Melanella adiastalta Souza & Pimenta & Barros 2021, sp. nov.

Melanella adiastalta sp. nov.

urn:lsid:zoobank.org:act: 3A3EAB3A-11EB-4CD3-8D7C-C50738650225

(Figure 15 A–G)

Type material. Holotype MNRJ 35250 † (Figure 15 A–G).

Type locality. Brazil: Off Rio Grande do Norte, REVIZEE Nordeste (06°14′04″S 34°52′33″W, 510 m), coll. RV Natureza, 26/xi/2001.

Etymology. From adiastaltos Gr., meaning not clearly defined, indefinite, referring to the inconspicuous features of the species.

Description (Holotype). Shell conical, straight, reaching about 3.6 mm long, 1.3 mm wide. Larval shell whitish, conical, with 2.75 convex whorls, 330 µm wide, 380 µm in height; smooth, transition to teleoconch marked by distinct incremental scar. Teleoconch with about 5.5 whorls, convex outline; suture well demarcated, sloping; subsutural zone indistinct; surface smooth, incremental scars well demarcated, appearing at intervals of about 0.7 whorl. Last whorl about 50% of the shell length; base rounded. Aperture short, pear shaped, acute posteriorly and rounded anteriorly; outer lip thin, opisthocline, sinuous, slightly retracting near the suture and slightly projecting in the uppermost third; inner lip sinuous, sloping, reflected and well demarcated. Not umbilicate. Shell vitreous, colorless.

Measurements. Holotype MNRJ 35250†, 8 whorls, SL= 3.6 mm; BWL= 1.9 mm; AL= 1.2 mm; SW= 1.3 mm; AW= 0.7 mm.

Remarks. Melanella adiastalta sp. nov. at first has inconspicuous features but can be distinguished from other similar species occurring in deep waters of the Atlantic. Despite the presence of only a single shell, we describe M. adiastalta sp. nov. based on the combination of consistent shell features that enable us to discriminate this species from remaining eulimids.

Melanella adiastalta sp. nov. can be distinguished from M. sarissa (Figure 12 A–K) because it has more convex teleoconch whorls, a wider spire angle (26° vs. 22° in M. sarissa) and a smaller number of protoconch whorls (2.75 vs. 4.0).

Melanella adiastalta sp. nov. can be distinguished from M. insculpta Bouchet & Warén, 1986, from the North Atlantic, by the faster increase in diameter of the spire. Both species reach about 8.5 whorls, but M. adiastalta sp. nov. has 1.3 mm wide while M. insculpta has 0.9 mm wide (Bouchet & Warén 1986: 376). Additionally, the outer lip of M. adiastalta sp. nov. protrudes gradually (Figure 15E) while it is more accentuated in M. insculpta (see Bouchet & Warén 1986: figs. 885, 887).

Melanella adiastalta sp. nov. differs from M. martynjordani (Jordan, 1895), M. obtusoapicata Bouchet & Warén, 1986, M. pseudoglabra (Dautzenberg & Fischer, 1912) and M. turrita Bouchet & Warén, 1986, all from the Northeast Atlantic area, mainly by the more convex teleoconch whorls (Figure 15A, B). Furthermore, M. adiastalta sp. nov. has a more protruding outer lip (Figure 15E) than M. martynjordani and a higher number of whorls in the protoconch (Figure 15G) than M. turrita (2.75 vs. 1.5).

Melanella adiastalta sp. nov. resembles Eulima anonyma Bouchet & Warén, 1986, from off Portugal, in the number and shape of protoconch whorls, but the protoconch is shorter (380 μm high vs. 500 μm high in E. anonyma) (Bouchet & Warén 1986: 467). Furthermore, M. adiastalta sp. nov. has a wider shell, despite reaching a smaller number of whorls (8 whorls, SW= 1.3 mm vs. 10 whorls, SW= 1.2 mm).

Geographical distribution. Only known from the type locality.

Bathymetric distribution. 510 m.