Emarginula retrogyra, Herbert, 2024

Herbert, David G., 2024, The Vetigastropoda (Mollusca) of Walters Shoal, with descriptions of two new genera and thirty new species, European Journal of Taxonomy 923, pp. 1-119 : 10-12

Emarginula retrogyra sp. nov.

urn:lsid:zoobank.org:act:AA5A658F-0F58-485C-9786-AE7C6CCACE17

Fig. 4

Diagnosis

Shell thin, relatively high and with apex strongly recurved such that its interior scarcely visible in basal view; apex not overhanging posterior margin of base; protoconch tucked under recurved apex, but largely remaining free of teleoconch due to strong apical recurvature; margins of selenizone distinctly elevated, but thin; sculpture fine and relatively uniform, comprising close-set, finely nodular radial ribs crossed by weaker, less elevated, concentric ridges; interstices quadrate and simple; pale straw brown with darker concentric bands.

Etymology

From the Latin “ retro ” – “backwards” and “ gyro ” – “to turn around”; in reference to the strongly recurved apex.

Material examined

Holotype

WALTERS SHOAL • empty shell; slopes; stn DW4881; 33°16′ S, 43°50′ E; depth 377–382 m; 2 May 2017; MNHN-IM-2000-38258 .

Paratypes

WALTERS SHOAL – slopes • 1 empty shell; same collection data as for holotype; MNHN- IM-2000-38259 • 1 empty shell, juvenile; stn DW4890; 33°09′ S, 43°59′ E; depth 492–588 m; 4 May 2017; MNHN-IM-2000-38260 • 2 empty shells, juvenile; stn DW4897; 33°09′ S, 43°59′ E; depth 490– 584 m; 5 May 2017; MNHN-IM-2000-38261 .

Other material

WALTERS SHOAL – slopes • 1 empty shell, juvenile (broken); stn DW4879; 33°17′ S, 43°52′ E; depth 288–300 m; 1 May 2017; MNHN • 1 empty shell, juvenile; stn DW4885; 33°17′ S, 43°55′ E; depth 272–380 m; 3 May 2017; MNHN .

Description

SHELL. Small and thin (basal length up to 10.5 mm); relatively high (H/L ± 0.64); anterior slope convex, but not strongly so and apex not extending over posterior margin of base; tip of apex nonetheless strongly recurved; basal outline evenly ovate (L/D ± 1.3), slightly concave in side view; anterior face not flattened; posterior slope for the most part straight, becoming strongly concave beneath recurved apex; protoconch tucked under recurved apex, but largely remaining free of ascending posterior face of teleoconch due to strong apical recurvature. Anterior slit moderately deep, approx. one-third of anterior slope; margins of selenizone distinctly elevated, but thin and mostly broken away in holotype, but partially remaining in paratypes; selenizone itself not raised, bearing coarse, evenly-spaced lunulae, these becoming more numerous and more close-set nearer to slit; remaining sculpture of fine, close-set radial ribs, of which approx. 22 primary with secondary and tertiary intermediaries between them; primary and secondary ribs of more or less equal size at shell margin, tertiaries usually slightly weaker; intervals between ribs narrow, but well defined; concentric sculpture of fine, close-set ridges; these weaker and not as markedly raised as radial ribs; ridges cross ribs rendering latter crisply nodular ( Fig. 4E ); interstices between ribs and ridges deep, quadrate, simple, with one or a pair of intritacalx pits; basal margin mostly damaged in holotype, but evidently finely crenulated at ends of radial ribs with shallow grooves on interior, underlying radial ribs, longer and more evident in juveniles. Interior with broad ridge underlying selenizone, approx. twice length of slit; ridge itself with shallow central furrow; interior of apex scarcely visible due to extent of recurvature.

PROTOCONCH ( Fig. 4F ). Typically emarginuliform; single whorl with thickened terminal varix; rotated 90° further with regard to teleoconch such that varix is parallel with plane of shell aperture; surface with sparse superficial granulation; maximum diameter ±180 μm.

COLOUR. Holotype rather dirty pale olive-brown, but fresher specimens with broad, darker greenish-brown concentric bands and occasional greenish-brown spots.

DIMENSIONS. Holotype, base 10.5 × 8.1 mm, height 6.7 mm (= largest specimen).

Distribution

Known only from the slopes of Walters Shoal, at depths of 300–492 m; living specimens unknown.

Remarks

The moderately high, finely sculptured shell of E. retrogyra sp. nov., with its strongly recurved apex, is distinctive. The Philippines specimen, illustrated by Poppe & Tagaro (2020: pl. 23 fig. 4a-c) as Emarginula choristes Dall, 1925 , closely resembles the new species, though its apex is not as strongly recurved and it has a more distinctly greenish hue. Japanese E. choristes has a more elevated shell (H/L=0.85–095) with a shorter slit and a less strongly recurved but more posteriorly positioned apex (level with the posterior margin of the base). In addition, it has a distinct sculpture in which the major radial ribs are fewer (±16) and remain more prominent ( Dall 1925; Habe 1955; Hasegawa 2018). Emarginula annielangleitae Poppe & Tagaro, 2020 from the Philippines has a similar lateral profile to that of E. retrogyra , but again it is not as strongly recurved. It also attains a considerably larger size (length up to 18 mm) and has more dominant primary radial ribs that remain prominent at the basal margin. Emarginula dubia Schepman, 1908 from Indonesia and the Philippines has a similar fine sculpture, but is neither as elevated nor as recurved as E. retrogyra . Of the South African species, E. viridicana Herbert & Kilburn, 1986 has a similar shape, but its apex is less strongly recurved and the shell is more robust with much coarser sculpture, whereas in E. connelli Kilburn, 1978 , the shell is taller, the apex overhangs the posterior basal margin and the primary radial ribs remain significantly stronger than the secondaries at the shell margin.

Figures

Fig. 4. Emarginula retrogyra sp. nov. A–D. Holotype, base 10.5 ×8.1 mm, height 6.7 mm (MNHN- IM-2000-38258). E. SEM showing detail of sculpture, stn DW4890, scale bar =0.5 mm (paratype, MNHN-IM-2000-38260). F. Protoconch, stn DW4890, scale bar = 100 μm (paratype, MNHN- IM-2000-38260).