Benthonellania aequatorialis ( Thiele, 1925 ) Amati & Giulio & Oliverio, 2022

Amati, Bruno, Giulio, Andrea Di & Oliverio, Marco, 2022, Deep-water Rissoidae of the genera Benthonella Dall, 1889 and Benthonellania Lozouet, 1990 (Gastropoda, Caenogastropoda, Rissooidea) from French Polynesia, Zoosystema 44 (12), pp. 335-389 : 356-358

Benthonellania aequatorialis ( Thiele, 1925) n. comb.

( Figs 11 A-G; 12D, E; 13B; 26H; Table 1 )

Rissoa aequatorialis Thiele, 1925: 84 , pl. 6, fig. 23.

TYPE MATERIAL. — Syntypes. Indonesia • 2 dd; Nias-Sudkanal (Sumatra), Valdivia Stn 199; 0°15.5’S, 98°4’E; 470 m depth; ZMB / Moll. no. 64973 .

DISTRIBUTION. — The species is known with certainty from Sumatra ( Indonesia) ( Fig. 13 ); Thiele (1925) reported it also from the Zanzibar Canal ( Tanzania) but we could not examine the relevant material.

DESCRIPTION BASED ON THE EXAMINED TYPES (in parentheses data from the original description)

Shell ( Figs 11 A-G; 12C, D)

Of medium for the genus, height 1.8-1.95 (3.3) mm width 1.35-1.37 (1.8) mm, height/width ratio 1.333 -1.423 (1.833), rather solid, ovate-conical.

Protoconch ( Figs 11G ; 12E )

Flat, multispiral, of 2-2.2 (2) whorls, height 0.275 mm, nucleus diameter 0.075 mm, first half whorl diameter 0.137 - 0.150 mm, maximum diameter 0.400 - 0.450 mm, 5 equidistant spiral zigzag cordlets on protoconch II ( Fig. 12E ) (protoconch I not observable). Protoconch-teleoconch boundary barely visible.

Teleoconch

Of 2.65-2.8 (3.3-4.75), barely convex whorls, suture canaliculate, crowned by small tubercles at tips of axial ribs, highlighted by a slight subsutural depression. Height last whorl 1.47-1.5 mm, height/height last whorl ratio 1.200 -1.326. Axial sculpture of 17-18 ribs on last whorl, narrower than the interspaces, prosocline and slightly curved, gradually vanishing at the base; last half whorl progressively devoid of axial sculpture. Spiral sculpture absent. Microsculpture of only growth striae. Umbilical chink present. Aperture pyriform, small, height 1-1.02 mm height/height aperture ratio 1.800 -1.911, peristome continuous, varix absent, outer lip sharp, internally smooth (broken). Columella arcuate, simple.

Colour

Coloration of teleoconch white, protoconch faded orange.

Operculum and soft parts

Unknown.

VARIABILITY

Only two juvenile specimens available (See Table 1 and Appendix 7).

REMARKS

Rissoa aequatorialis Thiele, 1925 was dubitatively suggested by Lozouet (1990) to belong in his new genus Benthonellania (see also Hasegawa 2005: 152). Based on the current systematic framework, we agree and propose Benthonellania aequatorialis ( Thiele, 1925) as new combination for Rissoa aequatorialis Thiele, 1925 .

Benthonellania aequatorialis n. comb. differs from B. africana n. comb. in its multispiral, spirally sculptured protoconch vs paucispiral and smooth in B. africana n. comb., and the lack of spiral cordlets on the base, present in B. africana n. comb.

As already noticed by Hasegawa (2005) a juvenile specimen of an unidentified Benthonellania from southwestern Japan, recalls Benthonellania aequatorialis n. comb. (Hasegawa: 152, fig. 5J). However, that specimen quite probably represents a distinct, undescribed species, differing from B. aequatorialis n. comb. by its wider base and more angled periphery, and the faint spiral sculpture on the base (absent in B. aequatorialis n. comb.).

See under Benthonellania bouteti n. sp. for diagnostic comparison.

Figures

FIG. 11. — Benthonellania aequatorialis (Thiele, 1925) n. comb., syntypes: A-C, height 1.8 mm; D, height 1.95 mm; E, original label; F, original drawing (Thiele 1925, 84, pl. 6, fig. 23); G, syntype, detail of the first whorls; Valdivia Stn 199, ZMB-64973. Scale bar: G, 0.25 mm.

FIG. 13. — Maps of the known distribution of: A, Benthonellania hertzogi (◯), and Benthonellania thielei n. sp. and Benthonellania africana (Thiele, 1925) n. comb. (both); B, Benthonellania aequatorialis (Thiele, 1925) n. comb.

FIG. 12. — Benthonellania spp. (SEM): A-C, Benthonellania thielei n. sp., holotype, East Africa, 693 m depth, height 3.05 mm, ZMB/Moll. No. 64955a: shell (A), detail of the protoconch (B); detail of the teleoconch axial sculture (C); D, E, Benthonellania aequatorialis (Thiele, 1925) n. comb., syntype, Sumatra (Indonesia), height 1.8 mm, ZMB/Moll. No. 64973: shell (D), detail of the protoconch (E). Scale bars: B, C, E, 100 μm.