Peltospira gargantua, Chen & Pradillon & Lorenzo & Alfaro-Lucas, 2025

Peltospira gargantua View in CoL sp. nov.

( Figs 2–6)

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

Diagnosis: A very large Peltospira , ≤ 25 mm SD. The shell is globose, rather tightly coiled, with a deep suture. The aperture is very large, expanding rapidly in the adult body whorl. The epipodium carries 24–28 thick, short, paddle-like epipodial tentacles along the posterior two-thirds of the foot. The operculum is lacking in adults. The oesophageal gland is greatly hypertrophied.

Etymology: From Gargantua, a giant king depicted 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). The word Gargantua has since been used to mean ‘huge’ or ‘immense’, as exemplified by the English adjective gargantuan. Thus, the species name refers to the very large size of the new species in relationship to other known congeners. Used as a noun in apposition.

Type locality: On black smoker chimneys of Hydra hydrothermal vent field ( 24.96°N, 45.57°W; ~ 3750 m deep) on the Grappe Deux non-transform offset, MAR ( Fig. 2) GoogleMaps .

Type material: Holotype ( MNHN-IM-2000-39943), SD 22.5 mm, SH 18.3 mm, 95% ethanol ( Fig. 3A); on black smoker chimney of Hydra vent field ( 24.9600°N, 45.5749°W; 3739 m deep), collected by the manipulator of HOV Nautile with a chimney sample during dive #2099 ( BICOSE3 dive #17), R / V Pourquoi pas? cruise BICOSE3 , 16 November 2023 GoogleMaps . Paratype 1 ( SMF 380299 About SMF ), SD 24.6 mm, SH 22.3 mm, 95% ethanol ( Fig. 3B), same data as the holotype GoogleMaps . Paratype 2 (NSMT-Mo 79606), SD 17.6 mm, SH 14.5 mm, 95% ethanol ( Fig. 3C), same data as the holotype. Paratype 3 (MNHN-IM-2000-39944), SD 19.7 mm, SH 16.9 mm, 95% ethanol ( Fig. 3D), same data as the holotype. Paratype 4 ( SMF 380300 About SMF ), SD 20.1 mm, SH 17.7 mm, 10% buffered formalin, specimen decalcified for anatomical examinations; on black smoker chimney of Hydra vent field ( 24.9600°N, 45.5748°W; 3740 m deep), collected by a suction sampler on HOV Nautile during dive #2097 ( BICOSE3 dive #15), R / V GoogleMaps Pourquoi pas? cruise BICOSE3 , 14 November 2023 . Paratype 5 ( SMF 380301 About SMF ), SD 16.9 mm, SH 14.7 mm, buffered formalin, specimen decalcified for anatomical examinations; same data as paratype 4.

Other material examined: Lot of three specimens ( SMF 380302 About SMF ), 95% ethanol, on a black smoker chimney of Hydra vent field ( 24.9598°N, 45.5749°W; 3799 m deep), sampled using the suction sampler of the ROV SuBastian during dive #498 on-board R / V Falkor (too), cruise FKt230303, 28 March 2023 GoogleMaps .

Lot of three specimens (NSMT-Mo 79607), 10% buffered formalin, 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.

Lot of three specimens (MNHN-IM-2000-39945), 10% buffered formalin, 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.

Seven planktonic larval specimens collected near Hydra vent field by an autonomous plankton pump (SALSA) ~ 30 m from the active edifice, R/V Pourquoi pas? cruise BICOSE3, used for SEM of protoconch, then DNA barcoding to confirm species identity.

Description: The shell ( Fig.3) is very large for the genus (≤ 25 mm diameter), globose, its shape transitioning from skeneiform to haliotiform along growth. Rather tightly coiled, with about two whorls; the suture is deep. The calcareous layer of the teleoconch is thin, pure white, and fragile. The diameter of the shell whorl rapidly expands, especially on the body whorl. The spire is depressed. The shape of the aperture is depressed oval, elliptic, very large, and holostomous. The outer lip is simple and not thickened even in adults, while the inner lip forms a thickened callus on the columella. The shell sculpture consists of regularly spaced but only weakly raised axial ribs, most evident on the first teleoconch whorl; this axial sculpture becomes more irregular on the second teleoconch whorl. In some specimens the axial ribs continue to form throughout the second teleoconch whorl ( Fig. 3B) but in others the ribbing becomes irregular ( Fig. 3A) and in some cases it almost completely vanishes ( Fig. 3C). Near the aperture of adult shells, the axial ribs often become intermittent, forming isolated semi-spherical nodes instead of continuous ribs ( Fig. 3A, B). Specimens from Falkor EMARK tend to have more extensive axial ribbing than those from Hydra. Shell microstructure of the teleoconch ( Fig. 4B) consists of two layers of equal thickness above the myostracum; the inner layer is complex cross-lamellar, and the outer layer is simple prismatic. Both layers exhibit numerous shell pores perpendicular to the shell surface, although the shell pores of each layer appear to have formed independently, and they do not seem to be connected. Protoconch morphology was obtained from pelagic larval stages between 265 and 280 µm in diameter ( Fig. 4A). The protoconch carries 16–18 narrow spiral ridges in the first two-thirds (of which seven to eight will be visible apically when the teleoconch develops), and the final one-third is smooth. The peristome of the larval shell is circular when viewed from the bottom and is ~150 µm in diameter; the lip is slightly flared. The periostracum is tough, thick, olive green in colour with a strong sheen; its edge is reflected along the shell margin around the entire aperture. The original coloration of the shell surface is usually obscured by dark brown to reddish layers of sulphide mineral deposits.

The radula ( Fig. 4C–E) is rhipidoglossate, the formula being ~50 + 4 + 1 + 4 + 50~. The rachidian tooth is solid, rectangular, and well reinforced by lateral ridges on either side of the shaft. The rachidian cusp is rectangular with a smooth cutting edge that is overhanging, serration lacking. Lateral projections at the base of the rachidian tooth interlock with those from the lateral teeth. The innermost laterals have sigmoidal, interlocking, well-reinforced shafts; the shafts each carry a basal protrusion or node. Lateral cusps are overhanging, blunt triangular in shape, with smooth cutting edges lacking in serrations. The second lateral is similar in morphology to the innermost lateral but slightly taller, with a larger cusp, and its shaft is more straight, with a weaker node. The third lateral is again similar to the second lateral but larger in size, with only a very weak node on the shaft. The outermost lateral is twice as large as the third lateral, with overhanging, sharply pointed triangular cusps bearing two to three denticles only on the outer side of the cutting edge ( Fig. 4D). The marginals are long, with smooth, flattened shafts. The inner marginals ( Fig. 4E) exhibit rake-like cusps serrated into 15–20 denticles distally; a major denticle is present near the centre, and the other denticles decrease in strength either side of it. The outer marginals are more elongate, with the cusps becoming increasingly finely serrated but weaker in strength outwards.

The animal is gonochoristic, with the external anatomy ( Fig. 5A–D, H, I) lacking in sexual dimorphism or specialized copulatory organs, such as modified cephalic tentacles. The head is large, with a thick and broad snout that expands slightly towards the mouth. The mouth is simple, with oral appendages lacking; a pair of thin, membranous jaws is present. A pair of cephalic tentacles extends slightly beyond the snout in preserved specimens; they are simple and broad at the base, tapering to a blunt tip distally. Sensory papillae are lacking on the cephalic tentacles. There is no trace of eyes visible externally, such as pigmentation. The foot is muscular and fleshy; it is rounded anteriorly but tapered posteriorly. The side of the propodium is covered in irregular tubercles and is demarcated from the epipodium by a narrow fissure. The epipodium carries 24–28 thick, short, and paddle-like epipodial tentacles ( Fig. 5B) along the posterior two-thirds of the foot; the tentacles are arranged in a single row. All adult individuals examined lacked an operculum. The mantle edge is simple, with no tentacles. The shell muscle is prominent and U-shaped; the posterior connection is through a narrow attachment and it is laterally expanded anteriorly on both sides, the right side being much larger than the left side ( Fig. 5H, I). The mantle cavity occupies about threequarters of the length.

A 3D reconstruction of the major organs is shown in Figure 6, with calculations of the organ volumes listed in Table 1; the 3D model is available in an interactive PDF file (Supporting Information, Supplementary Material 1), and a rotating video of the model is also available (Supporting Information, Supplementary Material 2). The circulatory system is dominated by a very large, bipectinate left ctenidium ( Figs 5C, 6A–E) situated at the left side of the mantle cavity, occupying 15.84% of the body volume in the specimen measured, terminating at the posterior edge of the mantle cavity. Approximately the posterior two-thirds of ctenidium are attached ventrally to the left body wall, whereas the anterior one-third is free. The osphradium is present underneath the ctenidium, taking the form of a weakly raised, pigmented band along the anteromedian wall of the left shell muscle. The efferent pallial vein runs along the left shell muscle, left of the ctenidium ( Fig. 5I). A large efferent branchial vein runs dorsally along the axis of the ctenidium, while the efferent branchial vein is also sizeable and is contained within the gill axis ( Fig. 5E). A large blood sinus is present beneath the left side of ctenidium, filled by bluish haemocoel.

The left kidney (nephridium) is situated immediately posterior of the ctenidium on the left side of the visceral mass ( Fig. 6A–C, E), separated into two lobes that envelop the pericardium ventrally. Externally, the posterior lobe appears larger ( Fig. 5I). The pericardium ( Figs 5F, 6A–C) is sizeable, containing a monotocardian heart where the auricle is slightly anterior to the ventricle. The auricle occupies 2.28% of the body volume in the specimen examined ( Table 1). The anterior aorta runs towards the head alongside ventral organs, while being embedded in the hypertrophied oesophageal gland; the posterior aorta branches into numerous blood vessels on the posterior mantle roof to feed organs in the visceral mass ( Fig. 5D). The ventricle occupies 2.21% of the body volume in the specimen examined ( Table 1), it is globular and exceptionally thickened by musculature, with muscle bundles running in irregular directions across the lumen ( Fig. 5G).

The digestive system posterior to the ventral mouth opening is overall characterized by a hypertrophied oesophageal gland occupying 13.87% of the body volume ( Table 1) in the specimen examined, in conjunction with a reduced size of the alimentary canal for its body size ( Fig. 5E). The radula ribbon is supported by a single pair of prominent cartilages that come into contact anteriorly ( Fig. 6F) with a pair of salivary glands dorsally. The buccal mass leads posteriorly to a simple, narrow oesophagus. The anterior portion of the oesophagus branches into the dorsal aspect of a fused, blind-ended, and hypertrophied oesophageal gland ( Fig. 6D). This oesophageal gland occupies a large portion of the ventral cavity of the animal, filling all available space and completely engulfing the alimentary canal and the anterior aorta, and comprises a network of semi-enclosed tubes within a glandular mass.

Beyond the oesophageal gland, the oesophagus runs further posteriorly to reach the stomach, embedded within the digestive gland at the postero-right side of the visceral mass ( Figs 5A, 6A–D). The stomach is clearly enlarged in comparison to the oesophagus or the intestine and is connected to the digestive gland occupying the posterior-most part of the visceral mass via several tubular openings ( Fig. 6C). The digestive gland is dark in colour and situated posterodorsal to the gonad, occupying 4.52% of the body volume in the specimen measured. The intestine is very narrow for the size of the animal, emerging from the ventral right side of the stomach and runs anteriorly to perform two loops while embedded in the oesophageal gland, before turning back to reach slightly left of the stomach, penetrating the digestive gland to emerge on the mantle roof dorsal to the stomach as the rectum ( Figs 5A, 6D). The rectum runs along the mantle roof anteriorly for about one-third of the mantle cavity, then ends in a simple anus, where faeces are expelled. The stomach and the hindgut were filled by a mixture of black, sediment-like matter and fine sulphide mineral particles, whereas the oesophagus was empty.

The reproductive system occupies a prominent part of the visceral mass on the right side, visible on the roof of the mantle cavity immediately anterior to the digestive gland and stomach ( Figs 5A, 6A, B). In both sexes, the gonad is voluminous and located directly adjacent to the right shell muscle, coiling slightly into the spire whorl posteriorly. In males, a prominent, lobulated prostate in the form of complexly folded ducts is present dorsal to the left part of the testis, which opens in a simple gonopore on the right side of the mantle roof. In females, the gonopore also opens in the same location. The ovary occupied 5.60% of the body volume in the female specimen measured from μ-CT data ( Table 1).

Distribution: Known only from Hydra and Falkor EMARK vent fields on the MAR between depths of ~3700 and 3950 m ( Fig. 1). It forms dense colonies directly on sulphide chimneys, and temperatures recorded from Hydra were between 6.28°C ± 2.26°C (2.83°C–14.28°C, 9 min recording with temperature probe of HOV Nautile, ambient temperature 2.5°C). It occurs close to Rimicaris exoculata Williams & Rona, 1986 shrimp aggregates, in a very similar habitat.

Remarks: Adults of Peltospira gargantua cannot be confused with any other known members of the genus owing to its very large size, averaging at ~ 20 mm SD, with the largest specimens reaching 25 mm; in comparison to P. smaragdina , P. delicata , and P. operculata that reach only 12 mm and P. lamellifera that barely exceeds 2 mm ( McLean 1989, Warén and Bouchet 2001). The shell of P. delicata and P. lamellifera cannot be confused with that of P. gargantua because they are not globose but more flattened haliotiform, with disjunct coiling ( McLean 1989, Warén and Bouchet 1989). Unlike P. lamellifera and P. operculata (Warén & Bouchet, 2001) , P. gargantua lacks an operculum. The shell of P. smaragdina is the most similar to P. gargantua when at a similar size, especially given that the development of axial sculpture is variable in both species, and both species lack an operculum (Warén and Bouchet 2001). The radula morphology and shell microstructure are also similar between these two species (Warén and Bouchet 2001, Kiel 2004). However, they are easily distinguished from external anatomy of the epipodium, where P. smaragdina possesses only ~10 small epipodial tentacles (Warén and Bouchet 2001), which is clearly different from P. gargantua , which has 24–28 thick, large, paddle-like epipodial tentacles in a single, continuous row. This arrangement of epipodial tentacles also serves to separate P. gargantua from P. operculata and P. delicata , which have multiple rows of densely packed epipodial tentacles ( Fretter 1989).

Furthermore, the condition of having such a hypertrophied oesophageal gland is not known from any other Peltospira species ( Fretter 1989) and is a clear signal for recognizing P. gargantua as a distinct species, although we note that the internal anatomy of P. smaragdina remains poorly known (Warén and Bouchet 2001) and the condition of the oesophageal gland in that species requires further studies. This feature, combined with its large size, means that P. gargantua superficially resembles giant, endosymbiotic peltospirids in genera Gigantopelta and Chrysomallon ( Chen et al. 2015a, c). The arrangement of epipodial tentacles and the radular characters would easily separate them upon closer examination, however. In addition, the lack of both a large operculum (as in Gigantopelta ) and dense sclerites on the foot (as in Chrysomallon ) also serves to distinguish P. gargantua from those two genera of giant peltospirids. The protoconch of P. gargantua is very similar in overall shape and sculpture to other Atlantic and Pacific Peltospira species ( Mullineaux et al. 1996, Warén and Bouchet 2001, Mills et al. 2009), although it appears to be slightly larger.