Alvania prosocostata Amati & Giulio & Oliverio 2023, n. sp.
Creators: name: Amati, Bruno
name: Giulio, Andrea Di
name: Oliverio, Marco
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Gastropoda
Littorinimorpha
Rissoidae
Alvania
Alvania prosocostata

description: Alvania prosocostata n. sp.
(Figs 16; 17; 49B; 53E; Tables 1; 2)
urn:lsid:zoobank.org:act: F00C4E5C-4A4E-407B-8D50-FEEB9E99589B
TYPE MATERIAL. — Holotype. Gambier • dd (height 2.05 mm, width 1.2 mm, Figs 16 A-D; 17A-C; 53E); Taraururoa; 23°6’25”S, 134°51’43”W; 1-3 m; J. Letourneux leg.; MNHN-IM-2000-38708.
Paratypes. Gambier • 5 dd (Fig. 16 E-J); same data as holotype; MNHN-IM-2000-38709.
TYPE LOCALITY. — Gambier, Taraururoa; 23°6’25”S, 134°51’43”W; 1- 3 m.
OTHER MATERIAL EXAMINED. — Gambier • 6 dd; Mangareva, Rikitea; 23°6’39”S, 134°58’1”W; beached; coll. JL • 8 dd; Mangareva, Rikitea; 23°6’39”S, 134°58’1”W; beached; coll. MB • 2 dd; Mangareva, Taku; 23°4’58”S, 134°56’56”W; 1-3 m; coll. JL • 8 dd; Tenoko; 23°4’26”S, 135°0’35”W; 1-3 m; coll. JL • 10 dd; Tenoko; 23°4’26”S, 135°0’35”W; 1-3 m; coll. JL • 5 dd; Totegegi; 23°5’2”S, 134°52’58”W; 1-3 m; coll. JL.
Australes • 2 dd; Rapa, Pararaki; 27°36’46”S, 144°19’1”W; beached; coll. MB.
DISTRIBUTION AND SYMPATRY. — The species is known in the South Pacific Ocean from the Gambier and Australes (Pararaki Bay, Rapa), with only empty shells collected in 1-3 m depth (Fig. 49B).
Alvania prosocostata n. sp. is sympatric with Alvania herosae n. sp. and Alvania uapou n. sp. in the Australes (Rapa); with Alvania herosae n. sp. in the Gambier (Table 2).
ETYMOLOGY. — From the Greek πΡόσω, forward, and the Latin costatus, ribbed; for the prosocline inclination of the axial ribs.
DIAGNOSIS. — Alvania with medium sized and robust shell; protoconch paucispiral; teleoconch with faint spiral sculpture on the body-whorl, well marked on the base; 3 spiral cordlets starting after protoconch-teleoconch boundary; prosocline axial ribs; colouration uniform dark brown with two whitish spots on the aperture (specimens from Gambier) or yellowish with small whitish spots scattered on the teleoconch, dark spot on the last whorl before the varix, columellar lip brown (specimens from Australes).
DESCRIPTION OF HOLOTYPE
Shell (Figs 16 A-C; 17A; 53E)
Small for the genus, height 2.05 mm, width 1.2 mm, height/ width ratio 1.71, robust, ovate-conical.
Protoconch (Figs 16D; 17B, C)
Paucispiral, with nucleus moderately intorted, of 1.25 rather convex whorls, height 0.237 mm, nucleus diameter 0.112 mm, first half whorl diameter 0.225 mm, maximum diameter 0.325 mm, nucleus with weak discontinuous sculpture, the remainder with five or six thin, irregular, very fragmented and widely spaced spiral threads. Protoconch-teleoconch boundary scarcely marked, prosocline.
Teleoconch
Of 3.5 slightly convex whorls, suture impressed, canaliculated. Axial sculpture on the last whorl of 24 prosocline ribs slightly narrower than interspaces, abruptly interrupting on the first basal spiral cordlet. Spiral sculpture of 13 flat cordlets on the last whorl, 6 above the aperture very faint, one on the suture line and 6 on the base more marked. Cordlets I and VI separated from other cordlets with a more marked groove. Cordlets I, II and VI starting immediately after the protoconch-teleoconch boundary. Microsculpture of growth lines and very faint, minute spiral threads (Fig. 17B, C). Umbilical fissure absent. Aperture piriform, height 0.90 mm, height/aperture height ratio 2.28, peristome continuous, outer varix broad and rather thick; outer lip sharp, internally smooth, prosocline.
Colour
Colouration of teleoconch uniform dark brown with two whitish spots on the aperture: one median, the other basal.
Operculum and soft parts
Unknown.
VARIABILITY
Height 1.77-2.05, width 1.10-1.25; height/width ratio 1.56- 1.71. Axial ribs on the last whorl 22-29. Colouration varies in the intensity of the background from dark brown to white. Two specimens from the Australes (numbers 8 and 9 in Appendix 5) have the inclination of the axial ribs less accentuated, the protoconch more densely sculptured (Fig. 17D) (see Table 1 and Appendix 5) and cordlets I, II and VII starting immediately after the protoconch-teleoconch boundary; colouration yellowish background with small whitish spots, dark blotch on last whorl before the varix. Columellar lip brown. (See Table 1 and Appendix 5).
REMARKS
Alvania prosocostata n. sp. is very similar to the European species of the group of Alvania lineata Risso, 1826 (Amati et al. 2019), and in particular to the Mediterranean Alvania schwartziana Brusina, 1866 for the similar outline, robustness of the shell and in particular for the uniform brown colour (Brusina 1866: 25, pl. III, fig. 9; Amati et al. 2019: fig. 57). The most evident differences are in the larger size (height c. 4 mm vs c. 2 mm in A. prosocostata n. sp.); in the sculpture of the paucispiral protoconch (1.3 whorls, maximum diameter 0.511 mm, sculptured by c. 12 undulated spiral cordlets, also on the nucleus vs 1.25 rather convex whorls, maximum diameter 0.325 mm, with nucleus moderately intorted and smooth, the remainder with five or six thin, irregular, very fragmented and widely separated spiral threads in A. prosocostata n. sp.); and for the inclination of the axial ribs (orthocline vs prosocline in A. prosocostata n. sp.).
description: Published as part of Amati, Bruno, Giulio, Andrea Di & Oliverio, Marco, 2023, Shallow-water Rissoidae of the genera Alvania Risso 1826 Haurakia Iredale 1915 Parashiela Laseron 1956 Simulamerelina Ponder 1985 and Subestea Cotton 1944 Gastropoda Caenogastropoda Rissooidea from French Polynesia with the description of a new deep-water genus, pp. 803-892 in Zoosystema 45 (25) on pages 821-824, DOI: 10.5252/zoosystema2023v45a25, http://zenodo.org/record/10458066