Lirapex felix, Chen & Han & Copley & Zhou, 2021

Chen, Chong, Han, Yuru, Copley, Jonathan T. & Zhou, Yadong, 2021, A new peltospirid snail (Gastropoda: Neomphalida) adds to the unique biodiversity of Longqi vent field, Southwest Indian Ridge, Journal of Natural History 55 (13 - 14), pp. 851-866 : 856-860

Lirapex felix sp. nov.

( Figures 2–4 )

ZooBank registration: urn:lsid:zoobank.org:act:7BE47DEC-FB80-473F-8658-2C268ED67627

Diagnosis

Lirapex with depressed spire (shell height to width ratio 1: 1.2 in both specimens known) and tightly coiled, non-disjunct final whorl. Sculpture consisting of weak axial ribs up to 100 μm apart on the teleoconch, strength somewhat variable but strongest at shoulder and base.

Type locality

‘Tiamat’ chimney (HOV Jiaolong Marker DFF 11), Longqi vent field, Southwest Indian Ridge ( Copley et al. 2016; Chen et al. 2017a).

Type material

Holotype ( RSIO 35733 ; Figures 2 (a–d,i), 3(a–b)), male, shell width 2.6 mm, shell height 2.1 mm, in 95% ethanol, Longqi vent field, Southwest Indian Ridge, 37.7838°S, 49.6502°E (‘Tiamat’ Chimney/ DFF11 ), 2761 m deep, R / V Xiangyanghong 9 expedition DY35, HOV Jiaolong Dive 94, 2015/i/11. Used for DNA sequencing and the examination of radula and teleoconch sculpture under SEM.

Paratype (NSMT-Mo 79160; Figures 2 (e–h), Figures 3 (c–f)), female, shell width 3.7 mm, shell height 3.1 mm, 99% ethanol, Longqi vent field, Southwest Indian Ridge, 37.7838°S, 49.6495°E (‘Tiamat’ Chimney/ DFF11 ), 2785 m deep, RRS James Cook expedition JC67, ROV Kiel6000 Dive 142, 2011/xi/29 . Shell dissolved for the examination of external anatomy after pieces broken and removed for SEM observation of microstructure.

Description

Shell ( Figure 2 ) fairly solid, depressed skeneiform, much wider than tall, shell height-towidth ratio approximately 1:1.2. Maximum dimensions: width 3.7 mm, height 3.1 mm (paratype specimen). Tightly coiled, no evidence of coil loosening even in final whorl. Typically encrusted with mineral deposits. Protoconch ( Figure 3 (a)) approximately 0.5 whorls, diameter 240 μm; 5 distinct spiral ridges with finer spiral wrinkles present on posterior half, anterior half completely smooth. Transition to teleoconch marked by clearly thickened ridge. Teleoconch with 3 whorls, white in colouration, covered by semitransparent, greenish periostracum. Teleoconch sculpture ( Figure 3 (b)) consisting of more or less equidistant, weak, axial ribs up to 100 μm apart. Strength of ribs varies somewhat, but ribs always evident on shoulder or base near umbilicus where it is strongest. Whorls nearly round in cross section, aperture not thickened in adults. Umbilicus opening of moderate size. Shell microstructure ( Figure 3 (f)) non-nacreous, comprising 2 layers. Granular layer on outside much thinner than cross-lamellar layer on inside, latter being partly penetrated by shell pores, which taper towards granular layer but diminish before transition.

Operculum ( Figure 3 (c)) multispiral, approximately 12 whorls in adult specimen (paratype), thin film-like, semi-transparent. Later volutions form clear extending fringes overlapping subsequent volution.

Radula ( Figure 3 (d–e)) rhipidoglossate, with formula ~25 + 4 + 1 + 4 + ~25. Rachidian and laterals well reinforced. Rachidian tooth triangular, sides slightly sigmoidal in outline, with triangular overhanging cusp, cutting edge smooth. Base of rachidian trifurcated. Laterals with bifurcating reinforcement at base, all have triangular, overhanging cusps with up to 5 minor serrations on both sides. All laterals with single minor protuberance near base. Laterals become less laterally bent with progressively broader cusps from innermost to outermost, outermost lateral being much broader than others. Marginal teeth long, thin, distally tapered. Innermost marginals larger in size, gradually decreasing outwards. Marginal cusps hook-like, serrated into approximately 15 denticles.

Soft parts ( Figure 4 ) just shy of three volutions. Sexes separate. Head large, eyes lacking. Snout even in breadth, short, mouth ventral. Cephalic tentacles about 1.5 times as long as snout when observed in contracted state, conical, rapidly decreasing in size distally. Both cephalic tentacles simple, smooth, no evidence of modification into copulation appendages in either sex. Cephalic lappets lacking, as well as neck-lobe. Foot well developed, transverse furrow separates propodium from mesopodium. Epipodial tentacles arranged in semi-circular fashion surrounding posterior two-thirds of opercular attachment, numbering 17 on either side, 34 in total. Pallial edge simple, lacking appendages. Columellar muscle reaching only approximately 0.3 whorls behind pallial edge, attachment area larger on right side than left side, two sides connected by a thick band of ventral muscular tissue.

Mantle cavity extending 0.4 whorls posterior of pallial edge. Ctenidium moderately large, bipectinate, approximately 35 leaflets. Digestive tract mostly filled with dark mineral deposits, especially intestine, which loops twice between stomach and left kidney. Stomach clearly enlarged compared to intestine, located 0.9 whorls posterior from pallial edge. Rectum curves slightly after entering mantle cavity, attached on mantle ceiling, becoming detached just posterior of anus. Heart monotocardian, ventricle located posteroventrally compared to auricle. Pericardium not penetrated by intestine (seen by transparency). Visceral mass dorsally occupied by digestive gland extending up to apex, ventrally occupied by gonad. Gonopore simple, located behind anus.

Distribution

Longqi hydrothermal vent field, Southwest Indian Ridge, Indian Ocean. It lives in association with the giant peltospirid species Chrysomallon squamiferum and Gigantopelta aegis on the active diffuse-flow chimney ‘Tiamat’.

Etymology

‘ Felix ’ (adjective in the nominative case, Latin), meaning lucky, happy or blessed. It is named in reference to the serendipitous discovery of the first specimen of this apparently rare species and the fortunate finding of the second specimen that allowed for a comprehensive description.

Remarks

The placement of L. felix sp. nov. in Lirapex is supported by (1) a coiled shell; (2) radula with outer marginals possessing serrated, hook-like cusps, and outermost lateral tooth much broader than the rest; (3) protoconch with distinct spiral ridges; (4) anterior part of the intestine visible in two coils between the stomach and the left kidney; (5) epipodial tentacles only present around the opercular attachment.

Four other species are currently recognised in the genus Lirapex . These are L. humatus Warén and Bouchet, 1989 and L. granularis Warén and Bouchet, 1989 from the East Pacific Rise; L. costellatus Warén and Bouchet, 2001 from the Mid-Atlantic Ridge; and L. politus Chen et al., 2017a also from the Longqi field, Southwest Indian Ridge. Lirapex felix sp. nov. can be distinguished from all four by its more depressed shell form and the fact that there is no evidence of the coil loosening in the final whorl, an unusual feature among Lirapex species. In L. humatus and L. costellatus the second half of the adult body whorl is clearly detached from the previous whorl ( Warén and Bouchet 1989, 2001), whereas in L. granularis and L. politus the whorl does not detach but visibly loosens ( Warén and Bouchet 1989; Chen et al. 2017a). Apart from L. felix sp. nov. which is wider than tall, all described Lirapex species are typically as tall as wide or taller than wide ( Warén and Bouchet 2001; Chen et al. 2017a). Although these results come from observing only two specimens of L. felix sp. nov., the characters are consistent between the two specimens and likely reliable.

The shell sculpture of L. felix sp. nov. is similar to that observed in L. humatus and L. costellatus . In both L. humatus and L. costellatus the sculpture is highly variable in development and may be present all over the shell surface or only between the shoulder and base ( Warén and Bouchet 1989, 2001). Even in the two available specimens of L. felix sp. nov. the sculpture differed in strength, being stronger in the holotype than in the paratype specimen, suggesting that the sculpture strength could be as variable as in the other two species. Unlike these three species, the granular sculpture of L. granularis is only present on the periostracum, and the teleoconch is smooth ( Warén and Bouchet 1989), making it impossible to confuse with L. felix sp. nov. Similarly, both teleoconch and periostracum of L. politus are completely smooth ( Chen et al. 2017a), and thus it cannot be confused with the co-occurring L. felix sp. nov. The radula of L. felix sp. nov. has a smooth rachidian cusp similar to L. costellatus and L. politus , but differs from L. humatus which has a serrated rachidian cusp ( Warén and Bouchet 2001; Chen et al. 2017a).

Figures

Figure 2. Lirapex felix sp. nov., type specimens. (a–d) Holotype (RSIO 35733), shell width 2.6 mm, shell height 2.1 mm; (e–h) paratype (NSMT-Mo 79160), shell width 3.7 mm, shell height 3.1 mm; (i) close-up of the holotype near the umbilicus, showing the axial ribbing sculpture. Scale bars: a–h = 1 mm; i = 200 μm.

Figure 3. Lirapex felix sp. nov., scanning electron micrographs. (a) Protoconch, holotype (RSIO 35733), arrows indicate the transition between protoconch and teleoconch; (b) shell sculpture, holotype (RSIO 35733); (c) operculum, paratype (NSMT-Mo 79160); (d–e) radula, holotype (RSIO 35733); (f) shell microsculpture, paratype (NSMT-Mo 79160), arrows indicate shell pores. Scale bars: a = 100 μm; b = 200 μm; c = 1 mm; d = 50 μm; e–f = 20 μm.

Figure 4. Lirapex felix sp. nov., soft parts, paratype (NSMT-Mo 79,160). (a) Right view; (b) anterior view with part of the mantle roof removed, visceral mass digitally removed at the dashed line; (c) ventral view; (d) left view. Abbreviations:a,anus; c, ctenidium; d, digestive gland;et, epipodial tentacle; f, foot; i, intestine; k, left kidney; lm, left columellar muscle; op, opercular attachment; ov, ovary; p, pericardium; pe, pallial edge; r, rectum; rm, right columellar muscle; s, stomach; sn, snout; t, cephalic tentacle. Scale bars: a, c–d = 1 mm; b = 0.5 mm.