Afrorhytida Möllendorff, 1903

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 : 75-79

Genus Afrorhytida Möllendorff, 1903

Afrorhytida: Möllendorff 1903: 61 . Type species: Helix knysnaensis Pfeiffer, 1846 , by original designation. Hyperrhytida Watson, 1934: 156 . Type species: Helix (Aerope) trimeni Melvill & Ponsonby, 1892 , by original designation.

Diagnosis: Shell of moderate size (adult diameter up to 31 mm), lenticular to globose, more or less uniformly yellowish or greenish brown, or brown, lacking spiral colour pattern; apical surface sculptured with close-set axial riblets, base smoother and more glossy; outer lip slightly thickened and without periostracal fringe; peristome interrupted in parietal region; umbilicus open, width narrow to moderate; protoconch of approx. 1.25 whorls, sculptured by axial riblets (sometimes weak), diameter 2.5–5.0 mm. Radula possessing rachidian tooth; outer lateral teeth conspicuously larger than marginal teeth; inner marginal teeth small but not vestigial. Labial palps present, but relatively small; left body lobe of mantle divided into two lobes, separated by a low ridge. Penis elongate and cylindrical, situated to left of retractor muscle of right optic tentacle; epiphallus well developed (approx. 50–75 % of penis length); epiphallus and lower vas deferens not joined to penis by connective tissue web; vagina long; oviduct caecum absent or little more than a small lateral pouch. Distal part of suprapedal gland sinuous but not convoluted, lacking a swollen terminal vesicle.

Radula ( Figs 58 , 64 , 67 , 71 ): Length up to 22.5 mm, with up to 85 V-shaped transverse rows of teeth, 3.2–4.5 rows per/mm in adult; formula varies considerably with species ( Table 2); rachidian present, but small; inner lateral teeth slender and acuminate, increasing slightly in size; outer 2 or 3 laterals considerably larger, each with a stout quadrate base-plate and curved, sharply pointed cusp; marginal teeth either rapidly or gradually decreasing in size and becoming vestigial at radula margin, but always with 1 or 2 of intermediate size adjacent to outermost lateral tooth, more in A. knysnaensis .

The boundary between the lateral and marginal series is easy to determine in Afrorhytida knysnaensis where the largest tooth (the outermost lateral) has a stout, quadrate base-plate and is followed by a much smaller (0.5–0.66× length of largest tooth) and far less robust tooth with a reduced base-plate (the innermost marginal). In the remaining species, however, the distinction is less clear and the largest tooth is followed by one which is only slightly smaller (0.75–0.9× length of largest tooth) and which retains a robust cusp and well-developed, quadrate base-plate. In this case, though it is not the largest tooth, we have included this latter tooth in the lateral series. For these species therefore, it is the penultimate lateral tooth that is the largest.

In A. trimeni and A. burseyae sp. n. the number of marginal teeth is usually less than 10 and there are more laterals than marginals. In A. knysnaensis and A. kraussi there are more than 10 marginals and they equal or outnumber the laterals.

External anatomy ( Figs 6A , 55 ): Left body lobe of mantle divided into two relatively small lobes separated by a low ridge ( Fig. 6A ); skin texture moderately granular; neck region usually with a paler longitudinal line extending backward from the base of each optic tentacle, often bordered laterally by a darker line; labial palps present but relatively small; optic and inferior tentacles with a ventrally distended apical bulb; tail region of foot short; genital pore ventral and just posterior to right optic tentacle.

Distal genitalia ( Figs 9B , 53 , 54 ): Penis long, elongate-cylindrical, usually tapering slightly toward its base, often somewhat sinuous in situ, but not strongly so; penis length and thickness of its wall variable and dependent on relaxation state during preservation; internal lumen lined by papillate epithelium ( Fig. 54A ), papillae coarser than those of Natalina and less tightly packed, somewhat angular and irregularly positioned, but alignment generally longitudinal rather than transverse, particularly in basal region. Apex of penis evidently lacking a distinct papilla, its lumen communicating directly with epiphallus lumen; epiphallus joins penis at U-shaped bend, to which penial retractor muscle is attached. Epiphallus well developed, 0.50–0.75 of penis length, tapering toward its junction with vas deferens, reflexed to lie beside penis, but not joined to it by connective tissue web. Internal morphology of epiphallus exhibiting considerable inter-specific variation ( Figs 54B–E ), mirroring morphology of spermatophore. In all species its inner wall (facing penis) bears two well-developed longitudinal folds which extend for the full length of the organ, between which is a narrow groove; remaining wall of lumen bears a series of longitudinal ridges of varying strength and length, with microscopic pits in their intervals, in a pattern which appears to vary with species. Pits generally occur where epiphallus wall is thickest and represent the openings of minute diverticulae in which the spermatophore spines are moulded; their arrangement thus determines the pattern of spines on spermatophore; diverticulae usually visible externally as diffuse white specks in epiphallus wall. Vas deferens runs alongside penis base, vagina and free oviduct, sometimes sinuous in the latter region, before it fuses with spermoviduct. It is closely applied to the vagina and oviduct and held in place by connective tissue, but its lower region is free of the penis base.

Vagina of moderate length (relatively shorter than in Capitina and Natalina ) generally of more or less even width, but basally swollen in Afrorhytida kraussi , attached laterally to body wall by a series of well-developed muscle fibres; vagina merges with free oviduct at point of origin of bursa copulatrix duct or just above this; an oviduct caecum is lacking or present merely as a short pouch-like vestibule into which the free oviduct opens, just above the origin of the bursa copulatrix duct (in Afrorhytida knysnaensis and Afrorhytida trimeni ); inner wall of vagina with longitudinal folds in 2–3 orders of magnitude; similar but somewhat thicker folds lining free oviduct. Bursa copulatrix duct long and slender, attached to free oviduct by connective tissue and then running beside convoluted spermoviduct (not within its convolutions) to bursa itself, situated posterior to pericardium and kidney; bursa elongate-ovate to pyriform, very thin-walled. Spermoviduct with distinct oviducal and prostatic portions, the former with superficial folds.

Spermatophores ( Figs 10 , 62 , 68 ): Allospermatophores have been found in the female tract of Afrorhytida burseyae sp. n. and A. kraussi and are discussed in detail under the respective species treatments. These differ considerably in shape, size and ornamentation, suggesting that spermatophore morphology is species specific and may ultimately provide useful supplementary morphological characters to test the validity of the species as currently delimited. However, at present very few spermatophores have been encountered and those of A. knysnaensis and A. trimeni are unknown. The spermatophores of Capitina and Natalina are of a very different form and are compared in the introductory section.

Notes: Afrorhytida has traditionally been considered a subgenus of Natalina (Connolly 1939) . However, the morphology of the radula, distal reproductive tract and spermatophores show that Afrorhytida exhibits several character states quite distinct from those present in the Natalina radiation. In addition, molecular data indicate that Afrorhytida represents highly divergent monophyletic lineage worthy of recognition as a separate genus ( Moussalli et al. 2009).

Characteristic features of the reproductive tract that separate Afrorhytida from Natalina are the fact that the penis lies to the left of the retractor muscle of the right optic tentacle (to its right in Natalina ), the epiphallus and lower vas deferens in Afrorhytida are not connected to the penis by a web of connective tissue, and a distinct oviduct caecum is absent, at most there is a small swelling just above the origin of the bursa copulatrix duct. In terms of the radula, whereas the transition between the lateral and marginal series is abrupt in Natalina , there being no teeth of intermediate size, in Afrorhytida there is always at least one tooth of intermediate size outside the largest lateral.

Afrorhytida comprises a group of medium-sized rhytidids endemic to the Cape region. They occur in a wide range of habitats from coastal fynbos and scrub, to montane fynbos, various types of thicket, Afrotemperate forest and even Nama Karoo. Shell proportions appear to vary considerably within species and to overlap between species, such that species identification based on shell characters alone is frustratingly difficult. We have been able to define species boundaries only through the use of a combination of shell and radula characters, and molecular data. The above notwithstanding, we are confident in our correlation of the species thus identified with the nominal taxa described to date, even though the latter were based on shell characters alone .

Watson (1934) proposed the taxon Hyperrhytida as a section within Afrorhytida , defined largely by radula characters (inner laterals with laterally flanged cusps), and included in it A. trimeni and A. arguta . As shown below these two names in fact refer to a single species and therefore the characters which Watson used to define Hyperrhytida are species-level characters. Although the low number of marginal teeth (<10) in both A. trimeni and A. burseyae sp. n. may represent a synapomorphy providing some support for the existence of an Afrorhytida lineage distinct from Afrorhytida s. s., molecular data do not support a sister group relationship between these two taxa.

Figures

Fig. 58. Afrorhytida knysnaensis (Pfeiffer, 1846), radula: (A–C) Fort Beaufort (NMSA W5211): (A) entire width of radula, scale bar = 0.5 mm; (B) rachidian and inner lateral teeth, scale bar = 150 µm; (C) outer lateral and marginal teeth, scale bar = 250 µm; (D) Cradock (NMSA W4248), outer lateral and marginal teeth, scale bar = 250 µm.

Fig. 64. Afrorhytida kraussi (Pfeiffer, 1846), radula (all NMSA): (A–D) A. k. kraussi: (A–C) Diepwalle, Knysna (W4845): (A) entire width of radula, scale bar = 0.5 mm; (B) rachidian and inner laterals, scale bar = 250 µm; (C) outer lateral base-plates and marginals, scale bar = 200 µm (l – outer lateral; m – inner marginal); (D) Grootvadersbosch (W3351), half-row, scale bar = 0.5 mm; (E, F) A. k. oraria subsp. n., De Hoop (paratype, W3292/T2363); (E) half-row, scale bar = 100 µm; (F) outer lateral base-plates and marginals, scale bar = 100 µm.

Fig. 67. Afrorhytida burseyae sp. n., radula (holotype, NMSA W5228/T2257): (A) half width of radula, scale bar = 250 µm; (B) rachidian and inner lateral teeth, scale bar = 100 µm; (C) lateromarginal transition zone, scale bar = 100 µm (l – outer lateral; m – inner marginal); (D) marginal teeth [7], scale bar = 50 µm.

Fig. 71. Afrorhytida trimeni (Melvill & Ponsonby, 1892), radula:(A–C) Kap River Nat. Res. (NMSA W4206): (A) entire width of radula, scale bar = 0.5 mm; (B) rachidian and inner lateral teeth, scale bar = 150 µm; (C) inner lateral teeth, scale bar = 50 µm; (D) Umtiza, Nat. Res., East London (NMSA W5253), outer lateral base-plates and marginal teeth, scale bar = 100 µm.

Fig. 6. Mantle edge anatomy of Afrorhytida, Capitina and Natalina, showing variation in left body lobe structure (all NMSA): (A) Afrorhytida knysnaensis (W5211); (B) Capitina calcicola sp. n. (W3201); (C) Natalina (Natalina) cafra natalensis subsp. n. (V8415); (D) Natalina (Tongalina) wesseliana (V7668).Abbreviations: lbl – left body lobe of mantle, pn – pneumostome, rbl – right body lobe of mantle.

Fig. 55. Afrorhytida species, living animals (all NMSA): (A) A. knysnaensis, Fort Beaufort (W5211), shell diameter 19.0 mm; (B) A. knysnaensis, Cradock (W4248), shell diameter 22.0 mm; (C) A. knysnaensis, Kompasberg, Nieu Bethesda (W6097), shell diameter 20.4 mm; (D) A. kraussi kraussi, Featherbed Nat. Res., Knysna (W4671), shell diameter 16.8 mm; (E) A. k. kraussi, Grootvadersbosch (W5803), shell diameter 23.8 mm; (F) A. kraussi oraria subsp. n., De Hoop Nat. Res. (paratype, W3292/T2363), shell diameter 13.4 mm; (G) A. burseyae sp. n., Wedgeley farm, Kei River Valley (paratype, W5229/T2258), shell diameter 19.3 mm; (H) A. trimeni, Kap River (W4206), shell diameter 24.5 mm.

Fig. 9. Distal genitalia of Natalina, Afrorhytida and Capitina (all NMSA): (A) N. cafra natalensis subsp. n. (E7682), (B) A. knysnaensis (W4643), (C) C. schaerfiae (W5672). Abbreviations: bd – bursa copulatrix duct, ep – epiphallus, ga – genital atrium, oc – oviduct caecum, p – penis, pr – penial retractor muscle, sod – spermoviduct, v – vagina, vd – vas deferens.

Fig. 53. Afrorhytida knysnaensis (Pfeiffer, 1846), reproductive tract (NMSA W4643), scale bar = 5.0 mm. Abbreviations: ag – albumen gland, bc – bursa copulatrix, bd – bursa copulatrix duct, ep – epiphallus, ga – genital atrium, hd – hermaphrodite duct, od – oviduct, ot – ovotestis, p – penis, pr – prostate gland, pr – penial retractor muscle, v – vagina, vd – vas deferens.

Fig. 54. Penis and epiphallus of Afrorhytida species, cut open longitudinally to show internal structure (all NMSA): (A) A. knysnaensis, penis (W4248, Cradock); (B) A. knysnaensis, epiphallus (W4643, Somerset East); (C) A. trimeni, epiphallus (W4206, Kap R.); (D) A. burseyae sp. n., epiphallus (paratype, W5231/T2260); (E) A. kraussi kraussi, epiphallus (V6578, Wilderness). Abbreviations: d – diverticulae in epiphallus wall, dp – digit-like pilaster at end of ridge, ep – epiphallus, er – internal epiphallic ridge, ga – genital atrium, lg – longitudinal groove between well-developed folds on inner side of epiphallus, p – penis, pl – penis lumen with papillate lining, pr – penial retractor muscle, v – vagina, vd – vas deferens.

Fig. 10. Spermatophores of Capitina, Afrorhytida and Natalina (all NMSA):(A) C. calcicola sp. n. (paratype, W5670/T2265), length 8.2 mm; (B) A. burseyae sp. n. (paratype, W5231/T2260), length 10.5 mm; (C) A. kraussi kraussi (W5803), length 20 mm; (D) N. cafra natalensis subsp. n. (paratype, E7682/T2375), length 16.3 mm; (E) N. quekettiana lucaris subsp. n. (paratype, W3305/T2559), length 26.5 mm; (F) N. wesseliana (V7668), length 32 mm. The head of the spermatophore is uppermost in all cases and the lengths given are for the spermatophore as if straightened out.

Fig. 62. Afrorhytida kraussi kraussi (Pfeiffer, 1846) spermatophore (NMSA W5803): (A) in situ in upper portion of vagina and free oviduct (wall opened); (B) extracted spermatophore, total length 20 mm (straightened). Abbreviations: bd – bursa copulatrix duct, fo – free oviduct, od – oviduct, pr – prostate, s – spermatophore, sh – head of spermatophore, sp – spines on anterior region of spermatophore, st – tail of spermatophore, v – vagina, vd – vas deferens.

Fig. 68. Afrorhytida burseyae sp. n., spermatophore (paratype, NMSA W5231/T2260): (A) in situ in dissected upper portion of vagina and lower free oviduct, (B) extracted spermatophore, total length 10.5 mm. Abbreviations: bd – bursa copulatrix duct, fo – free oviduct, m – mid-piece of spermatophore, s – spermatophore, sh – spermatophore head, sp – spines on outer surface of mid-piece, st – spermatophore tail, v – vagina.