Gulella nictitans, B & Rowson & C & N & Lange, 2007

B, Rowson, C, N & Lange, 2007, Two new species of Gulella (Mollusca Pulmonata: Streptaxidae) from the Taita Hills Kenya, African Invertebrates 48 (2), pp. 21-32 : 27-30

Gulella nictitans sp. n.

Figs 4–7 , 14–19

Etymology: From Latin nictitans (winking); with reference to the parieto-palatal sinus that recalls a closed eye.

Diagnosis: Shell small for the genus, distinctively ovate-biconical; sculptured with strong axial ribs; peristome complete, apertural dentition distinctive, arguably five-fold: a strong, palatally directed parietal lamella meeting a large palatal slab, the two almost meeting along a narrow parieto-palatal sinus, the other dentition consisting of three folds on the columella; embryonic shell finely granular; umbilicus closed.

Description: Adult shell ( Figs 4–7 ) small for the genus (height 3.6–3.9 mm, width 2.0– 2.2 mm), and ovate-biconical, of 5.5 whorls. Spire more conical than most other Gulella . Shell shape varies a little intraspecifically but other shell features almost constant. Penultimate whorl and body whorl together comprise about 67% of total shell height. Aperture (including peristome) comprises about 34% of total shell height. Peristome complete, thickened and reflected and flaring. Body whorl strongly constricted adjacent to the aperture, especially along the excision corresponding to the palatal slab, such that the palatal side of the body whorl is divided into two regions. Columella relatively straight but body whorl occludes any umbilicus, the visible suture being the junction of the body whorl and penultimate whorl. Apertural dentition well-developed, occluding much of the aperture, and consistently with five processes, although the three columellar processes are little more than folds on the columella and so the dentition could be considered less than five-fold.Apertural processes are as follows: (1) a large, thick, palatally directed and deeply entering parietal lamella projecting outwards a very short distance from the body whorl and corresponding to an outwardly and palatally directed V-shaped depression in the parietal callus; (2) a strong, slab-like palatal process, curved inwardly on the columellar side and nearly straight along its upper margin, and corresponding to a very long and deep excision on the palatal side of the body whorl; and (3–5) three slight, simple, and deeply set folds on the columella, the two upper folds being much smaller than the basal one. Resulting apertural dentition formula sensu Verdcourt (1962) can be given as 1; 1; 0; 3, or alternatively 1; 1; 0; 1(3). Parieto-palatal sinus narrow, tubular and directed palatally at an angle of 75–90° from the columella, almost closed in front by the near-meeting of the margins of the parietal lamella and palatal slab along a straight line, resulting in a conspicuous narrow slit. Parietal callus thick and clearly distinct from body whorl, being separated from it by a detectable groove. Outer columellar wall with a slight but long excision corresponding to the lowermost columellar fold. No sign of juvenile dentition in the single juvenile specimen (paratype 9) or inside the shells of dissected adults. Shell colourless and glassy, dirty white when corroded, with no obvious periostracum. Shell surface as follows: embryonic whorls smoothly, irregularly granular under a light microscope (60× magnification). Later whorls with major sculpture of strong, oblique, straight or slightly sinuous, regular axial ribs with sharply defined edges, interstices between ribs nearly smooth. On the holotype, ribs occur at a density of about 13 per mm (penultimate whorl) and about 12 per mm (body whorl). On palatal side of body whorl, ribs continue only as far as the long excision corresponding to the palatal slab, being replaced by much finer, irregular ribs beneath the excision. Finer, irregular ribs also replace the major axial ribs on the columellar side of the body whorl.

Body colour: Alcohol-preserved specimens are unicolorous pale cream, without differently coloured tentacles, patterning on the mantle, etc.

Salivary gland ( Fig. 14 ): Single, 1.2 mm long, fragile, distinctively Y-shaped, with anterior portion of posterior half of gland produced anteriorly. Anterior duct exiting gland apically; posterior duct exiting from the apex of the posterior half of gland. Both salivary ducts evenly narrow throughout their length.

Genitalia ( Figs 15–19 ): Penis rather long (3.25 mm when straightened or 1.5 whorls when bent) without appendices. Apical part of vas deferens bound to penis by an extremely thin, transparent sheath contiguous with wall of penis basally. Vas deferens substantially thickened distally, entering penis subapically, with penial retractor muscle attaching to apical part of penis and to vas deferens near the entry point and thus effectively bifid. Penial retractor originating from the free muscle system and not from the body wall. Interior surface of penis ( Fig. 15 ) consisting of soft structures and chitinous hooks (some highly modified) as follows. Apical part of penis: slightly constricted, with a single large (about 0.35 mm long), spatulate yellow-brown modified hook, its base embedded in fibres of penial retractor and projecting beyond constriction into main lumen of penis, with a row of thin extensions at its tip. This is flanked by two large (about 0.25 mm long) bifid yellow-brown hooks attached by very narrow bases. Main part of penis: dominated by a single central longitudinal blade-like pilaster, flanked by incomplete transverse ridges or septae and a single row of low, irregularly shaped pads bearing simple yellow-brown triangular hooks that vary little in size and shape (about 0.05 mm long). Basal part of penis: longitudinal pilaster, pads and septae interrupted, resulting in a more irregular surface, structures becoming very indistinct towards atrium. Total number of hooks in penis: 31. Albumen gland moderately sized and with a uniform structure of small and indistinct vesicles or acini. Hermaphroditic duct diverticulum (talon) enlarged, simple and not convoluted, not hidden within albumen gland. Bursa copulatrix (= gametolytic sac or spermatheca) small, ovoid, apparently empty, and attending albumen gland. Acini of oviductal gland elongate, flattened, large and distinct. Acini of prostate very indistinct. Vagina and atrium short, little muscularised. Holotype: KENYA: Taita Hills , Macha Forest Reserve (3°27'S: 38°22'E), leaf litter in indigenous forest at 1550 m altitude (site IB), leg. C.N. Lange, 27.ix.1998 ( NMK).

Paratypes: Collection data for all paratypes same as holotype; all specimens except paratype 5 and paratype 9 are live-collected adults (Table 2).

Other material examined: 9 additional specimens, as follows: KENYA: Taita Hills: 1 adult, Macha Forest Reserve (3°27'S: 38°22'E), leaf litter in indigenous forest at 1550 m altitude, leg. C.N. Lange, 27.ix.1998 ; 1 adult, Ndiwenyi Forest Reserve (3°26'S: 38°21'E), leaf litter in indigenous forest at 1580 m altitude, leg. C.N. Lange, 26.ix.1998 ; 6 adults, Yale Forest Reserve (3°24'S: 38°18'E), leaf litter in indigenous forest at 1900 m altitude, leg. C.N. Lange, 29.ix.1998 ; 1 adult, Fururu Forest Reserve (3°24'S: 38°20'E), leaf litter in indigenous forest at 1400 m altitude, leg. C.N. Lange, 26.ix.1998 ( NMK) .

Remarks: Comments made under G. ndiwenyiensis regarding generic placement also apply. Shell shape and dentition immediately suggest an affinity with certain other African Gulella , in particular G. radius (Preston, 1910) of the Shimba Hills and coastal Kenya and Tanzania. The Malawian G. meridithae van Bruggen, 2000 , the Comoran G. miniuscula (Morelet, 1877) and the South African–Mozambican G. browni van Bruggen, 1969 share with G. nictitans the biconical shell shape and coarse pattern of apertural dentition and may prove to form a monophyletic group. The Tanzanian G. cuspidata Verdcourt, 1963 might also be referrable to this group, but has more complex dentition than the aforementioned species. Assuming the features shared by all these species are in some way indicative of phylogenetic relatedness, we consider G. nictitans to be a northerly, endemic representative of a group otherwise distributed widely from eastern to south-eastern Africa, and possibly also on Comoros. Certain other land snail groups or taxa, including some streptaxids, have comparable distributions ( Herbert & Kilburn 2004; Rowson in press; Verdcourt 2000) but none has yet been reported from the Taita Hills.

Figures

Figs 1–7. New Gulella species from Kenya: (1–3) G. ndiwenyiensis sp. n., holotype shell, length 4.9 mm; (4–7) G. nictitans sp. n.: (4–6) holotype shell (length 3.8 mm), (7) aperture of paratype 1.

Figs 14–19. Gulella nictitans sp. n., anatomy of paratype 2: (14) salivary gland from columellar side; (15) penis opened dorsally; (16–18) hooks from penis; (19) genitalia from dorsal side. Abbreviations: bc – bursa copulatrix, p – penis, prm – penial retractor muscle, sgd – salivary gland duct, v – vagina, vd – vas deferens.