based on misidentified A. knysnaensis material (see also Schileyko 2000, fig. 971). He later omitted these records (Connolly 1939), suggesting that he had become aware of this error.An old radula slide in the NMSA labelled ‘ Natalina trimeni , Cradock’, clearly represents the radula of a specimen of A. knysnaensis .

Habitat: The limited habitat data available suggests that A. trimeni is primarily an inhabitant of indigenous forest; in leaf-litter and under logs.

Notes:Analysis of molecular sequence data ( Moussalli et al. 2009) indicates that material from East London, previously known under the name Natalina arguta Melvill & Ponsonby, 1907 shows little genetic divergence from Afrorhytida trimeni . Likewise there are no discernable differences in the radula. The reported differences in shell characters, namely more distinct axial sculpture on the base, wider umbilicus and smaller size in A. arguta (Connolly 1939) , appear to be consistent in the limited material available, but such differences could easily be accommodated within the variation of a single species. The colour difference mentioned by Melvill and Ponsonby (1907), namely olivaceous ( trimeni ) vs rufous-brown ( arguta ), is not apparent in the specimens available. The disparity in size appears to be the most striking difference between the two populations. Two evidently mature shells from East London (Umtiza Nat. Res.), each comprising ca 4.75 whorls, measure 19.7 mm and 20.3 mm in diameter, compared to 25.5 mm for a shell from Hamburg also comprising ca 4.75 whorls. The largest shell available from the East London area is 22.0 mm in diameter (ELM D15190), whereas many mature shells from other regions are larger than this. However, given that the East London material is genetically very close to that from other regions ( Moussalli et al. 2009 and Fig. 1 View Fig herein), it may be that the size difference relates to undetermined ecological factors. While acknowledging that this is an issue which needs further investigation, we consider it unlikely that the two nominal taxa represent specifically distinct entities and place the name Natalina arguta in synonymy with A. trimeni . The most that could be justified would be to recognise the junior name as an eastern subspecies of smaller size, but we refrain from doing this on account of the limited material available, much of which is juvenile or subadult.

Conservation: The distribution of Afrorhytida trimeni is somewhat limited, but not highly restricted. Its range spans 230 km in the coastal region and extends up to 40 km inland, but given that it appears to favour forest habitats, its distribution within this range is likely to be fragmented. There are relatively few formally protected conservation areas within this area, but populations are known from both Umtiza and Kap River provincial nature reserves.The preservation of southern coastal forest and southern mist-belt forest habitat within the broader Albany Thicket biome ( Mucina & Rutherford 2006) would appear to be important for the conservation of this species.