Imbricoscelis coronis (Barnard, 1963) 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 : 32-35

Imbricoscelis coronis (Barnard, 1963) gen. et comb. nov.

Figs 15–16

Brookula coronis Barnard, 1963a: 12 , pl. 2 figs 5–6.

Type loc.: Africana II , Station A1248 (approx. 36°48′ S, 52°08′ E) [Sapmer Bank Seamount], depth 400 m.

Brookula coronis – Kensley 1973: 44, fig. 114. — Giles & Gosliner 1983: 8.

Type material

Syntypes

SAPMER SEAMOUNT , SOUTHWEST INDIAN RIDGE • 9 empty shells; stn A1248; approx., 36°48′ S, 52°08′ E; depth 400 m; 9 Jul. 1961; R.S. Africana II leg.; SAMC A29928 (not seen). Although Barnard (1963a) mentioned 11 dead specimens, Giles & Gosliner (1983) recorded only nine. Repeated attempts to obtain photographs from the SAMC of a representative specimen from this syntype series have proved fruitless.

Material examined

WALTERS SHOAL – slopes • 3 empty shells; stn DW4877; 33°10′ S, 43°49′ E; depth 217– 256 m; 1 May 2017; MNHN • 5 empty shells; stn DW4881; 33°16′ S, 43°50′ E; depth 377–382 m; 2 May 2017; MNHN • 6 empty shells; stn DW4885; 33°17′ S, 43°54′ E; depth 272–380 m; 3 May 2017; MNHN • 21 empty shells; stn DW4886; 33°17′ S, 43°56′ E; depth 573–582 m; 3 May 2017; MNHN • 13 specimens, some living; stn DW4887; 33°17′ S, 43°57′ E; depth 599–640 m; 3 May 2017; MNHN • 3 empty shells; stn DW4890; 33°09′ S, 43°59′ E; depth 492–588 m; 4 May 2017; MNHN • 6 empty shells; stn DW4892; 33°12′ S, 44°01′ E; depth 624–646 m; 4 May 2017; MNHN • 4 empty shells; stn DW4893; 33°16′ S, 43°58′ E; depth 623–629 m; 4 May 2017; MNHN • 90+ empty shells; stn DW4898; 33°09′ S, 44°01′ E; depth 652–668 m; 6 May 2017; MNHN • 30+ specimens, some living; stn DW4899; 33°09′ S, 44°02′ E; depth 707–720 m; 6 May 2017; MNHN • 50+ empty shells; stn DW4900; 33°10′ S, 44°01′ E; depth 660–670 m; 6 May 2017; MNHN • 9 empty shells; stn CP4901; 33°09′ S, 44°01′ E; depth 647–672 m; 6 May 2017; MNHN • 11 specimens, some living; stn CP4902; 33°08′ S, 44°02′ E; depth 700–711 m; 7 May 2017; MNHN • 2 empty shells; stn CP4903; 33°11′ S, 44°01′ E; depth 620–642 m; 7 May 2017; MNHN • 1 empty shell; stn CP4908; 33°29′ S, 44°00′ E; depth 900–950 m; 9 May 2017; MNHN • 3 empty shells; stn CP4911; 32°46′ S, 44°18′ E; depth 964–965 m; 10 May 2017; MNHN .

Description

SHELL. Small (height up to 3.1 mm), globose-turbiniform, proportions variable but always somewhat wider than high (H/D =0.76–0.89); whorls evenly rounded, spire relatively low; suture strongly indented, inserted close to periphery (mid-whorl) on spire whorls, descending slightly below this on last adult whorl; sculpture predominantly axial; base rounded and umbilicate; teleoconch of up to 3.5 whorls, the last adult whorl with a strong subterminal varix. First teleoconch whorl evenly rounded, initial half smooth, sculptured only by microscopic granules; second half of first whorl with 11–14 rather ill-defined collabral ribs ( Fig. 16A ); rib intervals with traces of vermiform microsculpture caused by fusion of microgranules; ribs strengthen during second whorl, becoming forwardly sloping lamellae, almost overlapping base of rib in front by end of whorl; lamellae becoming tightly packed and overlapping during third and subsequent whorls, their crests frequently damaged; exposed portion of lamellae with indistinct spiral threads ( Figs 15E , 16C ); lamellae with microsculpture of fine close-set scratch-like axial marks ( Fig. 16D ), in places overlain with a chalky intritacalx-like deposit ( Fig. 16E ); final lamella behind outer lip greatly enlarged, forming strong, angular subterminal varix ( Fig. 15D, F ); lamellae continue around periphery, becoming progressively weaker on base and into umbilicus; spiral threads more distinct on base, close-set and numerous, present also in umbilicus; base evenly rounded; umbilicus of moderate width, lacking a distinct margin, but sometimes with a low, indistinct funicle spiralling into it from mid-point of columella. Aperture subcircular, strongly prosocline, almost tangential; peristome interrupted in parietal region; inner lip strongly concave below insertion of columella onto paries, thin and somewhat reflected over umbilicus in this region; inner lip thickened and somewhat flaring below mid-point of columella; basal and outer lips evenly rounded, edge beyond subterminal varix thin; interior smooth, lacking obvious nacre even in live-collected specimens.

PROTOCONCH ( Fig. 16B ). Translucent, pale greyish-white; comprising approx. 1.25 whorls, diameter approx. 290 μm; apical bulb rounded; surface for the most part smooth, but with traces of irregular granulation where unworn; terminal lip straight, slightly flaring, not thickened.

COLOUR. Uniformly dirty white.

DIMENSIONS. Height 3.0 mm, diameter 3.0 mm (fide Barnard 1963a); largest specimen, height 3.1 mm, diameter 3.5 mm.

OPERCULUM ( Fig. 16F ). Corneous; initially tightly multispiral, but whorls broadening with growth and becoming more openly multispiral.

RADULA ( Fig. 16G–I ). Formula ∞+3+1+3+∞, with ±50 transverse rows of teeth. Rachidian with lateral flanges of shaft slightly expanded creating a relatively weak hood; central cusp well developed, lanceolate and with two smaller lateral cusps on each side. Lateral teeth overlapping extensively, their cusps asymmetrically trigonal; central denticle largest, lateral denticles well developed on outer margin, progressively decreasing in size toward tooth shaft; inner margin with fewer denticles. Marginal teeth numerous and slender; cusps of inner ones recurved, narrowly and asymmetrically spathulate, inner margin straight and smooth, outer margin curved and finely pectinate for a considerable distance; outer marginals similar, but with pectinate outer margin extending further down shaft; outermost marginals somewhat broader.

EXTERNAL ANATOMY. Only dried bodies available, rehydration of which revealed limited detail; cephalic tentacles prominent; eyestalks with well-developed, pigmented eyes; right post-ocular peduncle present; left neck lobe digitate, right one entire; 3–4 epipodial tentacles evident on each side, perhaps more.

Distribution

Previously known only from the Sapmer Bank Seamount, adjacent to the Gallieni Fracture Zone of the Southwest Indian Ridge [± 900 km SE of Walters Shoal]. Here recorded from the slopes of Walters Shoal at depths of 256–964 m (live-taken material 640–707 m); dredged and trawled on coarse sandy substrata with abundant cnidarians, ophiuroids and bivalves.

Remarks

Although Barnard had 11 specimens of this species, his description of it is brief and inadequate; hence I have chosen to provide a more detailed description above. There can be little doubt, however, that the material from Walters Shoal belongs to the same species. It is evidently associated with seamounts in the vicinity of the Southwest Indian Ridge and appears to be a species with southern rather than tropical affinity.

The Pliocene Vetulonia philippinensis Kiel, Aguilar & Kase, 2020 is superficially similar, but it has fewer, more rounded, non-overlapping axial ribs, a narrower umbilicus and lacks a subterminal varix behind the outer lip.

Figures

Fig. 15. Imbricoscelis coronis (Barnard, 1963) gen. et comb. nov. A–D. Apertural, apical, basal and lateral views of typical specimen, height 2.8 mm, Walters Shoal stn DW4899 (MNHN). E. SEM of depressed specimen, rib crests mostly damaged at whorl periphery, DW4898, height 2.4 mm. F. Lateral view SEM to show detail of subterminal external varix, DW4898, height 2.9 mm.

Fig. 16. Imbricoscelis coronis (Barnard, 1963) gen. et comb. nov., SEM, Walters Shoal, stn CP4902 (MNHN). A. shell apex. B. Protoconch. C. Sculpture of overlapping lamellate ribs with fine spiral lirae. D. Microsculpture of slash-like axial marks.E. Microsculpture overlain by intritacalx layer.F. Operculum. G–I. Radula. G. Full width of radula. H. Rachidian and lateral teeth in central field. I. Latero-marginal area. Scale bars: A, F =500 μm; B, E=100 μm; C–D =200 μm; G= 25 μm; H–I =20 μm.