Benthonellania thielei, 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 : 353-354

Benthonellania thielei n. sp.

( Figs 10 A-N; 12A-C; 13A; 26E; Table 1 )

urn:lsid:zoobank.org:act:7534314B-BD6C-4596-A61B-115E09DA6079

TYPE MATERIAL. — Holotype. East AFrica • dd (height 3.05 mm, width 1.92 mm, Figs 10 A-C, 12A-C); off southern Somalia, Valdivia Stn 251 ; 1°40.6’S, 41°47.1’E; 693 m depth; ZMB/Moll. no. 64955a .

Paratypes. East AFrica • 198 dd ( Fig. 10 D-L); same locality data as holotype; ZMB /Moll. no. 64955b .

TYPE LOCALITY. — East Africa, off southern Somalia, Valdivia Stn 251; 1°40.6’S, 41°47.1’E; 693 m depth.

DISTRIBUTION AND SYMPATRY. — Benthonellania thielei n. sp. is known from the type locality, off southern Somalia (Indian Ocean), where it is sympatric with Benthonellania africana ( Thiele, 1925) n. comb. ( Fig. 13 ).

ETYMOLOGY. — Named after Karl Hermann Johannes Thiele, German author of “Gastropoden der Deutschen Tiefsee-Expedition ‘Valdivia’ 1898-1899”.

DESCRIPTION OF HOLOTYPE

Shell ( Figs 10 A-C; 12A)

Medium for the genus, height 3.05 mm width 1.92 mm, height/width ratio 1588, rather solid, ovate-conical.

Protoconch ( Figs 10 J-K; 12B)

Dome-shaped, paucispiral,of 1.1 whorls, height 0.337 mm, nucleus diameter 0.187 mm, first half whorl diameter 0.350 mm, maximum diameter 0.475 mm, glossy, smooth, except for a faint and confused spiral sculpture on the nucleus ( Fig. 12B ). Protoconch-teleoconch boundary distinct, slightly prosocline and marked by a fine thickening.

Teleoconch

Of 3.5 very little convex whorls, suture canaliculate, subscalariform outline, crowned by small tubercles at tips of axial ribs, highlighted by a slight subsutural depression. Height last whorl 2.2 mm, height/height last whorl ratio 1.386. Axial sculpture of 20 ribs on last whorl, slightly narrower than the interspaces, continuing on the base beyond the periphery, prosocline and slightly curved, curvature more pronounced on last whorl. Last 0.25 whorl progressively devoid of axials. Spiral sculpture of 6 cordlets on last whorl, narrower than their interspaces, slightly bulging at intersection with axial ribs, 1 above the aperture, 1 on the suture line and 4 on the base ( Fig. 10L ). Microsculpture of growth striae ( Fig. 12C ). Umbilical chink absent. Aperture pyriform, height 1.35 mm, height/height aperture ratio 2.259, peristome continuous, varix absent. Lip thin, orthocline, flexuose, smooth on the inside. Columella arcuate, simple.

Colour

Coloration of teleoconch and protoconch translucent white.

Operculum

Thin, light yellow with an eccentric nucleus.

Soft parts

Unknown.

VARIABILITY

The shell can attain a height of 3.4 mm and a width of 2.05 mm (height/width ratio 1.401 -1.700; height/height last whorl ratio 1.275 -1.454); with up to 3.6 whorls; number of axial ribs 13-20; spiral striae on the last whorl 4-6; height/ height aperture ratio 2.125 -2.461. A very narrow umbilical chink may be present. Protoconch: whorls number 1.1-1.25; height 0.300 - 0.350 mm; nucleus diameter 0.187 -0.212 mm; diameter of the first half whorl 0.337 -0.350 mm; maximum diameter 0.400 -0.475 (See Table 1 and Appendix 6).

REMARKS

Thiele mixed in the type material of his Rissoa africana , two species: one that we have placed in Benthonellania (see above B. africana n. comb.), and another that we describe here as Benthonellania thielei n. sp. He realised that there was a striking variability within that material but explicitly stated that he believed that “these differences [between his fig. 20 (= R. africana s.s.) and fig. 21 (= B. thielei )] fall within the limits of a single species” ( Thiele 1925: 83). Then, he stated: “Only for one shell that I show in figure 22 [again a B. thielei ] I have some doubts, and would like to call it a variety, with the name pluricostata ”. Eventually, he stated again that “the shells [of africana ] show variation in the number of axials”. Finally, Thiele did not separate the specimen of his figure 22 (the single specimen upon which he based Rissoa africana var. pluricostata ) and we have been unable to trace it among the 199 ‘pluricostate’ shells. We have pondered on the possibility of using the name ‘ pluricostata ’, but this is a paradigmatic case of an Author who introduced explicitly a variety, and “the content of the work unambiguously reveals that the name was proposed for an infrasubspecific entity”: ICZN, 1999: Art. 45.6.1).

Benthonellania thielei n. sp. differs from B. africana ( Thiele, 1925) n. comb. (among which it was mixed in Thiele’s collection, and which we classify in Benthonellania , see below) in its less slender shell, smaller size (height 3.4 mm vs height 4 mm in B. africana n. comb.), more numerous axial ribs on the last whorl (13-20 vs 8-10 in B. africana n. comb.), spiral cordlets more numerous and arranged differently ( Benthonellania thielei n. sp.: 4-6 well spaced, slightly bulging at the intersection with the axial ribs, 1 above the aperture, 1 on the suture line and 2-4 on the base; Benthonellania africana n. comb.: 3-4 very weak cordlets on the last whorl above the aperture and on the suture line, the uppermost suprasutural visible on all whorls, base smooth); umbilical fissure closed or barely noticeable vs open in B. africana n. comb.. Lower protoconch 0.300 - 0.350 mm vs 0.375 -0.450 mm in B. africana n. comb., lower maximum diameter 0.400 - 0.475 mm vs 0.525 -0.575 mm in B. africana n. comb., fewer whorls 1.10-1.25 vs 1.25-1.40 mm in B. africana n. comb.

B. thielei n. sp. differs from B. hertzogi ( Thiele, 1925) n. comb. in the more numerous whorls (3.5-3.6 vs 2.75-3.1 in B. hertzogi n. comb.), fewer axial ribs on the last whorl (13-20 vs 24-26 in B. hertzogi n. comb.), more evident subsutural crown of whorls, yielding a more scalariform profile, due to the absence of the fine subsutural cordlet on the first teleoconch whorls, which is present in B. hertzogi n. comb., the protoconch lower (height 0.300 - 0.350 mm vs 0.375 – 0.400 mm in B. hertzogi n. comb.) and smaller (maximum diameter 0.400 - 0.475 mm vs 0.500 mm in B. hertzogi n. comb.).

B. thielei n. sp. is also similar in the general shell outline to the species named ‘ Alvania sp. 2 ’ by Hasegawa (2005: 149, fig. 5I), differing for its smaller size (heigth 3.4 mm vs heigth 4.5 mm in ‘ Alvania sp. 2 ’) and the less slender shell. ‘ Alvania sp. 2 ’, as well as the similar ‘ Alvania sp. (s.s.)’ figured by Hasegawa (2001: 163, pl. 2, fig. F, G), represent quite probably two undescribed species.

Benthonellania thielei n. sp. differs from Benthonella basistriata n. sp. in its larger size (height 2.97-4.40 mm, width 1.82-2.05 mm vs height 1.95-2.60 mm, width 1.30-1.62 mm in B. basistriata n. sp.); absence of varix vs presence of varix in B. basistriata n. sp.; protoconch paucispiral, dome-shaped, glossy, smooth, with fewer whorls 1.10-1.25, vs multispiral, dome-shaped, of 2.3-2.9 whorls, protoconch I with 5 spiral threads, interspaces covered by microtubercles, and protoconch II with 2 fine spiral cordlets, with irregularly set microtubercles on the subsutural area in B. basistriata n. sp.

Figures

FIG. 10. — Benthonellania thielei n. sp., Valdivia Stn 251, East Africa, 693 m depth: A-C, holotype, height 3.05 mm width 1.92 mm, ZMB/Moll. No. 64955a; D-F, paratype,height 2.8 mm, ZMB/Moll.No. 64955b; G-L, paratype,height 2.97 mm, ZMB/Moll. No. 64955b; J, K, detail of the first whorls,L, detail of the spiral sculpture on the last teleoconch whorl; M, original label; N, original drawing (Thiele 1925, 83, pl. 6, fig. 21). Scale bars: J, K, L, 0.50 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.