Oxystele antoni

Herbert, David G., 2015, An annotated catalogue and bibliography of the taxonomy, synonymy and distribution of the Recent Vetigastropoda of South Africa (Mollusca), Zootaxa 4049 (1), pp. 1-98 : 55-58

Oxystele antoni nom. nov.

( Figure 7 K–M)

Trochus (Turbo) variegatus (non Gmelin, 1791: 3575 = Heliacus ) Anton, 1838: 57, no. 2072. Type loc.: not originally given; four syntypes in MTD (504) from ‘Kap der Guten Hoffnung’ [Cape of Good Hope] ( Schniebs 1996). On the premise that Anton’s type material was lost, Heller & Dempster (1991: 403, pl. 1m, n) designated a neotype for this species (SAMC A37549). Anton’s collection, however, is not lost and it contains the syntypes mentioned above. As a result Heller & Dempster’s neotype designation must be set aside.

Trochus (Oxystele) variegatus — Krauss, 1848: 96.

Monodonta sagittifera (non Lamarck, 1822)— Pilsbry, 1889 in 1889 – 1890: 114 (in part).

Trochus variegatus — Martens, 1904: 57 note 15.

Oxystele variegata (in part, includes O. impervia ( Menke, 1843) — Barnard, 1963a: 268. Day, 1969: 160. Kensley, 1973: 40, fig. 93. Richards, 1981: 36, pl. 9, fig. 67. Kilburn & Rippey, 1982: 42, pl. 9, fig. 4.

Oxystele variegata — Heller & Dempster, 1991: 403, pl. 1m –x, pl. 2d. Branch et al., 2010: 176, fig. 77.3. Van der Bank et al., 2013.

Diloma variegata —Steyn & Lussi, 1998: 24, fig. 76. Marais, 2011: 61.

Distribution. Northern Transkei (Mbotyi) to northern Namibia (Kunene River); intertidal.

Notes. This and the following species were for many years considered to belong to a single variable species under the name Oxystele variegata (e.g. Barnard 1963a; Kilburn & Rippey 1982). However, based on analysis of electrophoretic data, Heller & Dempster (1991) postulated that two species were involved, O. variegata itself and O. impervia , and they provided an indication regarding how these can be distinguished on the basis of shell coloration. More recently, Van der Bank et al. (2013), using mitochondrial DNA sequence data, found supporting evidence for two lineages within the ‘ O. impervia – variegata complex’, but noted inconsistencies in terms of the colour patterns supposedly distinctive of the species.

From a nomenclatural perspective, Trochus variegatus Anton, 1838 is regrettably a junior primary homonym of Trochus variegatus Gmelin, 1791 , a well-known species now referred to Heliacus d’Orbigny, 1842 (Architectonicidae) ( Bieler 1993; Bieler & Petit 2005). Given the uncertainty surrounding species recognition in the ‘ Oxystele impervia – variegata complex’ a major consideration in establishing the correct name to use in place of Anton’s variegatus should be the certainty with which the name can be applied to the material currently considered to represent the species.

The name Trochus indecorus Philippi, 1846 has been considered a junior synonym of Anton’s variegatus (G.B. Sowerby (III) 1892; Barnard 1963a; Heller & Dempster 1991), but Philippi’s figure of T. indecorus ( Philippi 1848 in 1846 –1855: pl. 24, fig. 5), reproduced herein ( Fig. 7A ), shows a species with a convex columella and a colour pattern unlike that of T. variegatus Anton, 1838 . In reality it is closer to the species currently known as Phorcus sauciatus (Koch, 1845) ( Ávila et al. 2015; Gofas 2015). T. indecorus was originally described without locality, but Philippi later stated that it belonged to a group of species occurring only in South Africa ( Philippi 1847: 18). This I consider was erroneous and I do not believe that this name can be considered a junior synonym of Anton’s T. variegatus . Like Pilsbry (1889 in 1889 – 1890) I believe T. indecorus to be a synonym of P. sauciatus from the subtropical north-eastern Atlantic and Macaronesia. Mention of T. indecorus in South Africa by Krauss (1848) was merely a repetition of Philippi’s earlier record. Philippi himself stated ‘Diese Art ist vielleicht nichts weiter als eine Varietät von Tr. sauciatus Koch …’ [This species is perhaps nothing more than a variety of Tr. sauciatus Koch ] (Philippi, 1851 in 1846–1855: 143).

However, like Martens (1904) and Heller & Dempster (1991), I do not agree with Pilsbry in considering Trochus sagittiferus Lamarck, 1822 to be a name that can be applied to the ‘ Oxystele impervia – variegata complex’. Again described without locality, the holotype of this species (MHNG, INVE 51508, Fig. 7 B–D) also has a convex columella and a colour pattern unlike that of T. variegatus . Neither the holotype nor the figure of T. sagittiferus given by Delessert (1841) correspond with Pilsbry’s interpretation of the species and it is in fact much closer to Phorcus sauciatus , of which it may well be a senior synonym. Thus neither T. indecorus nor T. sagittiferus are possible replacements for the homonymous T. variegatus of Anton (1838).

More recently, Kensley & Penrith (1972) proposed that Trochus fulguratus Philippi, 1849 , described from ‘ Gabon Guineae’, was a junior synonym of T. variegatus Anton, 1838 . Philippi had noted that his T. fulguratus was very close to T. impervius and particularly to T. variegatus ((Philippi, 1855 in 1846–1855), but he believed that it differed in lacking the articulated subsutural necklace of variegatus and had a yellowish rather than a greenish-grey apex. Subsequent to its description, this poorly known taxon seems only to have been recorded from southern Angola ( Nicklès 1950; Paes da Franca 1960; Gofas et al. 1985). It was not mentioned in a more recent publication discussing the molluscs of Gabon ( Bernard 1984) and was merely listed as having been recorded in West Africa by Ardovini & Cossignani (2004). Peter Ryall (pers. comm. viii/2015) has indicated that it does not occur in Gabon or the Gulf of Guinea as a whole, which is devoid of Oxystele -like and Phorcus -like species and believes, like Gofas et al. (1985), that it is restricted to southern Angola. Thus in all likelihood the original ‘ Gabon Guineae’ locality cited for T. fulguratus was erroneous and the specimens originated in southern Angola. The whereabouts of the type material of T. fulguratus , if still extant, are unknown, but photographs of southern Angolan material ( Fig. 7 E– H, courtesy of Peter Ryall) closely resemble the original figure of T. fulguratus ( Philippi, 1849 in 1846 –1855: pl. 39, fig. 10) reproduced herein ( Fig. 7 I), and there can be little doubt that the specimens represent that species. The distinctly concave columella and well-developed umbilical callus confirm that the species is referable to Oxystele , as Philippi (1849) originally proposed. Although Kensley & Penrith (1972) could not distinguish between southern Angolan and southern African material, I believe this southern Angolan O. fulgurata material is in all probability a distinct species. The dark green ground colour, fine close-set, dark red, zig-zag, axial lines and absence of any trace of a subsutural necklace represent a combination of features that I have not observed in any material from South Africa or Namibia. North of the Kunene River on the Namibia-Angola border, the cold, north-flowing Benguela Current moves off-shore ( Lange et al. 2014), as a consequence of which, intertidal habitats to the north of this, lying within the ‘southern alternance region’ (Le Loeuff & von Cosel 1998) are more influenced by the warm, south-flowing Angola Current. Kensley & Penrith (1973) demonstrated that southern Angola (Namibe) lies within the tropical West African faunal province and that the intertidal fauna, including 80% of the mollusc species, is of West African rather than southern African affinity. Spalding et al. (2007) similarly regarded the Angolan coast as lying within the tropical Gulf of Guinea marine province. Within this region, Le Loeuff & von Cosel (1998) have identified the area between Lobito and Moçâmedes (Namibe) [the focus of distribution of O. fulguratus ] as an important refuge area experiencing relatively stable conditions for much of the Neogene.

Another potential synonym of Anton’s T. variegatus is T. perdix Koch, 1851 , a further taxon of unknown provenance and one that Pilsbry (1889 in 1889 – 1890) listed as a synonym of his concept of T. sagittiferus [i.e. the South African ‘ O. impervia – variegata complex’]. Philippi’s figure of T. perdix ( Philippi 1848 in 1846 –1855: pl. 24, fig. 17, without legend), reproduced herein ( Fig. 7 J), shows a small, elevated specimen with a distinct subsutural necklace and a predominantly spiral colour pattern below this. Although specimens matching this figure occur in South Africa, morphologically they cannot be conclusively associated with the current concept of O. ‘ variegata ’ (cf. Heller & Dempster 1991). The largely spiral alignment of the colour pattern is more indicative of O. impervia and indeed specimens of similar appearance cluster with typical O. impervia specimens in barcoding analyses (Van der Bank et al. 2013). Thus again there is little support for employing this name for specimens currently identified as O. variegata .

A number of more recently described taxa are also probably referable to the ‘ O. impervia – variegata complex’. Described by Bartsch (1915) and Turton (1932), these taxa were based on juvenile specimens (height <10 mm), often in poor condition. Heller & Dempster (1991) considered Oxystele carinata Turton, 1932 and O. farquhari Turton, 1932 to be synonyms of O. impervia on account of their orange colour and predominantly spiral colour pattern, and I believe this to be correct. However, in the case of Gibbula rifaca Bartsch, 1915 , Oxystele distincta Turton, 1932 , O. sagittifera rufanensis Turton, 1932 and O. tabularis var. pulchra Turton, 1932 , I believe that one cannot with any degree of certainty ascribe these to either O. impervia or O. variegata . The shells of both of these taxa are simply too variable to draw a definitive conclusion. Even when adult it can be difficult to refer some specimens to either one of these species with confidence (Van der Bank et al. 2013) and it is only in adults that some of the more distinctive features become evident. Although O. distincta has a bold colour pattern, this comprises spiral bands of black rectangles rather than zigzag red/maroon axial lines, and there is no subsutural necklace. Given these difficulties and the resultant uncertainty, I consider that these four names must be regarded as nomina dubia .

In summary, I do not believe that there is an existing name that can be unambiguously considered a valid junior synonym of Trochus variegatus (non Gmelin, 1791) Anton, 1838. This being the case, the most pragmatic course of action is to designate one of Anton’s four syntypes to be the lectotype for his Trochus variegatus , and then to propose a replacement name for the species. However, the choice of which of the four syntypes to select in order to preserve current interpretation of the species is critical. These specimens, once considered lost ( Heller & Dempster 1991), are in the MTD and photographs supplied by Katrin Schniebs indicate that two of them, described as variety ‘Grund morgenroth’, are clearly specimens of O. impervia , another specimen, described as variety ‘Grund gelblich weiss, braun gefleckt, Basis roth punktirt’, is also probably referable to O. impervia . Only the fourth specimen described as variety ‘Grund weiss, mit dichten kirschrothen Ziczacstreifen’ is clearly consistent with the current concept of, and prevailing use of, the name O. variegata ( Heller & Dempster 1991; Van der Bank et al. 2013). To maintain this interpretation, I therefore designate this specimen as the lectotype of Trochus variegatus (non Gmelin, 1791) Anton, 1838 ( Fig. 7 K—M) and provide a new name, Oxystele antoni , on account of its homonymy. This will avoid the inevitable uncertainly that would remain had one of the above nomina dubia been proposed as a junior synonym instead and in so doing will preserve the current interpretation the species. Given the homonymy here exposed, the use of an alternative name for this species is unavoidable, but I believe that the solution proposed is the one that will result in the most stable nomenclature.

All of the above notwithstanding, there remain unresolved issues regarding the status of O. antoni and O. impervia as distinct species and their circumscription using morphological characters. Additional molecular studies utilising multiple genetic markers and employing a finer scale approach, including habitat-related data, are needed in order to shed further light on this issue.

Figures

FIGURE 7. A, Trochus indecorus Philippi, 1846, original figure reproduced from Philippi (1848 in 1846 – 1855: pl. 24, fig. 5), diameter 12 linien [= 26 mm]. B – D, Trochus sagittiferus Lamarck, 1822, holotype diameter 22 mm (MHNG, INVE 51508). E, F, Oxystele fulgurata, Saco Mar, Namibe, Angola, height 16.4 mm (images courtesy of P. Ryall). G, H, Oxystele fulgurata, Cabo de Santa Maria, Benguela, Angola, height 23 mm (images courtesy of P. Ryall). I, Trochus fulguratus Philippi, 1849, original figure reproduced from Philippi (1849 in 1846 – 1855: pl. 39, fig. 10), diameter 8.5 linien [= 18.5 mm]. J, Trochus perdix Koch, 1851, original figure reproduced from Philippi (1848 in 1846 – 1855: pl. 24, fig. 17, without legend), diameter 5.5 linien [= 12 mm]. K – M, Trochus variegatus Anton, 1838, lectotype, diameter 12.5 mm (MTD 504), (images courtesy of K. Schniebs). [Images not to scale.]