NEOLEPETOPSIS ARDUA SP. NOV.

(FIGS 2E, 5, 6)

Z o o B a n k r e g i s t r a t i o n: u r n: l s i d: z o o b a n k. org:act: 358436F9-1BAF-452D-A97A-935F1C36BEAC

Diagnosis: Medium-sized Neolepetopsis (SL ≤ 5.1 mm) with stronger concentric sculpture compared with radial sculptures, intersecting points of which are weakly drawn out to form nodes. Concentric sculptures between 50 and 150 µm apart. Shell profile low, flat limpet-form, with apex on the midline and slightly anterior to the centre. Greenish periostracum present.

Type locality: Apparently inactive sulfide deposits off the centre of venting activity at the Longqi hydrothermal vent field, Southwest Indian Ridge.

Type material: Holotype (RSIO 49001; Fig. 2E), the only specimen collected with both intact shell and animal, in 95% ethanol, SL 5.1 mm, SW 3.7 mm, Longqi vent field, Southwest Indian Ridge, 49°38.88′E, 37°46.80′S, 2778 m deep (station ‘49I-S011-TVG04’), R / V Xiangyanghong 10, by video-guided grab sampler, 1 June 2018 . Paratype 1 (NSMT-Mo 79219; Fig. 6), soft parts only, in 95% ethanol . Paratype 2 (RSIO 49010), soft parts only, in 95% ethanol . Paratype 3 (NSMT-Mo 79220), soft parts only, in 95% ethanol. Paratype 4 (RSIO 49011–49015), a lot of five individuals with soft parts only, in 95% ethanol . Paratype 5 (NSMT-Mo 79221), a lot of five individuals with soft parts only, in 95% ethanol. All paratypes are from the same lot as the holotype.

Description: Shell (Fig. 2E) thin, translucent, thickest near apex; only known adult shell with length 5.1 mm, width 3.7 mm (length-to-width ratio 1.38:1). Shell thickness decreases towards shell edge, with increasing transparency, without significant prismatic structural coloration. Oval in outline; anterior end slightly narrower than posterior end (based on holotype). Shell profile low. Apex (Fig. 5A) situated on midline, slightly anterior of centre. Shell sculpture (Fig. 5B) finely reticulate, formed by strong, closely spaced (interspacing ranging from 50 to 150 µm on holotype) concentric ribs crossing weaker, frequent radial ribs. Weak, almost indistinct nodes drawn out at points where concentric and radial ribs cross. Muscle scar clearly discernible. Protoconch unknown; holotype with seal along opening of protoconch (Fig. 5A). Thin, greenish periostracal layer present over shell.

Radula (Fig. 5C, D) with seven mineralized (rachidian, two pairs of laterals, pluricuspid) and four non-mineralized (two pairs of marginals) teeth per row, ~35 µm wide. Rachidian sturdy; base of long shaft slightly expanded laterally, tapering distally toward one overhanging, triangular cusp with smooth cutting edge. Laterals gradually descending in horizontal position outward. Inner laterals with minor indentation at base to accommodate expanded base, also with similar-sized overhanging, triangular, smooth cusp. Outer laterals with slightly studier shafts than inner laterals, carrying broader overhanging cusps. Pluricuspid teeth positioned much lower than laterals, with broad overhanging cusps with two sharp tips. Shafts of pluricuspids strongly basolaterally indented; sinuous shafts connecting to inner marginal. Inner marginals with broad, indented shaft; cutting edge broad. Outer marginals greatly reduced, with short, narrow shafts; cusps narrow. External anatomy (Fig. 6) similar to N. prismatica described above, but intestine packed with darker, black coloured faecal pellets.

Etymology: From Latin adjective arduus, meaning high, steep, difficult or troublesome. It is named in witness of the arduous efforts, through numerous attempts that the authors had to make in preparing and mounting the minute radula of this Neolepetopsis species.

Distribution: Known only from inactive chimney surfaces off the Longqi hydrothermal vent field on the Southwest Indian Ridge, Indian Ocean. The sulfide deposits from which the present new species was obtained were collected by a grab sampler and, as far as we could see from the video, they appeared inactive.

Remarks: Only one specimen (the holotype) of N. ardua was collected intact with the shell and soft parts. Shells of all other specimens available for study have apparently been lost during the sampling process; some had a few shell fragments still attached, indicating that they were not shell-less when alive. All descriptions of shell characters are therefore based solely on the holotype.

Neolepetopsis ardua is easily distinguished from the geographically closest species, N. prismatica, by its much stronger radial sculpture (compare Figs 3C, 5B) and the presence of drawn out nodes at the intersection between radial and concentric ribs. The shell shape of N. ardua is slightly wider at the posterior (based on the holotype), whereas that of N. prismatica is either wider at the anterior end or similar in width at both ends (Fig. 2). Furthermore, N. ardua does not possess the same level of iridescent structural colour on the shell interior seen on N. prismatica . The shell sculpture of N. ardua is generally similar to that of N. densata, N. verruca and N. occulta (McLean, 1990), but differs in that the axial ribs are much more frequent but weak in strength, leading to only insignificantly raised (as opposed to strongly raised) nodes being formed at the intersections. Neolepetopsis densata and N. occulta have fully transparent shells (McLean, 1990), whereas that of N. ardua is only translucent. The present new species is easily distinguished from both N. gordensis and N. nicolasensis by the concentric sculpture being stronger than the radial ones, and by the low shell profile and the much thinner, delicate shell. The radula of N. ardua is most similar to N. gordensis in having long shafts and possessing two cusps on the pluricuspid tooth (McLean, 1990), but the mineralized cusps are generally heavier than those in that species, and the marginals are more reduced in N. ardua .