Avakubia acuminata ( Thiele, 1933 ) de Winter & Vastenhout, 2013

de Winter, A. J. & Vastenhout, N., 2013, Revision of the Afrotropical land snail genus Avakubia Pilsbry, 1919, with description of Pseudavakubia gen. n. and eleven new species (Gastropoda: Pulmonata: Streptaxidae), African Invertebrates 54 (2), pp. 605-663 : 618-623

Avakubia acuminata ( Thiele, 1933) , comb. n.

Figs 4, 5A–F, 6, 7, 10A–D

Gulella (Avakubia) acuminata: Thiele 1933: 316 , fig. 61; de Winter & Gittenberger 1998: 239.

Diagnosis: Differs from other Avakubia species by the comparatively large (> 5 mm) shell with tapering spire and strongly acuminate apex.

Description:

Shell ( Figs 4, 5A–F, 10A–D, Table 1): Large for the genus (mean H 5.4 mm), elongate (H:D 1.88–2.1, median 1.97, in holotype 2.01), biconical, spire tapering. Coiling tightness 3.7–4.1, median 3.9, in holotype 4.0. Whorls moderately convex. Apex strongly acuminate. Protoconch comparatively high-spired, consisting of 2¼–2.5 whorls. Protoconch sculpture ( Figs 10A–D) consists of comparatively regularly spaced spiral cords, 10–12 on second whorl, each 6.6–8.5 µm wide. Cords composed of series of 13–22 µm long, elongate-rounded particles, that are individually just separated. Spiral sculpture starts ca ¼ whorl from nucleus. Teleoconch sculpture consists of slightly oblique, prosocline axial ribs, 6.4–9.2 ribs/mm on penultimate whorl, median 8 ribs/mm, in holotype 7.8 ribs/mm. Interstices with fine, solid, more or less irregular spiral lines. Body whorl proportionally small, BWH 41–47 % of H, median 44 %, in holotype 42%. Peristome complete in most specimens; some otherwise fully developed specimens with incompletely developed parietal-angular margin are probably subadult. Peristome somewhat higher than wide, PH:PW 1.08–1.24, median 1.18, in holotype 1.19. PH 29–34 % of H, median 33%, in holotype 31%. PW 49–60% of D, median 55%, in holotype 52 %. Apertural lip expanded and flaring, not strongly incrassate. Two apertural barriers visible in frontal view: a more or less pointed, tooth-like, thickening on mid-palatal wall and a conspicuous, somewhat projecting angular tooth that extends inwards as a deeply entering lamella. Internal wall of body whorl with a deep-set palatal fold ( Fig. 5), not visible in the aperture, but discernable in translucent specimens as short pale stripe ( Figs 4D, E). Short, little conspicuous lamella present on columella ( Fig. 5), but not visible in aperture. Umbilicus open but rather narrow .

Body colour: Live animal cream-whitish, without reddish pigment.

Anatomy ( Fig. 6; five specimens dissected, including two with fully grown shells but still poorly developed genitalia): Atrium mostly longer than wide, thin-walled. Penis more or less elongate, with a short apical caecum as well as a more conspicuous lateral diverticulum. In some (juvenile or contracted?) specimens diverticulum appears subapically, nearby apical caecum, giving penis a bifid appearance. Vas deferens in some specimens coiled around vagina. Vas deferens abruptly widens close to upper penis into a muscular terminal portion that enters upper penis laterally, cutting off a short apical caecum. Penis retractor muscle long and twisted, originating from columella muscle and inserting on penial caecum, extending under widened terminal vas deferens. Interior of penis distally without strong pilasters, internal wall with small irregular tissue pads. No chitinous spines were found inside penis.A pale, elongate spindle-shaped spermatophore-like structure ( Fig. 6E) with yellow content was encountered inside lower penis of one specimen. Bursa copulatrix arises from oviduct, cutting off short but distinct vagina, consisting of long narrow bursal duct with roundish oval sac. Free oviduct narrow, in two specimens strongly inflated to form a pouch­like uterus containing single, shelled, embryo. Hermaphrodite duct with long, tube-like, convolute talon.

Radula ( Fig. 7): Ribbon very long and narrow. Row formula 16-C-16, number of rows not counted (n=2). Central tooth distinctly smaller than laterals. Distinction between laterals and marginals gradual, latter appearing more slender. Viewed from above, mesocone of lateral and marginal teeth appear as elongate, sharply pointed, curved blades; in other views, endocone becomes visible as a lateral dilatation.

Holotype (examined): CAMEROON: “Kamerun, wahrscheinlich [probably] Johann Albrechtshöhe”, Conradt ( ZMB 72855 ).

Other material examined: CAMEROON: Sud Prov. : Minwo area ca 6 km NE Ebom, 15 km S Lolodorf, 3.10°N 10.73°E, 400–500 m, all collected within a single square kilometre of near-primary evergreen forest, from 1–3 m high understorey vegetation, except for three rather eroded empty shells (2 juv. and 1 ad.), viii–x.1995 and iii–iv.1996, A.J. de Winter & E.J. Semengue (16 samples containing 20 ad. & 6 juv. dry and 17 ad. in alcohol, of which 5 were dissected; RMNH) .

Distribution ( Fig. 8): For a long time A. acuminata was only known by the holotype shell, until the species was rediscovered near Lolodorf in Southwest Cameroon (de Winter & Gittenberger 1998). The type locality is uncertain [“Wahrscheinlich (probably) Johann Albrechtshöhe” ( Thiele 1933), presently Kumba, 4.63°N 9.45°E], which actually lies more than 200 km NW of Lolodorf. Although the occurrence of the species near the surmised type locality cannot be disproved, it may well be that the holotype was actually found closer to the second locality. Thiele (1933) mentioned Conradt as collector. It appears that Leopold Conradt, who collected especially insects, not only visited the surmised type locality, but also the Lolodorf area where he might have collected the type specimen while capturing, for example, stingless bees using a sweep net (as indicated in Eardley 2004).

Habitat: Found in a near-primary rainforest at 400–500 m elevation. Specimens were collected alive from 1–3 m high understorey vegetation, except for three rather eroded empty shells. The species lives syntopically with A. semenguei and A. fruticicola .

Remarks: A. acuminata is the largest species of Avakubia , and is readily recognizable by its characteristic elongate shell with tapering spire and acuminate apex. Its attribution to Avakubia is supported by a number of shell characters shared with A. avakubiensis , some of which may be synapomorphies, notably the beaded spiral sculpture on the protoconch, and the deep-set palatal lamella.