Pseudorimula leisei Beck, 2023

Chen, Chong & Sigwart, Julia D., 2023, The lost vent gastropod species of Lothar A. Beck, Zootaxa 5270 (3), pp. 401-436 : 415-418

Pseudorimula leisei Beck , sp. nov.

Figs 9 , 10

[ZooBank LSID: urn:lsid:zoobank.org:act:41162F08-EFEE-43B1-9847-86D40085CC3F ]

Type material. Holotype [MNHN-IM-2000-38680] and one paratype in MNHN [Paratype 1: MNHN-IM-2000- 38681], two paratypes in NSMT [Paratype 2: NSMT-Mo 79410; Paratype 3: NSMT-Mo 79411], two paratypes in SMF [Paratype 4: SMF 370357 ; Paratype 5: SMF 370358 ].

Type locality. Lau Basin (Hine Hina) [BIOLAU, BL03 ] .

Material examined. The type material and partly damaged specimens from BIOLAU BL03 [series of 5 specimens, SMF 370359 ] and 1 juvenile specimen from BIOLAU BL04 [MNHN-IM-2022-12941], 5 subadult specimens from STARMER II PL20 [NSMT-Mo 79412] .

Distribution. North Fiji Basin and Lau Basin in bathyal, sulfide-rich areas.

Etymology. Species is named after Dr. Thorsten Leise, entomologist, in remembrance of the good times [he and Beck] shared at the Dept. of Biology/Zoology at the Philipps-University of Marburg.

Description. Shell ( Figs 9A–E ). Shell moderately large for genus, maximum length 5.1 mm. External surface coated with blackish and brownish mineral encrustations. Periostracum light brown, projecting slightly past the shell margin. Aperture oval in outline, margin of aperture not protruding. Profile relatively high, height of holotype 0.40 to 0.45 times length. Highest elevation of shell at about one-half its length. Protoconch not preserved. Juvenile shells with open slit, in adult shells slit open about one-third of the length of anterior slope, slit strongly deflected to the right; anteriorly, the slit is in adults sealed by the periostracum, posteriorly, it is sealed by a calcitic callus. Sculpture consists of numerous very fine radial ribs and small, sharp periostracum-lamellae, which occur sporadically in approximately 20 radial lines. Shell interior opaque, muscles scars (2 lateral, 1 posterior) and pallial attachments well marked by darker areas. Apical whorl contains parts of the visceral mass apparently also present in adults; adjacent to shell slit and surrounding the foramen, a thickened callus strengthens the shell. Suture with a zig-zagged outline.

Radula ( Figs 10 A, B ). Rhipidoglossate, ribbon symmetrical, consisting of 60–70 rows of teeth in specimens of 4.3 mm shell length. Formula 10-12 x 4 x 1 x 4 x 10-12. Rachidian tooth with a broad base equipped with lateral ridges and with an extremely strong, overhanging cusp; cusp laterally with about 8 large denticles, at top acutely pointed. Lateral teeth similar in morphology to rachidian but somewhat narrower and denticulated at distal margins of cusps only. Marginal teeth relatively low in number, with broad tips, which are finely serrate, and have a long, tongue-like process distally.

Soft parts ( Figs 9 A , 10 C, D ). Anterior end of foot much broader in comparison to posterior end; anterior foot gland-opening represented by a very long, transverse slit. Three pairs of posterior epipodial tentacles with extended bases and two finger-like tentacles carrying micropapillae. Cephalic tentacles thickened and relatively long (in preserved condition) carrying micropapillae as well. Oral disc not folded and without clearly separated lappets, head without eyes. Pallial skirt deeply emarginated, which corresponds to the shell slit, oroficium has about 10–15 tentacles on each margin anteriorly, posteriorly the marginal membrane becomes the smooth, posterior end of the oroficium which is marked by a single tentacle directed outwards. Pallial margin slightly swollen, inner rim papillated, pallial skirt anteriorly attached to shell. One pair of lateral, equal-sized muscles and one isolated, posterior shell muscle present. Posterior muscle roughly in mid-line, covered on its left margin by the tip of the visceral mass. Arrangement of pallial cavity and visceral mass somewhat asymmetrical, reflecting a remnant of shell coiling which can be seen more clearly in juveniles. Pallial cavity deep, right ctenidium smaller than left. No sexual dimorphism detectable except granular ovary or somewhat striped testis. No operculum.

Remarks. Up to now, two species with an extremely disjunct distribution have been known, Pseudorimula marianae McLean, 1989 from Snail Pits vents, Mariana Back-Are Basin, and P. midatlantica McLean, 1992 , from Snake Pit, Mid-Atlantic Ridge ( McLean 1989, 1992). The new taxon, P. leisei sp. nov., is separated from either of the former by the following diagnostic characters: shell with apex close to posterior margin and with a much steeper posterior slope; shell surface without strong radial ribs; rachidian and lateral teeth with very coarse denticulation and sharply pointed cusps.

Figures

FIGURE 9. Pseudorimula leisei sp. nov., shell and soft parts. A. Paratype 5 (shell length 2.7 mm), ventral view. B–C. Holotype (shell length 4.5 mm). B. Apical view. C. Lateral view from left. D–E Paratype 1 (shell length 4.9 mm). D. Apical view. E. Ventral view.

FIGURE 10. Pseudorimula leisei sp. nov., radula and soft parts. A, B, C. Radula. A. Overview depicted in lateral view, thin arrow: rachidian tooth, bold arrow: tongue-like process at marginal tooth.B. Rachidian tooth with serrate, extremely overhanging cusp. C. Soft parts (critical-point dried). D. Oroficium (pallial slit) with many tentacles at the anterior slit, with thin membranes at opening, and a single tentacle (arrow) at its posterior end. [Scale bars: A, 50 μm; B, 10 μm; C, 1 mm; D, 500 μm.]