Fluxinella dufresneae, 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 : 40-44

Fluxinella dufresneae sp. nov.

urn:lsid:zoobank.org:act:E5F47BCC-5949-43E3-A122-646CA683873F

Figs 18A–C , 19A–B

Diagnosis

Shell depressed-trochiform, whitish, periphery angular, base flattened and broadly umbilicate; first teleoconch whorl with two strong spiral cords and evenly spaced axial riblets, sinuous above shoulder cord, opisthocline below it; third nodular cord subsequently arising below suture; shoulder cord weakens with growth and lies closer to periphery; whorl profile becomes convex above this core and concave below it; base with 1–2 subperipheral spiral lirae, mid-portion smooth, inner portion with 3–4 spiral lirae; umbilical margin angular, interior of umbilicus with an additional cord spiralling inward; aperture obliquely quadrate; outer lip with broad posterior sinus, protracted below this; inner lip concave either side of a central bulge.

Etymology

Named after the vessel Marion Dufresne II, aboard which the Walters Shoal survey (MD208) was conducted.Although the ship was named after the French explorer Marc-Joseph Marion du Fresne (1724– 1772), a man, the species is named after the ship. Since the Latin “ navis ” – “a ship”, is feminine, I employ the name in that gender.

Material examined

Holotype

WALTERS SHOAL • living specimen; south plain, stn CP4913; 33°52′ S, 44°05′ E; depth 1539–1615 m; 11 May 2017; DNA tissue sample; MNHN-IM-2013-67291 .

Paratypes

WALTERS SHOAL • 6 empty shells; same collection data as for holotype; SEM shell; MNHN- IM-2000-38058 • 1 empty shell; slopes, stn CP4918; 32°58′ S, 43°27′ E; depth 1295–1356 m; 14 May 2017; MNHN-IM-2000-38059 .

Description

SHELL. Of moderate size (diameter up to 6.0 mm), depressed-trochiform, width almost twice height (H/D ±0.52); whorls weakly convex, spire profile shallowly stepped by spiral sculpture; periphery angular; base somewhat flattened, broadly umbilicate; teleoconch of up to 5.5 whorls. First teleoconch whorl initially with two strong spiral cords, one midway between sutures (P1, shoulder cord), the second (P2) at whorl periphery, level with abapical suture ( Fig. 19B ); distinct axial sculpture of evenly spaced riblets (25–30) extending from suture to suture; riblets sinuous above shoulder cord, opisthocline below it; a third rounded cord (S1) develops below adapical suture toward end of whorl; sculpture of whorls 2–3 similar to that on first whorl, with S1 increasingly prominent and becoming nodular where crossed by axial riblets ( Fig. 19A ); whorls concave above and below P1; on fourth whorl P1 proportionally weaker and somewhat closer to periphery, with fine spiral thread arising above it; axial riblets becoming more strongly concave above P1 and almost orthocline below it; spiral cords on fifth whorl weakly nodular where crossed by axial riblets and peripheral cord shallowly scalloped; whorl profile becomes convex above P1, but remains concave below it; P1 scarcely evident on second half of last adult whorl, accompanied by numerous additional fine, supra-peripheral spiral lirae; periphery angular, somewhat flange-like. Base with 1–2 subperipheral spiral lirae, axial pliculae extend around periphery, reaching outermost lira but no further; mid-portion of base smooth, sculptured only by fine sinuous growth-lines; inner portion of base with 3–4 spiral lirae, progressively stronger toward umbilicus; umbilical margin angular, bordered by strong spiral cord that becomes raised as it spirals into umbilicus; growth-lines of base strengthen toward umbilical margin, becoming peri-umbilical pliculae and rendering the inner spirals weakly nodular; interior of umbilicus steep-sided with additional cord spiralling inward at mid-columella level. Aperture obliquely quadrate; outer lip with broad, U-shaped posterior sinus, and then protracted toward periphery and notched at peripheral keel; basal lip damaged in holotype (the only mature specimen available), but profile of growth-lines indicates shallow subperipheral sinus; columella lip oblique and slightly thickened, concave where it meets basal lip, projecting somewhat above this, but followed adapically by deeper concavity before its insertion on paries; no thickening of interior of outer lip evident.

PROTOCONCH ( Fig. 19B ). White; diameter 335–345 μm, comprising ca 1.25 whorls, apical cap evenly rounded; mostly smooth, but with traces of superficial granulation near apex; terminal lip weakly convex, not conspicuously thickened.

COLOUR. Uniformly milky-white, somewhat translucent; interior faintly nacreous.

DIMENSIONS. Holotype, height 3.1 mm, diameter 6.0 mm (= largest specimen).

Distribution

Known only from the slopes and surrounding plain of Walters Shoal, at depths of 1356–1539 m (live-taken specimen 1539–1615 m); obtained by beam trawl on fine sand with abundant echinoids and ophiuroids.

Remarks

Fluxina solarium Barnard, 1963 , from 300 km south of Madagascar (depth 875 m), which Marshall (1983) referred to Fluxinella , has reference here. Although Barnard (1963a) considered it extremely close to Fluxina marginata Schepman, 1909 , from Maluku, Indonesia, which is undoubtedly a species of Fluxinella , his description of F. solarium lacks precision and the figure provided is poor, illustrating only the apical surface. Since the type locality for the species lies only 650 km to the north-east of Walters Shoal, it is clearly important to compare this enigmatic taxon with the specimens from Walters Shoal. Unfortunately, it has not been possible to obtain photographs of the holotype in the South African Museum (Iziko) and comparison can only be based on the inadequate original description and figure, and the drawing provided by Kensley (1973: fig. 250). Nonetheless, neither Barnard’s description of the ontogeny of the spiral sculpture, nor his statement that the profile of the whorls is biconvex, is compatible with the present material. Likewise, his figure suggests that spiral P1 on the first teleoconch whorl persists as the subsutural cord on subsequent whorls. On this evidence, therefore, I conclude that the Walters Shoal specimens are not conspecific with Barnard’s species.

Fluxinella dufresneae sp. nov. resembles some of the other low-spired and relatively strongly sculptured species of Fluxinella such as F. acesta , F. euphanes , F. megalomphala , F. stirophora and F. tenera , all described by Marshall (1991) from the New Caledonian region. It differs from these, however, in that the shoulder spiral of the first teleoconch whorl does not disappear on subsequent spire whorls, but instead persists, albeit weaker and coming to lie progressively lower down the whorl, thus no longer forming a shoulder, but instead becoming a supra-peripheral cord that evanesces only during the latter part of the final whorl. In Fluxinella marginata the last adult whorl has a much less convex profile. Furthermore, none of the above species exhibits the nodular subsutural cord on the spire whorls evident in F. dufresneae . The southern African Fluxinella gelida (Barnard, 1963) , from a depth of over 2250 m, north-west of Cape Town, has an almost totally smooth shell.

Fluxinella stellaris Bozzetti, 2008 , described from beach-drift samples collected in southern Madagascar ( Bozzetti 2008), is not a seguenziid and the name is in fact a junior synonym of Agagus stellamaris Herbert, 1991 .

Figures

Fig. 18. A–C. Fluxinella dufresneae sp. nov., holotype, diameter 6.0 mm (MNHN-IM-2013-67291). D–F. Hadroconusscobinasp. nov.,holotype, diameter3.7mm (MNHN-IM-2000-38060).G. Hadroconus scobina sp. nov., holotype, SEM (uncoated) to show detail of sculpture. H. Hadroconus scobina sp. nov., specimen with shouldered whorls, diameter 4.0 mm, stn DW4898 (paratype, MNHN-IM-2000-38063).

Fig. 19. A–B. Fluxinella dufresneae sp. nov., juvenile paratype (MNHN-IM-2000-38058). A. Spire whorls. B. Protoconch. C–D. Hadroconus scobina sp. nov., paratype (MNHN-IM-2000-38065). C. Oblique view of spire whorls. D. Protoconch. E–J. Seguenzia cf. S. emmeles Marshall, 1991, Walters Shoal, stn CP4913, height 1.63 mm, diameter 1.50 mm (MNHN). E–G. Apertural, oblique basal and apical views of shell. H. Detail of sculpture showing more numerous axial riblets below shoulder keel. I. Side view of apex. J. Protoconch. Scale bars: A, C = 0.5 mm; B, D, J = 100 μm; E–G = 1.0 mm; H–I = 200 μm.