Emarginula nodulicostata, 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 : 7-8

Emarginula nodulicostata sp. nov.

urn:lsid:zoobank.org:act:DA82AFE9-2C80-4ED8-AEF2-6945D3329814

Fig. 3

Diagnosis

Shell very small, stout, uniformly milky white; apex strongly recurved, extending well beyond posterior margin; dorsal profile strongly convex, anterior slit narrow and deep; sculpture coarsely cancellate, radial and concentric elements of similar strength; radial ribs nodular where crossed by concentric ridges; sculptural interstices simple; basal margin broader at posterior, distinctly crenulated at ends of radial ribs.

Etymology

From the Latin “ nodus ” – “a knot or swelling”, dim. “ nodulus ”, and “ costa ” – “rib”, “ costatus ” “ribbed”; in reference to the nodose sculpture of the radial ribs.

Material examined

Holotype

WALTERS SHOAL • empty shell; summit area south-west, stn WS04; 33°12.2′ S, 43°50.8′ E; depth 40 m; 30 Apr. 2017; MNHN-IM-2000-38252 .

Paratypes

WALTERS SHOAL • 6 empty shells; summit area, south-west, stn WS03; 33°12.2S, 43°50.8′ E; depth 40 m; 30 Apr. 2017; MNHN-IM-2000-38253 • 11 empty shells; summit area south-west, stn WS04; 33°12.2′ S, 43°50.8′ E; depth 40 m; 30 Apr. 2017; MNHN-IM-2000-38254 • 1 empty shell; summit area, south, stn WS06; 33°15.1S, 43°54.5′ E; depth 26 m; 1 May 2017; MNHN-IM-2000-38255 • 10 empty shells; summit area, south, stn WS07; 33°15.4S, 43°52.2′ E; depth 30–33 m; 2 May 2017; MNHN- IM-2000-38256 • 3 empty shells; summit area, south-east, stn WS08; 33°13.7S, 43°55.9′ E; depth 30– 33 m; 3 May 2017; MNHN-IM-2000-3824657 .

Description

SHELL. Very small (basal length up to 3.45 mm), but relatively thick; apex strongly recurved, extending well beyond posterior margin of base; basal outline evenly ovate (L/W=1.37–1.40), slightly concave in side view; height moderate (H/L=0.52–0.65); dorsal profile strongly convex in side view, posterior slope comprising only one-third height of shell in adult; anterior face slightly flattened either side of slit; posterior slope almost vertical in largest specimens, much less so in smaller ones; protoconch tucked under recurved apex, slightly twisted to right. Anterior slit narrow and deep, approx. one-third of anterior slope; selenizone initially forming a broad mid-line rib (margins scarcely raised) with coarse lunulae that project above its margins ( Fig. 3E arrow), but selenizone becoming somewhat sunken nearer to slit in largest specimens; remaining sculpture relatively coarsely cancellate, comprising ± 16 primary radial ribs with narrower secondary ribs between them; secondaries stronger laterally, those between anterior ribs more slender; radial sculpture crossed by coarse concentric ridges; radial and concentric sculpture of similar strength; radial ribs roundly nodular where crossed by concentric ridges; interstices between ribs and ridges simple, square to concentrically elongate-rectangular, with paired intritacalx pits; basal margin distinctly crenulated at ends of radial ribs and noticeably broader posteriorly ( Fig. 3C ). Interior with broad, low ridge underlying selenizone posterior to anterior slit, approx. ⅔ length of slit; ridge itself with shallow central furrow; with interior of apex only just visible in basal view.

PROTOCONCH ( Fig. 3F ). Typically emarginuliform; single whorl with sharp, flared terminal lip; surface with finely flocculated superficial granulation, maximum diameter ±140 μm.

COLOUR. Uniformly milky white, somewhat translucent when fresh.

DIMENSIONS. Holotype, base 3.45 × 2.50 mm, height 2.25 mm (= largest specimen).

Distribution

A shallow-water species known only from the summit area of Walters Shoal, at depths of 26–40 m; living specimens unknown.

Remarks

Other strongly recurved species in which the apex lies beyond the posterior margin of the base include Emarginula adamsiana Sowerby II, 1863 from Japan, E. convexa Hedley, 1907 from Queensland, E. dahli Thiele, 1913 from the Bismarck Archipelago, and E. circumalbum Poppe & Tagaro, 2020 and E. galericulata A. Adams 1852 from the Philippines. All except E. convexa attain a larger size than E. nodulicostata sp. nov. (basal length> 4.5 mm); in addition E. adamsiana is pale pink, and E. galericulata has finer sculpture. Emarginula convexa has an indistinct concentric colour pattern of pale green and grey zones, more numerous radial ribs and finer concentric sculpture. Emarginula circumalbum is a deeper-water species (250–500 m) and has a proportionately broader basal outline and a much shorter slit. The poorly known E. dahli is perhaps the most similar species, but this also occurs in much deeper water (160–180 m).

Figures

Fig. 3. Emarginula nodulicostata sp. nov. A–D. Holotype, base 3.45 ×2.5 mm, height 2.25 mm (MNHN-IM-2000-38252). E. SEM of subadult specimen to show detail of sculpture, projecting lunulae of early selenizone arrowed, stn WS07, basal length 3.15 mm (paratype, MNHN-IM-2000-38256). F. Protoconch, stn WS07, scale bar =100 μm (paratype, MNHN-IM-2000-38256).