Vaceuchelus semilugubris ( Deshayes, 1863 )

Herbert, D. G., 2012, A revision of the Chilodontidae (Gastropoda: Vetigastropoda: Seguenzioidea) of southern Africa and the south-western Indian Ocean, African Invertebrates 53 (2), pp. 381-381 : 461-464

publication ID	https://doi.org/ 10.5733/afin.053.0209
persistent identifier	https://treatment.plazi.org/id/0D38CE4B-C24F-F72E-A24C-71CAFE36FA77
treatment provided by	Felipe
scientific name	Vaceuchelus semilugubris ( Deshayes, 1863 )
status

Treatment

Vaceuchelus semilugubris ( Deshayes, 1863) View in CoL

Figs 4L View Fig , 50 View Fig , 51 View Fig

Turbo semilugubris: Deshayes 1863: 72 , pl. 9 [36], figs 9, 10; Martens 1880: 294; Herbert 1996: figs 70, 71 (syntype). Type loc.: Réunion.

Leptothyra semilugubris: Pilsbry 1888: 252 , pl. 58, fig. 52a; Viader 1937: 55.

Vaceuchelus semilugubris: Poppe et al. 2006: 47 View in CoL .

Description:

Shell: Small, turbiniform, robust; length more or less equal to diameter (L/D=0.92–1.04); teleoconch of approx. 3.5 whorls, but apex usually badly eroded; sculpture cancellate; first whorl strongly shouldered at mid-whorl, almost horizontal above shoulder and vertical below it; shoulder develops into a strong spiral cord at end of whorl, another cord evident at level of abapical suture; axial pliculae do not develop until near end of whorl; this sculpture strengthening during second whorl with an additional cord arising between first cord and adapical suture; these two cords becoming the large peripheral and supra-peripheral cords of last adult whorl; 1–2 additional secondary cords arising above supra-peripheral cord during third whorl; axial pliculae stronger on second and subsequent whorls, prosocline and raised into granules where they cross spiral cords, granules somewhat angular on second whorl, but becoming progressively more rounded and bead-like on subsequent whorls; interstices quadrate; last adult whorl with 25–30 pliculae. Base with 5 spiral cords, the first level with suture, the fifth small and spiralling into umbilicus; umbilicus narrowly patent even in adults. Peristome oblique and nearly tangential, aperture subcircular; columella shallowly concave with a rounded tooth at its base; interior of outer lip of mature individuals subterminally thickened and set with approx. 10 ridge-like denticles, with an additional row of smaller granules just inside lip edge; apertural dentition absent in immature specimens, in these the outer lip is strongly notched at ends of spiral cords; interior nacreous, weakly angled beneath external cords, but labral denticles not extending into aperture as in-running ridges.

Microsculpture ( Fig. 51A, B View Fig ): Juvenile microsculpture comprising irregular vermiform spiral threads; subsequent microsculpture mostly granular, but with traces of scratch-like markings.

Protoconch ( Fig. 51C View Fig ): White, slightly exsert, eroded in most material; diameter ca 285 µm; strongly sculptured with an irregular superficial reticulation showing traces of axial alignment, but no spiral element. Terminal lip more or less straight, but perhaps evincing signs of a broken beak-like projection; becoming concave toward abapical suture.

Colour: Shell white, variously patterned with black spots, blotches or axial bands, orientation of pattern prosocline in some specimens, opisthocline in others; markings generally finer on base; pattern fading to brown in old material. Occasional specimens with almost no black markings ( Fig. 50F View Fig ). Coloration of fresh specimens somewhat obscured by dirty buff intritacalx deposit, superficial calcareous encrustations and debris. Dimensions: Largest specimen, length 3.55 mm, diameter 3.40 mm; smallest specimen with mature apertural dentition, length 2.6 mm, diameter 2.7 mm.

Operculum ( Fig. 4L View Fig ): Relatively tightly multispiral throughout.

Radula ( Fig. 51D, E View Fig ): Formula ∞+3+1+3+∞; ca 50 transverse rows of teeth; transition from lateral to marginal series clear. Rachidian with broad, trigonal cusp, well-developed hood, and a distinct transverse basal ridge; cutting edge with a slender, acuminate central denticle and 2 or 3 progressively smaller, similarly shaped denticles on each side. Lateral teeth all of similar size, their cusps with coarse lateral denticles on both margins (3–5), those on the third lateral particularly strong. Marginals closely resembling those of Herpetopoma scabriusculum , the cusps of the inner ones with a strongly pectinate outer margin; outermost marginals shorter and with a slightly dilated, fringed cusp.

External anatomy: Only dried material available; evidently chilodontid but insufficient detail evident.

Type material: Two syntypes of Turbo semilugubris Deshayes, 1863 , in MNHN, the one in better condition is here figured and designated lectotype ( MNHN 24658 View Materials ) ( Fig. 50A, B View Fig ) , length 3.10 mm, diameter, 3.05mm. Other material examined: RÉUNION: not further localised (M. Jay coll’n, MNHN; NMSA G4264 View Materials , J269 View Materials ) ; off Cap La Houssaye (21.01797°S 55.23709°E), - 12 m, hand-dredged sand, J. Drivas, i.1988 ( NMSA K3035 View Materials ) GoogleMaps .

Distribution and habitat: Known only from the island of Réunion; Jay (2009) recorded it as living under stones at - 10–15 m, but noted it to be rare.

Remarks:There appear to have been no published records of this species since its original description. Subsequent mentions of the name are either simple listings ( Martens 1880; Viader 1937) or mere translation of the original description ( Pilsbry 1888), and there is no evidence of new material having been examined. However, additional samples clearly referable to this species have been found on Réunion, confirming that the original provenance was correct. The occurrence of the species in Mauritius requires confirmation. Pilsbry (1888) and Viader (1937) mentioned this island, but it is not clear whether this was a generalisation or specifically intended to mean Mauritius rather than Réunion. Despite extensive searching for micro-molluscs in beach-drift samples and hand-dredged sand from near-shore reefs on Mauritius (by Kilburn and Herbert), no specimens of V. semilugubris have been collected. Nonetheless, it is clearly not common, even on Réunion. Given the proximity of Réunion and Mauritius, it would be surprising if the species did not occur on both islands.

Whether this species should be referred to Vaceuchelus or Herpetopoma requires further study. I have chosen to refer it to Vaceuchelus on account of the shape of the shell and its relatively coarse, cancellate sculpture. The presence of denticles inside the outer lip is a feature common to both Vaceuchelus and Herpetopoma . However, in the present species there is also a distinct denticle at the base of the columella that delineates a notch (albeit weak) at the junction of the basal and columella lips. This is a feature more typical of Herpetopoma .

The species is distinctive amongst small chilodontids of the south-western Indian Ocean on account of its bold, axial colour bands and relatively wide umbilicus. In fresh shells these bands are very dark purplish brown to almost black, but fade to a paler maroon-brown in old and sun-bleached specimens. They are clearly part of the shell, unlike the greyish axial bands seen in fresh material of other species (e.g. V. natalensis ), which are part of the superficial intritacalx deposit. Rarely, specimens are almost devoid of black markings and may easily be confused with the sympatric V. jayorum (above). That species, however, has strong close-set axial pliculae on the first teleoconch whorl, has intermediary spiral cords on the last quarter of the final whorl, and does not develop labral dentition when mature. I suspect V.semilugubris may exhibit ecological variation in shell morphology, ranging from typical boldly marked specimens with coarse sculpture, to somewhat smaller specimens with little or no colour pattern and finer sculpture ( Fig. 50F View Fig ). However, since accurate information on habitat is not available, this remains a speculative observation.