Prestonella bowkeri ( Sowerby, 1890 )

Herbert, D. G., 2007, Revision of the genus Prestonella (Mollusca: Gastropoda: Orthalicoidea: Bulimulidae s. l.), a distinctive component of the African land snail fauna, African Invertebrates 48 (2), pp. 1-19 : 6-10

Prestonella bowkeri ( Sowerby, 1890)

Figs 2–7 , 9–11 , 19 , 25

Bulimus ( Mesembrinus ?) bowkeri: Sowerby 1890: 581 , pl. 56, fig. 5. Type loc.: ‘ Somerset (East), Cape

Colony [ South Africa]. Forest about 3,000 ft above sea-level (Col. Bowker)’.

Buliminus (Pachnodes) bowkeri: Kobelt 1902: 657 , pl. 101, fig. 2.

Ena bowkeri: Connolly 1912: 165 .

Prestonella bowkeri: Connolly 1929: 232 ; 1939: 292, pl. 9, figs 25, 29, text-fig. 24 (radula); Barnard

1948: vii.

Description ( Figs 2–7 ): Shell succineiform to elongate-conical, thin; suture weakly to moderately indented.Aperture broadly drop-shaped; upper part of columella lip reflected over and partially covering umbilical region, umbilicus remaining patent but rather narrow; apertural rim strongly oblique, adapical part of outer lip extending almost to mid point of ventral part of body whorl (tangential) and the peristome thus effectively in one plane ( Fig. 6 ); aperture without teeth, denticles or varices.

Sculpture somewhat variable and superficial features frequently badly eroded, such that shells appear more or less smooth and lustreless. In fresh specimens, early part of first teleoconch whorl sculptured for the most part only with irregular, collabral growth-ridges, but these supplemented by uneven, close-set spiral threads soon thereafter; growth-ridges and threads interacting to produce a fine, irregularly granular sculpture over adapical surface of whorls, most noticeable on spire whorls (appearing pitted in worn shells); growth-ridges strongest immediately below suture; occasional intervals between spiral threads stronger than others and appearing as incised spiral lines; some features of spiral sculpture evidently scar-like growth flaws resulting from damage to mantle edge; sculpture of later whorls and base weaker, essentially of growth-ridges only.

Colour: Apical whorls corneous orange-brown, becoming paler with growth; surface layer of body whorl whitish, underlying layers apricot-coloured; fresh specimens with a straw-brown periostracum. Periostracum, sculpture and whitish superficial shell layer often completely eroded away ( Fig. 4 ). Head-foot buff to yellow.

Dimensions: Length up to 23.25 mm, width up to 13.5 mm; adult L/W 1.53–2.0; body whorl length = 78.5–87.3% of shell length; aperture length = 45.3–58.3% of shell length

(N=77). The material examined comes from two widely separated regions with the following dimensions in the two populations:

• Somerset East: length up to 23.25 mm, width up to 13.5 mm; adult L/W 1.53–1.98; body whorl length = 79.2–87.3% of shell length; aperture length = 46.0–57.7% of shell length (N=50).

• Beaufort West: length up to 19.4 mm, width up to 11.3 mm; adult L/W 1.63–2.0; body whorl length = 78.5–84.5% of shell length; aperture length = 45.3–58.3% of shell length (N=27).

Protoconch ( Fig. 7 ): Diameter approx. 1.6–1.85 mm (Somerset East); 1.85–2.2 mm

(Beaufort West), comprising approx. 2.0 whorls; first whorl set at a pronounced angle,

rising to start of second whorl which is initially flat-sided with an almost ridge-like apical angle, becoming more evenly convex toward end of whorl; sculptured with widely spaced collabral axial riblets, approx. 0.20 mm apart, riblets about one fifth the width of their intervals; intervals sculptured by numerous close-set spiral striae; riblets usually straight, but somewhat wavy in occasional individuals; specimens from Beaufort West area show strongest protoconch sculpture, in which the change from embryonic to adult sculpture is moderately abrupt and the protoconch–teleoconch junction thus relatively distinct; those from Somerset East (particularly above the Glen Avon falls) with weaker, sometimes almost obsolete protoconch sculpture, but many specimens from this site badly eroded.

Type material: Connolly indicated that the ‘type set’ was in the BMNH, but the type lot contains only one specimen, BMNH 89.11 .4.9, ‘ Somerset ( East ), Cape Colony’ [ South Africa], length 19.5 mm ( Fig. 2 ). It is here designated lectotype.An additional two paralectotypes are present in RMCA (Glen Avon, Miss Bowker, 97477, 97478) .

Type locality: Sowerby stated that the original specimens were collected in ‘forest about 3000 ft [about 900 m] above sea-level’, near Somerset East. Connolly (1912) localised this more precisely as ‘ Glen Avon Falls’ and labels associated with additional NMSA material collected in the early 1900s give the locality as ‘ The Gorge, Somerset East’ —another name for the Glen Avon Falls . It seems likely that Colonel J.H. Bowker and/or his niece Mary Layard Bowker collected the original specimens at this locality. Indeed, descendants of the Bowker family still own ‘ Glen Avon’ farm .

Additional material examined: SOUTH AFRICA: Eastern Cape: Glen Avon Falls, Somerset East (32°40.887'S: 25°38.13'E), 1000 m a.s.l., on large boulders in forest beside base of falls, D. Herbert, M. Bursey & G. Redman, 19.i.2002 ( NMSA V9816 , W753 ) ; same locality, 1100 m a.s.l., on rock wall above falls, D. Herbert, M. Bursey & G. Redman, 19.i.2002 ( NMSA V9818 ) ; Somerset East, J. Farquhar and Mrs J.H. Howard ( NMSA 2978 , B115, W392, W887-8) ; ‘ The Gorge’ Somerset East ( NMSA 2977-8 , B110-4, B116, W392, W887, mostly collected by Mrs J.H.Howard,1915) . Northern Cape: Oukloof, Nuweveldberg, between Fraserburg and Beaufort West [estimated position 32°10'S: 21°45'E, approx. 1200–1500 m a.s.l.], Drs Hesse & Boonstra, 1946 ( SAM A8171 ) . Western Cape: Beaufort West area, Karoo National Park, Rooivalle (32.32553ºS: 22.449ºE), 1130 m a.s.l., and Fonteintjieskloof (32.29553ºS: 22.17314ºE), 1150 m a.s.l., M. Bursey, i.2004 ( NMSA W1479 , W1480 ) .

Distribution ( Fig. 9 ): Endemic to South Africa, recorded from Eastern Cape, Northern Cape and Western Cape provinces. Currently known only from three localities, all situated on the southern section of the Great Escarpment, in the vicinity of Somerset East and Beaufort West, at altitudes of 1000–1500 m.

Habitat ( Figs 10, 11 ): Early records provided no habitat information, but recently collected samples suggest very specific habitat requirements. Specimens from both the type locality and the Karoo National Park were found where rivers, in traversing sharp topographical gradients at the edge of escarpments, form waterfalls and have cut steep-sided, wooded/forested gorges or ‘kloofs’. The snails are found almost exclusively on near-vertical rock walls above the water course, primarily up-stream of the falls, and on the shaded (south-facing) side. The rock faces are mostly bare of macroscopic vegetation and superficial encrustations, the snails hiding in crevices and under overhangs in the otherwise smooth rock, sometimes in aggregations of more than 100 individuals. The populations seem restricted to the environs of the waterfall, mostly ranging less than 200 m above or below it. The third locality, at Oukloof in the Nuweveldberg, appears also to be a site where a river (Koekemoersrivier) descends an escarpment, but no habitat details were recorded with the specimens.

Although situated in a relatively dry area (rainfall about 300–500 mm per annum, Schulze 1997), orographic phenomena associated with areas of sharp topographical change would be likely to concentrate rainfall and mist at these escarpment-edge localities. The habitats will thus be less xeric than the surrounding eastern mixed Nama Karoo and valley thicket vegetation ( Low & Rebelo 1996), and hence the presence of afrotemperate forest and wooded ravines on south-facing slopes. Cooling orographic breezes combined with wind-blown spray from the waterfall and the shaded aspect of the habitat are further likely to ameliorate conditions.

Notes: In the Glen Avon population there appear to be two cohorts of shells, suggesting a biennial lifecycle. In one, the shell is relatively thin, uneroded, and the periostracum is for the most part still present. In the second, the shell is somewhat thicker, and the periostracum and sculpture almost completely eroded. The former probably represent snails hatched during the season of collection and the latter may perhaps be snails that are more than one year old.

Figures

Figs 2–6. Prestonella bowkeri (Sowerby, 1890): (2) lectotype of Bulimus (Mesembrinus?) bowkeri Sowerby, 1890, length 19.5 mm (BMNH 89.11.4.9); (3) fresh specimen, Glen Avon Falls, length 18.5 mm (NMSA V9816); (4) eroded specimen, Glen Avon Falls, length 20.5 mm (NMSA 2977); (5) Beaufort West specimen, length 17.7 mm (SAM A8171); (6) side view, length 20.5 mm (NMSA 2977).

Figs 7, 8. Prestonella protoconchs: (7) P. bowkeri, Nuweveldberg, Beaufort West (SAM, A8171); (8) P. nuptialis, protoconch. Scale bars = 0.5 mm.

Fig. 9. Prestonella bowkeri distribution.

Figs 10, 11. Habitat of Prestonella bowkeri, above Glen Avon falls, Somerset East: (10) flat river bed with near vertical rock wall on south-facing bank, (11) cluster of more than 100 individuals under an overhang in the rock wall.

Figs 19, 20. Prestonella bowkeri and Prestonella nuptialis, living specimens: (19) P. bowkeri, shell length 18.0 mm, Glen Avon Falls (NMSA V9816); (20) P. nuptialis, shell length 13.0 mm, Fenella Falls (NMSA W5507).

Figs 21–25. Prestonella radulae:(21–24) P. nuptialis (NMSA V9778): (21) central field and inner marginals (rachidian and laterals numbered), scale bar = 50 µm; (22) rachidian (R) and inner laterals, scale bar = 25 µm; (23) outer marginals, scale bar = 25 µm; (24) latero-marginal transition, scale bar = 25 µm; (25) P. bowkeri (NMSA V9816) latero-marginal transition, scale bar = 25 µm.