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Ascetostoma providentiae (J.C. Melvill, 1909)

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Obrázky

Ascetostoma providentiae

Autor: Herbert, D.G.

Ascetostoma providentiae

Autor: Herbert, D.G.

Taxon v check-listech zemí*

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Popis

Trochoid-turbiniform, moderately elevated (L/D=1.04—1.26); teleoconch of 5.0-5.5 rounded whorls; suture indented and somewhat channelled, that between last adult whorl and penultimate whorl inserted at level of second subperipheral cord, but descending below this just prior to aperture; exterior of outer lip with a broad low subterminal thickening. First teleoconch whorl with approx. 20 axial pliculae; 3 spiral cords develop during second whorl (lowest level with abapical suture), and a fourth arising beneath adapical suture near end of whorl; subsequent whorls with further cords arising through intercalation; penultimate whorl with 7-9 cords, sometimes alternating a little in strength, sometimes not; seventh cord usually peripheral; cords well defined, equal to or wider than their intervals. Axial pliculae of first whorl persist on later whorls rendering spiral cords beaded where they cross them; beads of early whorls more or less rounded, but later becoming somewhat axially elongate, those on subsutural cord usually largest; axial pliculae well developed on spire whorls rendering sculpture cancellate, usually less prominent on last adult whorl. Base with 6-7 primary spiral cords and occasional intermediaries, sculptured as above. Peristome markedly oblique, more or less in one tangential plane; aperture subcircular to D-shaped, flattened parietally; columella lip protrudes into aperture as a thickened pillar which bears 2 well-developed teeth separated by a concavity; basal tooth often squarish, upper one somewhat smaller and more rounded; a deep U-shaped notch separates basal tooth from first denticle of basal lip; parietal region with glossy, translucent inductural callus, sculptured with a variable number of ridges extending into aperture; ridges sometimes bifid terminally; a well-developed parietal tooth projects from paries over umbilical de¬pression; umbilicus present, lined and apically plugged with smooth, white callus when mature, conventionally patent in juveniles; umbilical margin thickened, also covered with callus and bearing ridge-like denticles (only in fully mature of specimens); callus slightly raised basally and confluent with flaring margin of outer lip; interior of outer lip with 2 rows of ridge-like denticles (at maximal maturity), the inner row stronger and lying on subterminal thickening of lip interior, the outer row (on non-nacreous flaring lip margin) weaker and less distinct; 2 denticles of inner row, nearest basal columella notch, usually larger; a small rounded or elongate granule may be present on columella near bottom of basal columella notch; interior of aperture nacreous, somewhat angled beneath spiral cords of shell exterior, but not spirally lirate. Dimensions: Greatest length 9.0 mm, greatest diameter 8.0 mm, but size at maturity evidently variable; some specimens of length ca 7.0 mm possess mature apertural den¬tition.
Microsculpture : Early teleoconch whorls with fine vermiform spiral threads; microsculpture of subsequent whorls often completely obscured by encrusting organisms; little evidence of any superficial intritacalx deposit and scratch-like sculpture scarcely evident; microsculpture instead comprising irregular, somewhat oblique, ver¬miform threads, most noticeable on the spiral cords Protoconch: Translucent white, peripherally tinged with orange; diameter ca 240 µm; somewhat globose and protruding slightly above first teleoconch whorl; sculptured with 3 fine, widely spaced, spiral threads, between which lie numerous, fine, close-set, oblique, axial threads; terminal lip with a well-developed angular projection just above mid-whorl.
Colour: Most specimens uniformly rich orange-brown with darker spots on spiral cords of second and third whorls; occasional specimens with alternating darker and lighter blotches below suture; cord intervals faintly iridescent; umbilical region white; pigmentation of old, dead shells frequently rather more pinkish/purplish brown. Shell exterior of live-taken specimens usually more or less entirely covered with a thin, brownish or blackish, spiculiferous, encrusting sponge. Operculum: Initially tightly multispiral, but whorls broadening with growth and becoming more openly multispiral.
Radula: Formula oo+4+1+4+oo; ca 75 transverse rows of teeth. Rachidian with trigonal cusp and well-developed hood; base of cusp with weak transverse ridge; cutting edge coarsely dentate, central denticle largest, lanceolate and with 3 or 4 smaller denticles on each side. Lateral tooth cusps with an elongate, rather spathulate central denticle and relatively coarse secondary denticles on outer margin, inner margin with few if any denticles. Inner marginals more slender with a recurved, dentate cusp, the denticles on outer margin extending some way down shaft; middle marginals very slender, but outermost ones shorter, broader and very delicate.
Herbert, D.G., 2012. A Revision of the Chilodontidae (Gastropoda: Vetigastropoda: Seguenzioidea) of Southern Africa and the South-Western Indian Ocean.

Možné záměny

This species is very similar to Euchelus ringens Schepman, 1908 from the Sulu Archipelago, Indonesia, and the two may eventually prove to be synonyms. The columella lip of the holotype of E. ringens is somewhat damaged (perhaps due to occupation by a hermit crab) and looks rather deformed. Compared with southwestern Indian Ocean material it differs in being slightly less elevated and in having a narrower umbilicus; there are also minor differences in terms of coloration and aperture characters. In the absence of data on variation in Indonesian material, I have chosen, to err on the side of caution and maintain Ascetostoma providentiae as a distinct species. There can be no doubt, however, that E. ringens is also a species of Ascetostoma.
A. providentiae is easily distinguished from the other southern African chilodontid taxa by its rounded whorl profile, very strong, complex apertural dentition and patent, callus lined umbilicus. Clanculus crassilabrum Sowerby, 1905 from Sri Lanka has a similar overall facies, but lacks the unusual umbilical features of Ascetostoma and is probably closer to Herpetopoma. Also similar is Herpetopoma rubrum (A. Adams, 1853), from Japan to SE Asia, which is likewise often vividly coloured, but in that species the inductural callus does not extend over the umbilical region and the umbilicus remains conventionally patent even at full apertural maturity. The Japanese Euchelus lischkei Pilsbry, 1904 also resembles A. providentiae in size, shape and sculp¬ture, but has a much weaker basal columella tooth and has a simple, patent umbilicus. The Philippine material identified under this name (as Clanculus) by Kosuge and Chino (1998) is not in fact referable to this species.
The Cretaceous Chilodonta (Agathodonta) africana Rennie, 1930 from the Pondoland coast, Eastern Cape (holotype, SAMC 8630) is superficially similar to A. providentiae, but it has finer granules above the periphery, distinct prosocline pliculae below the suture and a smoother base. It has a well-developed tubercle on the basal lip separated from the columella by a U-shaped notch, but no details of its umbilicus are apparent.
Herbert, D.G., 2012. A Revision of the Chilodontidae (Gastropoda: Vetigastropoda: Seguenzioidea) of Southern Africa and the South-Western Indian Ocean.

Rozšíření

Distribution and habitat: Islands of the western Indian Ocean (southern Seychelles group and Mascarenes) and the Mozambique Channel south to north-eastern South Africa (28.3°S, off Mission Rocks); -35-195 m (living specimens -50-85 m). In Zululand this species appears to inhabit hard substrata beyond the line of the near-shore reef system, where the sea floor is composed of fragmented sandstone or coral rubble, and is generally rich in marine life. Evidently a mid to outer continental shelf species throughout its range and not part of the shallow-water coral reef biota.
Herbert, D.G., 2012. A Revision of the Chilodontidae (Gastropoda: Vetigastropoda: Seguenzioidea) of Southern Africa and the South-Western Indian Ocean.

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