Lirapex pantagruel, Chen & Pradillon & Lorenzo & Alfaro-Lucas, 2025

Lirapex pantagruel View in CoL sp. nov.

( Figs 2, 7–10)

ZooBank registration LSID: urn:lsid:zoobank.org:act:

Diagnosis: A very large Lirapex , ≤ 7.6 mm SD; shell sculpture of weak, irregularly spaced, intermittent axial ribs. The final whorl expands more rapidly than previous whorls, with the last 0.25 whorls exhibiting loosening of coiling, leading to the adult aperture detaching slightly from the previous whorls. The opercular attachment is surrounded by 22–26 short epipodial tentacles. Central tooth with smooth cutting edge.

Etymology: From Pantagruel, son of the giant Gargantua in the French writer François Rabelais’ novel series The Five Books of the Lives and Deeds of Gargantua and Pantagruel ( Les Cinq livres des faits et dits de Gargantua et Pantagruel) ( Rabelais 1534). This species is the largest of all known Lirapex species, hence the name of a giant is fitting. Used as a noun in apposition.

Type locality: On black smoker chimneys of Falkor EMARK hydrothermal vent field ( 23.47°N, 44.99°W; 3993 m deep) located on the south intersection of the MAR axis with the Kane Fracture Zone ( Fig. 2D–F) GoogleMaps .

Type material: Holotype ( MNHN-IM-2000-39946), SD 7.5 mm, SH 6.9 mm, 95% ethanol ( Fig. 7A), on a black smoker chimney of Falkor EMARK vent field ( 23.4735°N, 44.9866°W; 3993 m deep), sampled using the suction sampler of the ROV SuBastian during dive #495 on-board R/V Falkor (too), cruise FKt230303, 23 March 2023. Paratype 1 (SMF 380303), SD 7.6 mm, SH 7.5 mm, 95% ethanol ( Fig. 7D). Paratype 2 (NSMT-Mo 79608), SD 6.5 mm, SH 6.1 mm, 95% ethanol ( Fig. 7B). Paratype 3 (MNHN-IM-2000-39947), SD 7.2 mm, SH 7.8 mm, 95% ethanol ( Fig. 7C). Paratype 4 (SMF 380304), SD 7.5 mm, SH 7.3mm, 95% ethanol. Paratype 5 (NSMT-Mo 79609), SD 6.2 mm, SH 7.0 mm, 95% ethanol. Paratype 6 (SMF 380305), SD 7.3 mm, SH 6.8 mm, 10% formalin; female specimen decalcified for examination of external anatomy. Paratype 7 (SMF 380306), SD 7.0 mm, SH 6.4 mm, 10% formalin ( Fig. 10); male specimen decalcified and used for µ-CT scanning. Paratype lot 8 (SMF 380307), three specimens, 95% ethanol. Paratype lot 9 (NSMT-Mo 79610), three specimens, 95% ethanol. Paratype lot 10 (MNHN-IM-2000-39948), three specimens, 95% ethanol.

Description: The shell ( Fig. 7) is solid, skeneiform, and very large for the genus (≤ 7.6 mm in diameter). Adult shells consist of 3.5–4 whorls, white in colour. Tightly coiled with deep suture, the coiling abruptly loosens in the final 0.25 whorls, leading to the slight detachment of the adult aperture from the previous whorl. The last adult body whorl also expands more rapidly than the previous whorls. The spire height is rather variable among individuals, and some individuals appear to be depressed ( Fig. 7D). Cross-section of the whorls almost round, although a little irregular. The aperture is weakly opisthocline and not significantly thickened. Umbilicus narrow, usually filled by sulphide mineral deposits. Shell sculpture on the teleoconch consisting of weak, irregular axial ribs that are typically stronger on the earlier whorls. Each rib is more often than not intermittent, taking the appearance of dotted lines. The strength of sculpture varies among individuals. Protoconch ( Fig. 8A) about half a whorl and 200 µm in length. The protoconch is sculptured with five very strong, angular spiral ribs, with some weak, irregular axial sculpture between them. This sculpture disappears about halfway through the protoconch, and the distal half is entirely smooth. Teleoconch shell microstructure ( Fig. 8B) of two distinct layers above the myostracum, including a thick cross-lamellar layer on the inner side and a much thinner granular layer outside of it. Shell pores are frequent on the cross-lamellar layer but do not penetrate into the granular layer. The periostracum is thin, semi-transparent, and greenish. A layer of reddish to blackish sulphide mineral deposit typically overlays the periostracum ( Figs 7D, 8B).

The operculum ( Fig. 8C) is multispiral, comprising>20 volutions in adult snails. It is thin, film-like, and semi-transparent; the colour is yellowish. The edge is free and detached, extending over the next volution to form a fringe. The central part of the operculum is often covered in a thick layer of tablet-like mineral deposit ( Fig. 7C, D).

The radula ( Fig. 8D, E) is rhipidoglossate, with a formula of ~50 + 4 + 1 + 4 + 50~. The rachidian tooth is solid, rigid, and well reinforced. The shaft of the rachidian is triangular, with a triangular overhanging cusp lacking serrations. The three inner laterals are also solid and rigid, similar in shape, with bifurcating reinforcement near the base. The cusp of the innermost lateral is smooth, the second lateral is very weakly serrated, and the third lateral carries relatively stronger serrations. The fourth, outermost lateral is much broader in comparison to the rest and carries clear serrations on the cusp. The laterals each exhibit a minor protuberance on the shaft, near the base. The marginal teeth are long and thin, tapering distally; the size of marginal teeth decreases outwards. The inner marginals have wider, triangular, rake-like cusps that are serrated, forming ~12 denticles. The outer marginals have smaller, hook-like cusps that are serrated into fine denticles.

The soft parts ( Fig. 9) are overall typical of the genus Lirapex . The head is large and without pigmented eyes. The animal is gonochoristic and lacking external sexual dimorphism; the cephalic tentacles are not modified into copulation organs in both sexes. The snout is short, flattened, with the mouth opening ventrally. A pair of thin, film-like jaws are present. The cephalic tentacles are smooth, conical, and gradually decreasing in size distally; the tentacles are ~1.5 times as long as the snout when contracted. Both cephalic lappets and neck lobes are lacking. The mantle edge is smooth and lacks tentacles.The columellar muscle extends only ~0.3 whorls behind the mantle edge, with the right shell muscle being larger than the left shell muscle. The two shell muscles are connected by a thick band of ventral muscular tissue. The foot is well developed, with a distinct transverse furrow separating the propodium from the mesopodium. Underneath the operculum, the opercular attachment is surrounded by ~22–26 short epipodial tentacles arranged in a semi-circular fashion around the posterior two-thirds of the opercular attachment. The epipodial tentacles are longest near the posterior edge of the opercular attachment.

A 3D reconstruction of the major organs is shown in Figure 10. Calculations of the organ volumes resulting from the reconstruction are listed in Table 1; the 3D model is available in an interactive PDF file (Supporting Information, Supplementary Material 3) and as a rotating video rendering (Supporting Information, Supplementary Material 4). The mantle cavity extends ~0.5 whorls from the mantle edge, mainly occupied by a sizeable ctenidium on the left occupying 13.72% of body volume in the specimen measured ( Figs 9C, 10A). The bipectinate gill carries ~60 pairs of gill leaflets. The ctenidium is connected to the left body wall in the posterior two-thirds, and the osphradium is present as a weakly raised and pigmented band underneath the ctenidium. The efferent pallial vein runs parallel and immediately dorsal to the left shell muscle ( Fig. 9G), posteriorly connecting to the pericardium. The pericardium ( Figs 9D, 10D) contains a monotocardian heart, with the ventricle situated posteroventral to the auricle; it is not penetrated by the intestine. The ventricle occupies 0.43% of the body volume and the auricle 0.99% in the specimen measured ( Table 1), the ventricle is weakly muscular. The left kidney (nephridium) is situated dorsal to the pericardium and is sizeable.

The digestive tract ( Figs 9E, 10E) is typical of Lirapex species. The radula ribbon is well sized and supported by a pair of cartilages that contact each other in the anterior two-thirds; a pair of salivary glands is situated above the radula ribbon ( Fig. 10B). The oesophagus runs directly towards the posterior to reach the stomach, situated about one whorl posterior to the mantle edge. The first half of the oesophagus is annexed with a small oesophageal gland that occupies 0.36% of the body volume in the specimen examined ( Table 1). The stomach is rather large and connected to the digestive gland by a series of tubular openings ( Fig. 10E). The digestive gland is light grey in colour and fills much of the dorsal part of the visceral mass ( Fig. 9). The intestine is long, emerging from the stomach on the left side, and initally runs anteriorly to reach the radula, then abruptly turns back to posterior-left of the stomach, where is loops twice between the stomach and the nephridium (visible from external anatomy, Fig. 9E). Then it emerges as the rectum immediately to the right of the nephridium ( Fig. 10F) and runs along the mantle roof towards the anterior-right side of the pallial cavity, where it ends in the anus. Much of the stomach and the intestine were filled with organic material mixed with dark, shiny mineral particles. The oesophagus was empty.

The gonad is very voluminous, occupying the ventral part of the entire visceral mass ( Fig. 10) and taking up 15.20% of the body volume in the male specimen examined using μ-CT ( Table 1). In males, the prostate is present as a series of complexly folded ducts with a lobulated appearance, situated immediately to the right of the rectum. In females, the ovary is of a similar size to the testis, but there is no similar complex glandular structure in the same relative location of the prostate, and the space is instead taken up by the ovary itself. The gonopore in both sexes opens on the right side of the mantle roof.

Distribution: Known only from EMARK vent field on the MAR at 3993 m depth ( Fig. 1), forming dense colonies on black smoker chimneys ( Fig. 2F).

Remarks: The coiled, skeneiform shell combined with a protoconch carrying distinct spiral ridges support the placement of L. pantagruel in Lirapex , alongside anatomical characters such as the two coils of the intestine visible from the external anatomy and the presence of epipodial tentacles only around the opercular attachment ( Warén and Bouchet 1993, Chen et al. 2021). Although the anatomy of L. pantagruel does not deviate from typical members of the genus and its radular characters are very similar to those of L. politus ( Chen et al. 2017b) , its shell characters are clearly distinct from all other known Lirapex species. The only species with sculpture similar to the intermittent, weak axial ribs of L. pantagruel is L. humatus from the East Pacific Rise, but the shell of L. humatus is more loosely coiled, and the aperture usually detaches more significantly from the previous whorl ( Warén and Bouchet 1993). The size of L. pantagruel is also much larger, at a maximum of 7.6 mm, in comparison to 3.4 mm in L. humatus . The only other Atlantic species, L. costellatus , has much broader, denser axial ribs when present and again a smaller, more loosely coiled shell (Warén and Bouchet 2001). The protoconch of L. pantagruel (200 µm) is also much smaller than that of L. costellatus (250 µm).

Genetic support

Our consensus tree from phylogenetic reconstruction using Bayesian inference ( Fig. 11) recovered a strongly supported [Bayesian posterior probability (BPP) =.96], monophyletic genus Peltospira containing all four species included: P. operculata , P. delicata , P. smaragdina , and P. gargantua . Within Peltospira , P. gargantua was fully supported (BPP = 1) as being sister to P. smaragdina ; this pair was, in turn, most closely related to the P. operculata and P. delicata pair, which was moderately supported (BPP =.86). The genus Peltospira was recovered as being closest to a clade comprising Rhynchopelta and Nodopelta , although the support for this was weak (BPP <.6). The tree also shows that sequences from both Hydra and Falkor EMARK vent fields, in addition to the pelagic larval stages collected at Hydra, all belong to the same species-level clade of P. gargantua . Lirapex was also recovered as a monophyletic genus including all three species with COI barcode available: L. politus , L. felix , and L. pantagruel . However, the support for this was weak (BPP <.6). Within Lirapex , L. pantagruel was fully supported (BPP = 1) as the sister species of L. politus . Lirapex was recovered as sister to Pachydermia , with the two genera forming a strongly supported (BPP =.95) clade.

Our consensus tree also recovered the order Neomphalida as a fully supported clade (BPP = 1). The family Melanodrymiidae was also found to be monophyletic, although with only weak support (BPP =.62). Neomphalidae was found to be a fully supported (BPP = 1) clade, but it was nested within Peltospiridae , rendering the latter paraphyletic. All neomphaline genera with multiple species included in the tree were recovered as monophyletic clades, although the support value varied from weak to strong (BPP =.66–1).

Pairwise interspecific K2P genetic distances of COI between pairs of the four Peltospira species with genetic data ( Table 2) averaged 15.0% (range 9.4%–18.0%). Meanwhile, the average intraspecific genetic distance was 0.3% (0.0%–0.5%), revealing a clear barcoding gap. In line with the phylogenetic reconstruction, P. gargantua was found to be closest to P. smaragdina , with an average K2P distance of 9.5%; the average distance from P. operculata was 17.4% and from P. delicata 16.0%. Likewise, the pairwise interspecific K2P distance between pairs of the three Lirapex species averaged 15.9% (range 9.3%–20.0%), showing a clear barcoding gap with the interspecific mean of 0.4% (range 0.0%–0.7%). As in the tree, L. pantagruel was closest to L. politus , with an average K2P distance of 9.3% (range 9.1%–9.5%).