Cyathopoma pembense Rowson

Rowson, B, Warren, B. H. & Ngereza, C. F., 2010, Terrestrial molluscs of Pemba Island, Zanzibar, Tanzania, and its status as an " oceanic " island, ZooKeys 70, pp. 1-39 : 6-8

1. Cyathopoma pembense Rowson sp. n. Figs 1627

Type material:

(all from TANZANIA: Zanzibar: Pemba Island). Holotype (NMW.Z.2009.013.00001): adult shell stored dry; in leaf litter, near Wete (Locality 10 in Fig. 1 and Table 1), 13 February 2009, leg. B. Rowson, B. H. Warren & C. F. Ngereza. Paratypes (NMW.Z.2009.013.00002-00032): 31 adults and juveniles in 80% ethanol; other data as holotype. Paratypes (NMW.Z.2009.013.00033-00077): 45 adults and juveniles stored dry; other data as holotype. Paratypes (NMW.Z.2009.013.00078-00079), 2 adults gold-coated for SEM; other data as holotype. Paratypes (NMT): 2 adults stored dry; collection data as holotype. Paratypes (BMNH.20100582): 1adult & 1 juvenile stored dry; collection data as holotype. Paratypes (MNHN): 1 adult & 1 juvenile stored dry; collection data as holotype. Paratypes (NMSA.L8207/T2591): 2 adults stored dry; collection data as holotype. Paratypes (RMNH): 2 adults stored dry; collection data as holotype. Paratypes (NMW.Z.2009.013.00080-00096): 17 adults and juveniles in 80% ethanol; in leaf litter, Msitu Mkuu FR (Locality 8 in Fig. 1 and Table 1), 10 February 2009, leg. B. Rowson, B. H. Warren, C. F. Ngereza & paid local collectors. Paratypes (NMW.Z.2009.013.00097-00174): 78 adults and juveniles stored dry; other data as previous. Paratype (NMW.Z.2009.013.00175): 1 adult gold-coated for SEM; other data as previous. Paratypes (NMW.Z.2009.013.00175-00176): 2 adults in 80% ethanol; in leaf litter, near Matuleni (Locality 12 in Fig. 1 and Table 1), 15 February 2009, leg. B. Rowson & C. F. Ngereza. Paratypes (NMW.Z.2009.013.00177-00210): 33 adults and juveniles stored dry; other data as previous. Paratype (NMW.Z.2009.013.00211): 1 adult stored dry; in leaf litter, near Wambaa (Locality 13 in Fig. 1 and Table 1), 13 February 2009, leg. B. Rowson, B. H. Warren, C. F. Ngereza & paid local collectors.

Diagnosis:

Shell relatively large (to 4.20mm wide) and strongly depressed. When fresh, with spirally-ridged operculum and characteristic periostracum of radial lamellae peripherally extended into long hairs gathered into points, or much shorter hairs gathered into fringes. When denuded, with relatively few spiral keels.

Description of holotype: Adult shell (Figs 16-18) relatively large for the genus in Africa, 2.25mm x 3.95mm including periostracum, strongly depressed, of approximately 4.5 regularly expanding whorls, with wide, perspective umbilicus. Peristome effectively complete, slightly thickened and flaring, especially basally. Aperture and operculum effectively circular. Operculum calcareous, outer surface with multispiral, blade-like raised lamella of approximately 9 revolutions, weakly convex as a result; inner surface smooth. Protoconch smooth, with irregular malleation discernible only at extreme magnification (Fig. 25). Teleoconch periostracum of fine, extremely close (<0.025 mm apart) radial lamellae, running from suture to suture. Lamellae each prolonged into long, flat periostracal extensions ( “hairs”) extending well beyond the whorl periphery, forming spiral keels (up to four on the body whorl), with less-pronounced periostracal keels continuing into umbilicus. Periostracal hairs (in life and in fresh shells, whether wet or dried) regularly gathered at their tips to form bunches of six or more hairs (Fig. 25).

Further description from paratypes:

The periostracum of C. pembense forms a continuum of variation. At one extreme are individuals with hairs gathered together at their tips (as in the holotype). At the other are those in which the periostracal lamellae form instead a rough, raised periostracal fringe where the lamellae appear cemented together (Figs 19-21; 27). These extremes are more frequent than intermediate forms, but such intermediates do occur, in which the bunches of hairs are irregularly missing, probably worn away (see below). The two extreme forms are sympatric at three of the species’ four localities - i.e., at the type locality, at Msitu Mkuu FR, and near Matuleni (Localities 10, 8 & 12 respectively in Fig. 1 and Table 1). At the fourth locality near Wambaa (Locality 13 in Fig. 1 and Table 1), only one individual was found, and was of the fringed form. Both forms include both live- and dead-collected individuals, and both adult and juvenile shells. The size ranges overlap, although the fringed form seems to reach a slightly larger maximum (4.00-4.20mm wide with 4.25-4.5 whorls). Other features of the shell (shape, operculum, and protoconch; Fig. 26) are consistent across all individuals. Shells from which the periostracum has been lost were common but always empty, and cannot be assigned to either form. Such denuded shells (Figs 22-24) have relatively few (up to 8) weak spiral keels on the body whorl (including umbilical part), with fine, extremely close incised radial lines between the keels. All nine live-collected individuals of the fringed form were dissected and a penis was detected in six of them. A penis was not detected in any of nine individuals of the hairy form.

Remarks:

This species is attributed to Cyclophoridae: Cyathopoma sensu lato following Emberton (2003). All forms of Cyathopoma pembense differ from the few other East African Cyclophoridae in being larger and more depressed than Cyathopoma azaniense Verdcourt, 1978 (Figs 13-15), an undescribed azaniense-like species from the East Usambaras ( Verdcourt 2006; NMW material examined), and the Malawian Cyathopoma tres van Bruggen, 2008 ( van Bruggen 2008). They are also larger than the Central African Cyathopoma papillaris (von Martens, 1892) and have fewer spiral keels (see van Bruggen 1986). The elaborate periostracum appears to be unique among East African taxa but similar features occur in some southeast African and western Indian Ocean island taxa. Cyathopoma pembense differs from species of the southeast African Chondrocyclus Ancey, 1898 either in the operculum or in periostracal features; Cyathopoma putealis Connolly, 1939 and Cyathopoma trifimbriatus Connolly, 1939 have fringes like Cyathopoma pembense but very different opercula (see van Bruggen 1986; Herbert and Kilburn 2004). Cyathopoma pembense is more depressed and differs in periostracum from the Seychelles Cyathopoma blanfordi H. Adams, 1868 (see Gerlach 2006a, b). Photographs of Cyathopoma pembense were compared with the BMNH types of several Comoros species attributed to "Cyclotopsis" ( Cyathopoma nevilli Morelet, 1877, Cyathopoma ilicum Morelet, 1877, and Cyathopoma horrida Morelet, 1887). Although worn, none of these were an exact match for Cyathopoma pembense . Nor does it agree with the descriptions or figures of any other cyclophorid of the Comoros (see Fischer-Piette and Vukadinovic 1974), Madagascar ( Emberton 2003, 2004), the Mascarenes ( Griffiths and Florens 2006), the Seychelles ( Gerlach 2006a, b), nor any Asian species known to us.

The variation shown by this species is striking. One might consider the extreme periostracal forms separate species, albeit indistinguishable when the shells are denuded. However the presence of intermediates suggests that this is not the case. The variation could result from sexual dimorphism (hairy forms female, fringed forms male) which would explain their occurrence in sympatry. However, sexual dimorphism would not explain the existence of intermediate forms. It would also demand that the three fringed individuals without a penis were immature males rather than females, when equally possible is that fringed forms consist of three females and six males while and all nine hairy individuals were immature. Natural wear and corrosion on the periostracum, presumably from hairy to fringed forms, would explain the latter possibility and account for the continuum of variation. It would not, however, easily explain the existence of live animals of both types (in each case both adults and juveniles in sympatry, where presumably the whole population is exposed to similar factors causing wear and corrosion. Possibly both sexual dimorphism and wear on the shells play a part in this unusual pattern. More speculatively, other alternatives include incomplete speciation or hybridisation between two closely related species.

Distribution:

Apparently endemic to Pemba island.

Etymology:

pembense, from Pemba island, a noun in the generative case.