Ilanga cosmia, Vilvens & Williams, 2020

Vilvens, Claude & Williams, Suzanne T., 2020, New species of Ilanga (Gastropoda: Trochoidea: Solariellidae) from the Indo-West Pacific, Zootaxa 4732 (2), pp. 201-257 : 238-240

Ilanga cosmia n. sp.

( Figs 20 A–R, Table 11, Key 2: 4)

Type material. Holotype (6.5× 10.7 mm) MNHN IM-2000-34404 . Paratypes: 2 MNHN IM-2000-34405 (as listed below) .

Type locality. Tonga Islands , BORDAU 2, stn DW1589, 18°39’S, 173°54’W, 281 m.

Material examined. Tonga Islands. BORDAU 2: stn DW1540, 21°15’S, 175°14’W, 317–329 m, 1 dd, 1 dd sub . — Stn DW 1567, 21°02’S, 175°19’W, 351–356 m, 2 sub lv . — Stn DW 1589, 18°39’S, 173°54’W, 281 m, 4 dd, 2 dd sub (with holotype MNHN IM-2000-34404 . — Stn DW 1602, 20°49’S, 174°57’W, 263–320 m, 4 dd, 1 dd sub (with para-

types MNHN IM-2000-34405).— Wallis and Futuna. MUSORSTOM 7: stn DW509, 14°15’S, 178°12’W, 200–240 m, 1 dd . — Stn DW 526, 13°13’S, 176°16’W, 355–360 m, 1 dd.— Stn DW 585, 13°10’S, 176°13’W, 415–475 m . — Stn DW 601,

13°19’S, 176°17’W, 350 m, 6 juv sub.— Stn DW 610, 13°22’S, 176°09’W, 286 m, 1 juv sub.— Solomon Islands. SA- LOMONBOA 3: stn DW2775, 09°23’S, 160°57’E, 282–427 m, 1 dd . — Stn CP 2804, 09°15’S, 161°21’E, 150–175 m, 2 lv . — Stn CP 2809, 09°19’S, 161°27’E, 788–1042 m, 2 dd . — Stn DW 2811, 09°42’S, 161°30’E, 228–238 m, 2 dd . — Stn DW 2853, 09°47’S, 160°54’E, 264–285 m, 1 dd sub . — Stn DW 2855, 09°45’S, 160°50’E, 167–183 m, 1 dd.

Distribution. Tonga Islands, 281–351 m, lv at 351–356 mm; Wallis and Futuna, 240–415 m (dd); Solomon Islands, 175–788 m, lv at 150– 175 m.

Diagnosis. A rather small Ilanga species with a depressed, conical spire, a subangulate periphery with some faint spiral cords, 4 spiral cords on first teleoconch whorl and 1–3 additional cords on the second, subsutural pleats on median whorls, rounded to slightly subangulate umbilicus, possibly with faint axial pleats here and there around, bordered by an internal smooth spiral cord and 7–8 spiral cords inside.

Description. Shell: Rather small in size for genus (H up to 6.5 mm, W to 10.7 mm), wider than high, shape conical with large apical angle, glossy; spire depressed, height 0.55–0.62×width, about 1.97×to 2.42×aperture height; subangulate periphery; umbilicus broad and deep. Protoconch ca. 230–280 μm wide, 1.25 whorls, rounded, with a straight terminal lip. Teleoconch up to 4.6 slightly convex whorls; early whorls with 5 smooth spiral cords; intermediate whorls with subsutural axial pleats; last whorls smooth with some faint spiral cords on periphery. Suture canaliculated on first whorls, impressed on last ones. First whorl convex, with 4 smooth spiral cords Pi appearing immediately after protoconch except P1 usually appearing at first half teleoconch whorl; weak thin axial threads; P2 strongest than P1; subsutural ramp forming at end of first teleoconch whorl with P1 on rim; distance between cords about 1×to 1.5×width of cords. On second whorl, all cords widening and flattening; S2 and/or S3 appearing in some specimens, S4 emerging from suture; axial threads no longer visible; low axial pleats appearing at end of second whorl or at beginning of next one, covering adapical third of whorl. On third whorl, subsutural pleats stronger especially in area close to suture; subsutural ramp and all spiral cords. On fourth whorl, axial pleats weakening and vanishing. Last whorl completely smooth, except a few very faint spiral cords on periphery of whorl. Aperture subelliptical; peristome incomplete; outer and inner lip thin; inner lip with a weak thickening against umbilical rim. Base moderately convex, smooth; a few faint axial pleats might be present in some parts around umbilicus on subadult samples. Umbilicus broad (diameter 20–25% of shell width), central, with perspective to apex, with rounded to slightly subangulate rim, bordered by an internal smooth spiral cord, with 7–8 spiral cords inside, cords mainly visible on abapical part of wall.

Colour: Teleoconch nacreous white with two horizontal bands of chevron-shaped brownish flames; base paler, with two rows of brownish patches.

Operculum: Unknown.

Remarks. The combination of the subsutural axial pleats on the median whorls and the rounded umbilical rim with at most a few faint axial pleats in some parts around it make I. cosmia difficult to confuse with other similarly sized Ilanga species. There is rather a large gap in its geographic distribution, between Tonga Islands, Wallis and Futuna and the Solomon Islands. This is reflected in slight morphological differences, with specimens from the Solomon Islands slightly taller than other specimens and lacking spiral cords around the inside of the umbilicus. In the absence of molecular evidence, we conservatively treat these differences as intraspecific variation within a single species.

Etymology. Decorated (Ancient Greek: χοσμιος, υια, όν)—with reference to the fine, chevron-like colour pat- tern of the shell.

Figures

FIGURE 20. Ilanga cosmia n. sp. A–H. Tonga Islands, BORDAU 2. A–D. Holotype IM-2000-34404, stn DW1589, 281 m, 6.5×10.7 mm. E–H. Paratype MNHNIM-2000-34405, stn DW1602, 263–320 m, 5.7×10.3 mm. I–L. Wallis and Futuna, MUSORSTOM 7, stn DW509, 200–240 m, 5.5×9.9 mm.M–R. Solomon Islands, SALOMONBOA 3, stn DW2811, 228–238 m. M–O. 7.4×12.6 mm. P–R. 5.3×9.9 mm.