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Extinct even before scientific recognition: a remarkable radiation of helicinid snails (Helicinidae) on the Gambier Islands, French Polynesia

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Abstract

Recent literature abounds with reports of the decline and extinction of the endemic species of Achatinellidae and Partulidae in the Hawaiian and Society Islands, respectively, resulting from the introduction of the predatory snail Euglandina rosea. Here, we describe a previously unrecognised radiation of helicinid land snails from the Gambier Islands of French Polynesia, with up to seven species co-occurring in a single locality and up to eight species on a single island. This radiation had already become extinct (nine of ten species) several decades before the expansion of E. rosea in the Pacific, and even before the species were collected for scientific study. The Gambier Islands case study shows that massive extinctions of endemic land snails had already taken place in the nineteenth century, but have remained largely unrecognised and undocumented. Nine of the ten species are new to science and are described here almost entirely based on empty shells collected from the shell bank of the soil after the extinction had already taken place. This helicinid radiation alone increases the number of documented global mollusc extinctions by almost 2 %. Most of the species are minute and, at 1.5 mm, rank among the smallest, if not the smallest, species in the family. Several have apertural barriers and one has opercular apophyses—character states not previously documented in Pacific helicinids. Whereas the only surviving Gambier species belongs anatomically to the genus Sturanya, representative helicinid species from the Austral, Society and Cook Islands are not congeneric with it, and the generic name Nesiocina is here established for the latter taxa. It is hypothesised that the extinct Gambier species were also Nesiocina.

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Acknowledgments

The second author is most grateful to Bruno Schmidt of Rikitea, Mangareva, for his extensive help during the 1997 field work, and sharing his knowledge on the recent history of his native island. In the lab, Ahmed Abdou picked the snails from the residues as part of his master’s research project, and did the initial segregation of helicinid material to morphospecies. We also thank the curators and collection managers at BPBM (Robert Cowie, Regina Kawamoto, Carl Christensen), Museo Nacional de Ciencias Naturales, Madrid (Rafael Araujo) and Muséum d’Histoire Naturelle de Bordeaux (Laurent Charles) for access to material under their care and Richard C. Preece for providing specimens of Nesiocina hendersoni for comparison. He and an anonymous reviewer provided helpful comments on the manuscript. Vollrath Wiese, Haus der Natur—Cismar, Germany, is acknowledged for sharing his excellent photographic equipment. This paper was supported by the “Sixième Extinction” Grant from Agence Nationale de la Recherche (ANR) to Philippe Bouchet (Project “Losers”).

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Correspondence to Ira Richling.

Appendices

Appendix 1: Species descriptions

In the species descriptions that follow, the shape of the aperture and outer lip, as well as whorl counts, are described following Richling (2009, p. 260, Fig. 2b). Shell measurements are given in the following sequence: height/greatest diameter/minor diameter, as in Richling (2009, p. 260, Fig. 2a). Unlike most other snails, helicinids resorb major parts of the internal walls, with the remaining structure called an internal septum (Fig. 7).

To facilitate the recognition of multiple species co-occurrences, we have conserved in the lists of material examined the numbering system of the Mangarevan expedition collecting stations, which are referred to by their botany number ("Bot xxx"; e.g. Bot 142).

Abbreviations: BPBM Bernice Pauahi Bishop Museum, Honolulu, USA; MNHN Muséum National d’Histoire Naturelle, Paris, France; spm(s) specimen(s)

Nesiocina nom. nov.

Synonym: Discoidea Wagner 1905: p 413; invalid: junior homonym of Discoidea Agassiz 1834 (Echinodermata).

Type species: Helicina discoidea Pease 1868, by tautonymy.

Nomenclatural remarks: The name Discoidea was attributed by Agassiz to “Klein”, a pre-Linnean author, whose names are not available under the Code of Zoological Nomenclature. However, Gray (1825) had first made Klein’s name available with the spelling Discodea. Technically, Discoidea of Agassiz is an incorrect subsequent spelling of Discodea Gray 1825, and thus not available. However, Discoidea Agassiz 1834 has been treated in the literature (e.g., Wagner and Durham 1964) as an unjustified emendation (and thus an available name), and it is in current use in the paleontological literature. Discoidea Wagner 1905 is thus an invalid name, and we here establish the replacement name Nesiocina nom. nov. under Article 67.8 of the Code, such that the type species of Discoidea Wagner 1905 is also the type species of Nesiocina.

Etymology: The name Nesiocina is a contraction of nesios (Greek noun for island) and the suffix -cina as in Helicina, the type genus of the family Helicinidae.

Species with parietal apertural structures

Nesiocina trilamellata n. sp.

Type material: Holotype MNHN 24980, 2.6/2.8/2.3 mm (collected by Bouchet, 16 September 1997) (Figs. 1a, 6).

Fig. 6
figure 6

Nesiocina trilamellata n. sp., holotype, Gambier Islands: Taravai, MNHN 24980: height: 2.6 mm, arrow indicates the end of the internal septum; scale bar 1 mm

Paratypes: MNHN 24981, 6 spms, same collection data as holotype.

Type locality: French Polynesia, Gambier Islands: Taravai, east side of village, 23°08.6′S, 135°01.8′W, altitude 5 m.

Material examined: MNHN (coll. Bouchet): Mangareva: Ganhutu, lawn in coconut plantation, 23°04.6′S, 134°56.6′W, 17. & 22.9.1997 (about 517 spms); Mangareva: Gatavake, open, disturbed ground, 23°06.95′S, 134°58.75′W, 17. & 22.9.1997 (34 spms); Aukena: Pointe Mata Kuiti, light soil mixed with white marine sand, 23°08.05′S, 134°55.1′W, altitude 2–3 m, 18.9.1997 (1 spm); Aukena: central isthmus, bare sandy ground, 23°07.6′S, 134°54.0′W, altitude 7–8 m, 18.9.1997 (60 spms); Aukena: north-east part of island, 23°07.5′S, 134°53.9′W, 18.9.1997 (1 spm); Taravai: east side of village, fallow land (ancient gardens) and open ground, 23°08.6′S, 135°01.8′W, altitude 5 m, 16.9.1997 (7 spms); Akamaru: small deposit of drift material on sandy ground, 23°10.6′S, 134°54.9′W, 19.9.1997 (1 spm).

BPBM: Mangareva: Ganhutu, inland 150 ft., 6 ft., flat (Bot. 142), 3.6.1934, coll. Kondo & Cooke (BPBM 138961: 2 + 1 broken); Ganhutu, Northeast end of island, inland 100–200 yards, on open ground (=Bot. 142) (Bot. 277), 26.6.1934, coll. Anderson (BPBM 138996d: 5 spms, BPBM 138997b: 1 spm, BPBM 138998a : 10 spms); Mangareva: Ngahutu, Northwest valley, inland 5–200 ft., 3–25 ft., flat, medium, under stones & on dead leaves (Bot. 139), 3.6.1934, coll. Kondo & Cooke, Jr. (BPBM 141304: 1 spm); Mangareva: Ngahutu, Northwest valley, inland 5–200 ft., 3–25 ft., flat, medium, under stones & on dead leaves (Bot. 139), 3.6.1934, coll. Kondo & Cooke, Jr. (BPBM 141705: 1 spm); Mangareva: Northeast of Vaituatai Bay, inland 2–6 ft., 1–3 ft., flat (Bot. 197), 9.6.1934, coll. Anderson & Cooke, Jr. (BPBM 139028b: 1 spm juvenile); Aukena: 1st cove East of gap (Bot. 102), 28.5.1934, coll. Anderson (BPBM 138786b: 5 spms juvenile; BPBM 138789: 16 spms, BPBM 138790: 6 spms juvenile); Aukena: 2nd cove East of gap (Bot. 103), 28.5.1934, coll. Anderson (BPBM 138813: 1 spm); Aukena: inland 100 ft., on trail near gap, 6 ft., flat (Bot. 88), 28.5.1934, coll. Anderson & Cooke (BPBM 138741b: 1 spm juvenile, BPBM 138743b: 1 spm, BPBM 138745a: 7 spms [3 juvenile], BPBM 138746: 52 spms).

Diagnostic features: Small, shiny, conical-shaped, whitish-yellowish shell with one strong and two weaker parietal lamellae inside the aperture.

Description: Shell (Figs. 1a, 6, 7): Conical with a well rounded periphery, moderately thick, small and shiny. Best preserved specimens whitish-yellowish without any pattern, slightly translucent. Shell surface rather smooth and shiny, with a pattern of oblique diverging lines. Embryonic shell about 1 whorl and 550 μm in diameter; subsequent 3.1 (holotype) whorls nearly straight and evenly rounded at the periphery. Whorls regularly expanding, evenly descending except for the ultimate part before the adult aperture which descends more strongly; forming a straight spire with a pointed apex. Suture only slightly impressed. Aperture oblique and considerably curved backwards, last whorl usually inserting a little below the periphery. Parietal side of the aperture with a strong lamella a little above the middle which extends for about 0.5 whorls nearly as long as the internal septum and two weaker lamellae at equal space below the upper strong lamella reaching nearly half of its length and ending a little earlier. Inside of the base of aperture smooth without ridges. Outer lip not thickened, but slightly expanded and roundly reflected. Lower part of the lip with a straight transition to the columella, which is remarkably bent forwards in its lower part. Basal callus whitish and moderately developed, rather smooth.

Fig. 7
figure 7

Internal structures of Nesiocina trilamellata n. sp., specimen from Ganhutu, MNHN: greatest diameter: 2.7 mm; scale bar 1 mm

Internal shell structures: Internal septum expanding for nearly 0.6 whorls. See Figs. 6 and 7.

Operculum: None preserved in the present material.

Distribution: Nesiocina trilamellata was fairly widespread on the Gambier Islands occurring on Mangareva, Aukena, Taravai and Akamaru.

Remarks: Except for very few shells most of the material is rather old and faded, suggesting that extinction took place probably several decades ago. Nesiocina trilamellata is unique with its three parietal lamellae and cannot be confused with any other helicinid species known.

Etymology: The name refers to the three characteristic lamellae inside the aperture.

Nesiocina unilamellata n. sp.

Type material: Holotype MNHN 24982, 1.9/2.2/1.9 mm (collected by Bouchet, 16 September 1997) (Figs. 1b, 8).

Fig. 8
figure 8

Nesiocina unilamellata n. sp., holotype, Gambier Islands: Taravai, MNHN 24982: height: 1.9 mm, arrow indicates the end of the internal septum; scale bar 1 mm

Paratypes: MNHN 24983, 20 spms, same collection data as holotype.

Type locality: French Polynesia, Gambier Islands: Taravai, east side of village, 23°08.6′S, 135°01.8′W, altitude 5 m.

Material examined: MNHN (coll. Bouchet): Taravai: east side of village, fallow land (ancient gardens) and open ground, 23°08.6′S, 135°01.8′W, altitude 5 m, 16.9.1997 (21 spms); Akamaru: coastal fallow land near former village, 23°10.7′S, 134°54.7′W, altitude 2 m, 19.9.1997 (1 spm); Akamaru: small deposit of drift material on sandy ground, 23°10.6′S, 134°54.9′W, 19.9.1997 (3 spms).

BPBM: Akamaru: north-west side, flat, medium, inland 20–200 ft.; elevation 6 ft., on cliffs, fossil (Bot. 97), 28.5.1934, coll. Anderson (BPBM 138846a: 2 spms).

Diagnostic features: Minute, rather solid-shelled, whitish shell with a single long lamella in the middle of the parietal side of the aperture.

Description: Shell (Figs. 1b, 8, 9): Conical-globose with a very slightly shouldered but otherwise rounded periphery, rather thick and size. Faded specimens uniformly whitish and obviously without any pattern. Shell surface smooth and probably originally shiny. Embryonic shell about 1 whorl and 520 μm in diameter; subsequent 2.75 (holotype) whorls slightly convex. Whorls regularly expanding, evenly descending and forming a rather straight but slightly stepped spire with a blunt apex. Suture very slightly impressed. Aperture oblique and only slightly bent backwards, last whorl usually inserting a little below the periphery. Parietal side of the aperture with a single central lamella which extends for about 0.4 whorl and ends directly in a line with the aperture. Inside of the base of the aperture smooth without ridges. Outer lip not differentiated from the whorl, neither noticeably thickened nor reflected. Lower part of the lip protruded towards the transition to the columella, which is remarkably bent forwards in its lower part. Basal callus whitish and regularly developed, slightly granulated.

Fig. 9
figure 9

Internal structures of Nesiocina unilamellata n. sp., paratype, Gambier Islands: Taravai, MNHN 24983: greatest diameter: 2.3 mm; scale bar 1 mm

Internal shell structures: Internal septum expanding for about 0.5 whorl. See Figs. 8 and 9.

Operculum: None preserved in the present material.

Distribution: Nesiocina unilamellata was only found on the islands Taravai and Akamaru.

Remarks: In shape and size N. unilamellata is nearly identical with N. superoperculata except for the clearly differentiating apertural structures.

Etymology: The name refers to the characteristic single parietal lamella inside the aperture.

Nesiocina gambierensis n. sp.

Type material: Holotype MNHN 24984, 1.5/1.8/1.5 mm (collected by Bouchet, 16 September 1997) (Fig. 10).

Fig. 10
figure 10

Nesiocina gambierensis n. sp., holotype, Gambier Islands: Taravai, MNHN 24984: height: 1.5 mm, arrow indicates the end of the internal septum; scale bar 1 mm

Paratypes: MNHN 24985, 5 spms, same collection data as holotype.

Type locality: French Polynesia, Gambier Islands: Taravai, east side of village, 23°08.6′S, 135°01.8′W, altitude 5 m.

Material examined: MNHN (coll. Bouchet): Mangareva: Gatavake, open, disturbed ground, 23°06.95′S, 134°58.75′W, 17. & 22.9.1997 (2 spms); Taravai: east side of village, fallow land (ancient gardens) and open ground, 23°08.6′S, 135°01.8′W, altitude 5 m, 16.9.1997 (6 spms); Akamaru: small deposit of drift material on sandy ground, 23°10.6′S, 134°54.9′W, 19.9.1997 (1 spm).

BPBM: Akamaru: north-west side, flat, medium, inland 20–200 ft.; elevation 6 ft., on cliffs, fossil (Bot. 97), 28.5.1934, coll. Anderson (BPBM 138846b: 2 spms).

Diagnostic features: Minute, solid shell with two short equally spaced lamellae at the parietal side inside the aperture.

Description: Shell (Figs. 1c, 10): Conical-globose with a well rounded periphery, rather thick and size. Faded specimens uniformly whitish. Shell surface eroded and in old specimens rather dull. Embryonic shell about 1 whorl and 460 μm in diameter; subsequent 2.4 (holotype) whorls convex and evenly rounded at the periphery. Whorls regularly expanding, evenly descending and forming a slightly convex spire with a blunt apex. Suture only slightly impressed. Aperture oblique and only slightly bent backwards, last whorl inserting a little below the periphery. Parietal side of the aperture with two rather short equally spaced lamellae that extend for about 1/8 whorl and end directly in a line with the aperture. Inside of the base of the aperture smooth without ridges. Outer lip almost undifferentiated from the whorl, but final part of the last whorl continuously slightly expanding in diameter. Basal part of the lip slightly protruded towards the transition to the columella, which is slightly bent forwards in its lower part. Basal callus well developed and rather smooth.

Internal shell structures: Not visible through the intact shell.

Operculum: None preserved in the present material.

Distribution: Only eleven shells were found, three from Akamaru, two from Mangareva and six from Taravai.

Remarks: The shells are very poorly preserved, faded and eroded. N. gambierensis is the smallest of all known Gambier Island helicinids and clearly among the smallest, if not the smallest, world-wide.

Etymology: The name refers to Gambier Islands to which the species is endemic.

Nesiocina superoperculata n. sp.

Type material: Holotype MNHN 24986, 1.9/2.3/2.0 mm (collected by Bouchet, September 1997) (Figs. 1d, 11).

Fig. 11
figure 11

Nesiocina superoperculata n. sp., holotype, Gambier Islands: Mangareva, Gatavake, MNHN 24986: height: 1.9 mm. The arrow indicates the end of the internal septum (based on specimens from Ganhutu); scale bar 1 mm

Paratypes: MNHN 24987, 50 spms, same collection data as holotype.

Type locality: French Polynesia, Gambier Islands: Mangareva, Gatavake, 23°06.95′S, 134°58.75′W.

Material examined: MNHN (coll. Bouchet): Mangareva: Ganhutu, lawn in coconut plantation, 23°04.6′S, 134°56.6′W, 17. & 22.9.1997 (about 690 spms including some juveniles of uncertain determination); Mangareva: Gatavake, open, disturbed ground, 23°06.95′S, 134°58.75′W, 17. & 22.9.1997 (51 spms); Aukena: Pointe Mata Kuiti, light soil mixed with white marine sand, 23°08.05′S, 134°55.1′W, altitude 2-3 m, 18.9.1997 (97 spms); Aukena: central isthmus, bare sandy ground, 23°07.6′S, 134°54.0′W, altitude 7–8 m, 18.9.1997 (61 spms); Aukena: SE of Pointe Mata Kuiti, little humid cliff, sandy ground, 23°08.1′S, 134°55.0′W, altitude 5 m, 18.9.1997 (1 spm); Taravai: east side of village, fallow land (ancient gardens) and open ground, 23°08.6′S, 135°01.8′W, altitude 5 m, 16.9.1997 (8 spms); Akamaru: small deposit of drift material on sandy ground, 23°10.6′S, 134°54.9′W, 19.9.1997 (3 spms).

BPBM: Mangareva: Ganhutu, Northeast end of island, inland 100-200 yards, on open ground (= Bot. 142) (Bot. 277), 26.6.1934, coll. Anderson (BPBM 138996a: 10 spms, BPBM 138997a: 1 spm); Mangareva: North end of Rikitea, inland 6 ft., 2 ft., flat (Bot. 189), 8.6.1934, coll. Kondo & Cooke, Jr. (BPBM 141701b: 1 spm, BPBM 141704: 1 spm); Mangareva: North part of Rikitea, 6 ft. (Bot. 187), 7.6.1934, coll. Kondo (BPBM 141671b: 3 spms); Aukena: inland 100 ft., on trail near gap, 6 ft., flat (Bot. 88), 28.5.1934, coll. Anderson & Cooke (BPBM 138741c: 9 spms, BPBM 138742d: 1 spm, BPBM 138744b: 1 spm, BPBM 138745b: 110 spms); Aukena: 1st cove East of gap (Bot. 102), 28.5.1934, coll. Anderson (BPBM 138786c: 68 spms); Akamaru: north-west side, flat, medium, inland 20–200 ft.; elevation 6 ft., on cliffs, fossil (Bot. 97), 28.5.1934, coll. Anderson (BPBM 138846c: 1 spm).

Diagnostic features: Minute, shiny, rather solid, yellowish to whitish shell with a knob-like structure on the parietal side just before the aperture, operculum possessing an outer duplication with hole corresponding to the knob structure.

Description: Shell (Figs. 1d, 11): Conical-globose with a very slightly shouldered but otherwise rounded periphery, rather thick, of minute size and shiny. Fresh specimens uniformly whitish to light yellowish without any pattern. Shell surface smooth and shiny. Embryonic shell about 1 whorl and 520 μm in diameter; subsequent 3.0 (holotype) whorls slightly convex. Whorls regularly expanding, evenly descending and forming a rather straight but slightly stepped spire with a blunt apex. Suture very slightly impressed. Aperture oblique and only slightly bent backwards, last whorl usually inserting clearly below the periphery. Parietal side of the aperture just in front of position of a closed operculum with a more or less prominently developed knob which is part of the basal callus. Inside of the base of the aperture smooth without ridges. Outer lip only slightly thickened and weakly reflected. Lower part of the lip protruded towards the transition to the columella, which is remarkably bent forwards in its lower part. Basal callus whitish and well developed, apart from the knob structure thickened near the insertion of the body whorl, slightly granulated.

Internal shell structures: Internal septum expanding for nearly 0.4 whorls. See Fig. 11.

Operculum: Comparably broad with a nearly central nucleus and a strongly curved S-line, outer side with a singular rounded protruding duplication with a tooth-like structure in its upper part thus leaving a hole which corresponds to the knob-like structure on the shell when the animal would be extended and crawling (Fig. 12a).

Fig. 12
figure 12

Preserved opercula, left outer side, right inner side. a Nesiocina superoperculata n. sp., Gambier Islands, Aukena, SE de la Pointe Mata Kuiti, MNHN, below upper side; b Nesiocina abdoui n. sp., Gambier Islands, Mangareva, Ganhutu, MNHN; c Nesiocina pazi, Aukena, central isthmus, MNHN; d Sturanya makaroaensis n. sp., holotype; scale bars 0.4 mm (a), 0.5 mm (bd)

Distribution: Nesiocina superoperculata was collected on Mangareva, Aukena, Taravai and Akamaru.

Etymology: The name was chosen on account of the outstanding and unusual external structures on the operculum.

Species without parietal apertural structures

Nesiocina abdoui n. sp.

Type material: Holotype MNHN 24988, 2.6/2.3/2.1 mm (collected by Bouchet, 17 and 22 September 1997) (Figs. 2a, 13).

Fig. 13
figure 13

Nesiocina abdoui n. sp., holotype, Gambier Islands: Mangareva, Gatavake, MNHN 24988: height: 2.6 mm. The arrow indicates the end of the internal septum (based on specimens from Ganhutu); scale bar 1 mm

Paratypes: MNHN 24989, 4 spms, same collection data as holotype.

Type locality: French Polynesia, Gambier Islands: Mangareva: Gatavake, 23°06.95′S, 134°58.75′W.

Material examined: MNHN (coll. Bouchet): Mangareva: Ganhutu, lawn in coconut plantation, 23°04.6′S, 134°56.6′W, 17. & 22.9.1997 (46 spms); Mangareva: Gatavake, open, disturbed ground, 23°06.95′S, 134°58.75′W, 17. & 22.9.1997 (5 spms); Aukena: Pointe Mata Kuiti, light soil mixed with white marine sand, 23°08.05′S, 134°55.1′W, altitude 2–3 m, 18.9.1997 (1 spm + 1 cf.); Aukena: central isthmus, bare sandy ground, 23°07.6′S, 134°54.0′W, altitude 7–8 m, 18.9.1997 (14 spms); Taravai: east side of village, fallow land (ancient gardens) and open ground, 23°08.6′S, 135°01.8′W, altitude 5 m, 16.9.1997 (4 spms); Akamaru: small deposit of drift material on sandy ground, 23°10.6′S, 134°54.9′W, 19.9.1997 (2 spms); Agakauitai: Pointe Nord, degraded secondary forest, humid rocks, 23°09.4′S, 134°02.1W, altitude 5 m, 15.9.1997 (1 spm).

BPBM: Mangareva: Ganhutu, inland 150 ft., 6 ft., flat (Bot. 142), 3.6.1934, coll. Kondo & Cooke (BPBM 138960b: 1 spm cf.); Mangareva: Ganhutu, Northeast end of island, inland 100–200 yards, on open ground (=Bot. 142) (Bot. 277), 26.6.1934, coll. Anderson (BPBM 138996e: 1 spm [freak?], BPBM 138997c: 1 spm); Mangareva: North part of Rikitea, 6 ft. (Bot. 187), 7.6.1934, coll. Kondo (BPBM 141673b: 2 spm); Mangareva: North end of Rikitea, inland 6 ft., 2 ft., flat (Bot. 189), 8.6.1934, coll. Kondo & Cooke, Jr. (BPBM 141702: 2 spms); Mangareva: Northeast of Vaituatai Bay, inland 2–6 ft., 1–3 ft., flat (Bot. 197), 9.6.1934, coll. Anderson & Cooke, Jr. (BPBM 139029b: 2 spms); Aukena: inland 100 ft., on trail near gap, 6 ft., flat (Bot. 88), 28.5.1934, coll. Anderson & Cooke (BPBM 138743a: 29 spms, BPBM 138744a: 2 spms, BPBM 138745f: 9 spms); Aukena: 1st cove East of gap (Bot. 102), 28.5.1934, coll. Anderson (BPBM 138786d: 4 spms, BPBM 138787: 9 spms including 1 freaky); Aukena: inland 20 ft., by side of trail, 10 ft., flat (Bot. 82), 28.5.1934, coll. Anderson & Cooke (BPBM 138695b: 1 spm broken cf); Taravai: along trail through village, inland 200 ft., flat, dry, on open ground (Bot. 123), 1.6.1934, coll. Anderson (BPBM 138881: 5 spms); Taravai: in sand, sweeping (Bot. 126), 1.6.1934, coll. Anderson (BPBM 138893a: 3 spms); Agakauitai: Northwest side of island, back of beach, inland 100+ ft., hillside, medium, flat, on sandy soil (Bot. 195), 8.6.1934, coll. Anderson (BPBM 138924: 78 spms); Akamaru: flat, elevation 3–5 ft., in wave cutting, fossil (Bot. 107), 30.5.1934, coll. Anderson & Cooke, Jr. (BPBM 138865c: 1 spm); Akamaru: north-west side, flat, medium, inland 20–200 ft.; elevation 6 ft., on cliffs, fossil (Bot. 97), 28.5.1934, coll. Anderson (BPBM 138845a: 2 spms).

Diagnostic features: Minute, highly elevated, white and glossy, rather solid shell with a remarkably small aperture with both a slight ridge along the parietal side and inside at the base parallel to the lower lip.

Description: Shell (Figs. 2a, 13): Highly elevated with a well rounded periphery, rather thick, small and glossy. Fresh specimens uniformly white to slightly yellowish, slightly translucent. Shell surface smooth and glossy. Embryonic shell about 1 whorl and 540 μm in diameter; subsequent 3.3 (holotype) whorls slightly convex and more strongly rounded towards the base of the periphery. Whorls slowly expanding, rapidly descending and forming a convex spire with a blunt apex. Suture only slightly impressed. Aperture oblique and strongly curved backwards, comparably small, last whorl inserting decidedly below the periphery. Aperture with a ridge along the line from the insertion of the body whorl towards the base, which is fading in the upper part. Inside at the base of the aperture another well developed internal lamella starting from the columella and slightly diverging from the outer lip. Outer lip in all specimens studied not further differentiated, neither expanded nor reflected. Lower part of the lip with a rather straight transition to the short and hardly bent columellar area. Basal callus whitish and more or less strongly developed, towards its centre often granulated.

Internal shell structures: Internal septum expanding for nearly 0.4 whorls. See Fig. 13.

Operculum: Simple, comparably narrow and evenly shaped, rather straight along the parietal side, S-line only slightly bent, outer surface slightly granular (Fig. 12b).

Distribution: Nesiocina abdoui was the only helicinid collected during the 1997 field work on Agakauitai. It was also found on Mangareva, Aukena, Taravai and Akamaru.

Remarks: The characters of the two ridges inside the aperture are similar in this and the following species and clearly differentiate them from other Gambier Island helicinids. For differences from N. mangarevae, see under that species.

Etymology: The species is dedicated to Ahmed Abdou who, as a master student, did much of the preliminary work for the present paper and co-authored the research papers that form the baseline of our knowledge on the land snail fauna of the Gambier Islands.

Nesiocina mangarevae n. sp.

Type material: Holotype MNHN 24990, 2.3/2.4/2.1 mm (collected by Bouchet, 17 and 22 September 1997) (Figs. 2b, 14).

Fig. 14
figure 14

Nesiocina mangarevae n. sp., holotype, Gambier Islands: Mangareva, Gatavake, MNHN 24990: height: 2.3 mm; scale bar 1 mm

Paratypes: MNHN 24991, 2 spms, same collection data as holotype.

Type locality: French Polynesia, Gambier Islands: Mangareva: Gatavake, 23°06.95′S, 134°58.75′W.

Material examined: MNHN (coll. Bouchet): Mangareva: Ganhutu, lawn in coconut plantation, 23°04.6′S, 134°56.6′W, 17. & 22.9.1997 (9 spms); Mangareva: Gatavake, open, disturbed ground, 23°06.95′S, 134°58.75′W, 17. & 22.9.1997 (3 spms).

BPBM: Taravai: in sand, sweeping (Bot. 126), 1.6.1934, coll. Anderson (BPBM 138893b: 3 spms).

Diagnostic features: Minute, moderately elevated, whitish-yellowish and glossy, rather solid shell with a straight and slightly stepped spire, a remarkably small aperture with both a slight ridge along the parietal side and inside at the base parallel to the lower lip.

Description: Shell (Figs. 2b, 14): Moderately elevated with a well rounded periphery, rather thick, small and glossy. Fresh specimens uniformly whitish-yellowish. Shell surface smooth and glossy. Embryonic shell about 1 whorl and 500 μm in diameter; subsequent 3.0 (holotype) whorls convex and equally rounded. Whorls regularly expanding and descending and forming a straight spire with a rather blunt apex. Suture remarkably impressed, producing a slightly stepped appearance. Aperture oblique and strongly curved backwards, comparably small, last whorl inserting decidedly below the periphery. Aperture with a ridge along the line from the insertion of the body whorl towards the base, which is fading in the upper part. Inside at the base of the aperture another well developed internal lamella starting from the columella and slightly diverging from the outer lip. Outer lip slightly expanded and very shortly reflected. Lower part of the lip with a rather straight transition to the short and hardly bent columellar area. Basal callus whitish and more or less strongly developed, towards its centre often granulated.

Internal shell structures: Not visible through the intact shell.

Operculum: None preserved in the present material.

Distribution: Nesiocina mangarevae was only found at two localities on Mangareva and one on Taravai.

Remarks: Nesiocina mangarevae most closely resembles Nesiocina abdoui, but it is less elevated, has a stronger impressed suture and thus a stepped general appearance. Its co-occurrence with the former species at the two localities known for Nesiocina mangarevae and the absence of intermediate forms support its recognition as a separate species.

The alternative interpretation would involve the possibility of sexual dimorphism. Richling (2004) showed in detailed studies on Costa Rican helicinids that in some species males are significantly smaller than females with only up to 2/3 of the shell volume, e. g. in Helicina beatrix Angas 1879 or H. talamancensis. But contrary to the presently observed differences, in these species the relation of height to diameter and general shape proved to be rather constant. Therefore, and in face of absence of anatomical material, the two forms are described as two different species. The great differences in abundance of the two species provide further evidence, although the sex ratio is often not approximately 1:1.

The great similarity in apertural features of N. abdoui and N. mangarevae points to a close relationship.

Etymology: The name refers to the geographical origin of the holotype, used in the genitive.

Nesiocina grohi n. sp.

Type material: Holotype MNHN 24992, 2.7/2.8/2.3 mm (coll. Bouchet, 17and 22 September 1997) (Figs. 2c, 15).

Fig. 15
figure 15

Nesiocina grohi, holotype, Gambier Islands: Mangareva: Gatavake, MNHN 24992: height: 2.7 mm. The arrow indicates the end of the internal septum (based on specimens from Ganhutu); scale bar 1 mm

Paratypes: MNHN 24993: 14 spms, same collection data as holotype.

Type locality: French Polynesia, Gambier Islands: Mangareva: Gatavake, 23°06.95′S, 134°58.75′W.

Material examined: MNHN (coll. Bouchet): Mangareva: Ganhutu, lawn in coconut plantation, 23°04.6′S, 134°56.6′W, 17. & 22.9.1997 (96 spms); Mangareva: Gatavake, open, disturbed ground, 23°06.95′S, 134°58.75′W, 17. & 22.9.1997 (15 spms); Taravai: east side of village, fallow land (ancient gardens) and open ground, 23°08.6′S, 135°01.8′W, altitude 5 m, 16.9.1997 (1 spm); Akamaru: small deposit of drift material on sandy ground, 23°10.6′S, 134°54.9′W, 19.9.1997 (1 spm).

BPBM: Mangareva: Ganhutu, inland 150 ft., 6 ft., flat (Bot. 142), 3.6.1934, coll. Kondo & Cooke (BPBM 138960c: 1 spm); Mangareva: Ganhutu, Northeast end of island, inland 100–200 yards, on open ground (=Bot. 142) (Bot. 277), 26.6.1934, coll. Anderson (BPBM 138996b: 7 spm); Mangareva: North part of Rikitea, 6 ft. (Bot. 187), 7.6.1934, coll. Kondo (BPBM 141673a: 1 spm); Mangareva: North end of Rikitea, inland 6 ft., 2 ft., flat (Bot. 189), 8.6.1934, coll. Kondo & Cooke, Jr. (BPBM 141703: 1 spm); Mangareva: Northeast of Vaituatai Bay, inland 2–6 ft., 1–3 ft., flat (Bot. 197), 9.6.1934, coll. Anderson & Cooke, Jr. (BPBM 139029a: 3 spms); Aukena: inland 100 ft., on trail near gap, 6 ft., flat (Bot. 88), 28.5.1934, coll. Anderson & Cooke (BPBM 138742c: 1 spm); Akamaru: north-west side, flat, medium, inland 20–200 ft.; elevation 6 ft., on cliffs, fossil (Bot. 97), 28.5.1934, coll. Anderson (BPBM 138846d: 1 spm juvenile).

Diagnostic features: Small, highly elevated, shiny, rather solid, yellowish-whitish shell with prominent narrowly spaced rounded axial ribs continuing down to the basal callus, aperture simple with smooth continuation at columellar side and base, but remarkably curved backwards.

Description: Shell (Figs. 2c, 15): Highly elevated with a well rounded periphery, rather thick and small. Fresh specimens whitish, only towards the suture yellowish, slightly translucent. Surface sculptured with prominent narrowly spaced rounded axial ribs parallel to the growth lines that continue down to the basal callus, otherwise smooth and shiny. Embryonic shell about 1 whorl and 520 μm in diameter; subsequent 3.25 (holotype) whorls convex and evenly rounded at the periphery. Whorls regularly expanding, rapidly descending and forming a straight spire with a pointed apex. Suture remarkably impressed. Aperture oblique and remarkably curved backwards in its middle portion, whorls inserting distinctly below the periphery. Inside the aperture no protruding structures with a smooth continuation towards the columellar side and the base. Outer lip expanded and roundly reflected. Lower part of the lip only slightly protruding towards the transition to the columella which is slightly bent forwards. Basal callus whitish and rather weakly developed, towards its centre often granulated.

Internal shell structures: Internal septum expanding for nearly 0.6 whorls. See Fig. 15.

Operculum: None preserved in the present material.

Distribution: The species was collected at various localities on Mangareva and at single sites on Aukena, Taravai and Akamaru.

Remarks: Nesiocina grohi is closely related to N. pauciplicata, sharing the possession of axial folds on the shell as well as the absence of any apertural ridges or lamellae. N. grohi differs by the more highly elevated spire together with a more stepped appearance and more narrowly standing axial ribs which continue down to the basal callus instead of vanishing at and below the periphery.

Etymology: The species is dedicated to our friend and colleague Klaus Groh. The first author especially wants to acknowledge his friendly, excellent and inspiring collaboration during recent years.

Nesiocina pauciplicata n. sp.

Type material: Holotype MNHN 24994, 2.3/2.5/2.1 mm (coll. Bouchet, 17 and 22 September 1997) (Figs. 2d, 16).

Fig. 16
figure 16

Nesiocina pauciplicata n. sp., holotype, Gambier Islands: Mangareva: Ganhutu, MNHN 24994: height: 2.3 mm. The arrow indicates the end of the internal septum (based on paratypes); scale bar 1 mm

Paratypes: MNHN 24995: about 1000 spms, same collection data as holotype.

Type locality: French Polynesia, Gambier Islands: Mangareva: Ganhutu, lawn in coconut plantation, 23°04.6′S, 134°56.6′W.

Material examined: MNHN (coll. Bouchet): Mangareva: Ganhutu, lawn in coconut plantation, 23°04.6′S, 134°56.6′W, 17. & 22.9.1997 (about 1000 spms); Aukena: Pointe Mata Kuiti, light soil mixed with white marine sand, 23°08.05′S, 134°55.1′W, altitude 2–3 m, 18.9.1997 (1 spm); Aukena: central isthmus, bare sandy ground, 23°07.6′S, 134°54.0′W, altitude 7–8 m, 18.9.1997 (2 spms).

BPBM: Mangareva: Ganhutu, inland 150 ft., 6 ft., flat (Bot. 142), 3.6.1934, coll. Kondo & Cooke (BPBM 138960a: 2 spms); Mangareva: Ganhutu, Northeast end of island, inland 100–200 yards, on open ground (=Bot. 142) (Bot. 277), 26.6.1934, coll. Anderson (BPBM 138996c: 44 spms, BPBM 138998b: 1 spm); Mangareva: Ganhutu, W. Bay, flat, damp, under stones (Bot. 106), 29.5.1934, coll. Kondo (BPBM 140996: 1 spm); Aukena: inland 100 ft., on trail near gap, 6 ft., flat (Bot. 88), 28.5.1934, coll. Anderson & Cooke (BPBM 138742b: 19 spms, BPBM 138745c: 6 spms); Aukena: inland 100 ft., on trail near gap, 6 ft., flat (Bot. 88), 28.5.1934, coll. Anderson & Cooke (BPBM 138785b: 1 spm); Aukena: 2nd cove East of gap (Bot. 103), 28.5.1934, coll. Anderson (BPBM 138812b: 1 spm).

Diagnostic features: Small, moderately elevated, shiny, rather solid, yellowish-whitish shell with prominent widely spaced rounded axial folds on the upper part of the whorls, aperture simple with smooth continuation at columellar side and base, but remarkably curved backwards.

Description: Shell (Figs. 2d, 16): Moderately elevated with a well rounded periphery, rather thick and small. Fresh specimens whitish with a tinge of yellowish, slightly translucent. Surface sculptured with prominent widely spaced rounded axial folds parallel to the growth lines that vanish towards the periphery, base rather smooth, overall shiny. Embryonic shell about 1 whorl and 550 μm in diameter; subsequent 3.0 (holotype) whorls convex and evenly rounded at the periphery. Whorls regularly expanding and evenly descending, forming a straight spire with a pointed apex. Suture moderately impressed. Aperture oblique and remarkably curved backwards in its middle portion, whorls inserting below the periphery. Inside the aperture no protruding structures with a smooth continuation towards the columellar side and the base. Outer lip slightly expanded and roundly reflected. Lower part of the lip only slightly protruding towards the transition to the columella which is slightly bent forwards. Basal callus whitish and rather weakly developed, towards its centre often granulated.

Internal shell structures: Internal septum expanding for 0.5 to nearly 0.6 whorls. See Fig. 16.

Operculum: None preserved in the present material.

Distribution: The species was at one locality on Mangareva and two on Aukena.

Remarks: For differences from N. grohi see under that species. It differs from all other species of the Gambier Islands in the axial sculpture.

Etymology: The name refers to the widely spaced axial folds characteristic of the species.

Nesiocina pazi (Crosse 1865)

Type material: Syntypes: MNHN 24719 (now lectotype) and MNHN 24979 (now paralectotypes), Îles Gambier, 16 spms. The original figure is too general on account of the small size of the species and the specimen figured in the original description cannot be singled out. Therefore the best preserved, fully grown specimen, with its operculum still in place, is here chosen as the lectotype (Fig. 3a) to enhance the stability of the name. It is slightly chipped at the base.

Dimensions: Lectotype (Fig. 3a) MNHN 24719: 2.2/2.9/2.5 mm (with operculum and dried animal inside). Figured specimen (Fig. 3b, 17): 2.3/2.9/2.4 mm (empty shell with operculum)

Fig. 17
figure 17

Nesiocina pazi, Aukena, central isthmus, MNHN: height: 2.3 mm, dashed line attachment area of retractor muscles; scale bar 1 mm

Type locality: “Gambier” [French Polynesia: Gambier Islands].

Material examined: MNHN (coll. Bouchet): Mangareva: Ganhutu, lawn in coconut plantation, 23°04.6′S, 134°56.6′W, 17. & 22.9.1997 (2 spms); Aukena: Pointe Mata Kuiti, light soil mixed with white marine sand, 23°08.05′S, 134°55.1′W, altitude 2–3 m, 18.9.1997 (44 spms); Aukena: central isthmus, bare sandy ground, 23°07.6′S, 134°54.0′W, altitude 7–8 m, 18.9.1997 (about 232 spms); Aukena: north-east part of island, 23°07.5′S, 134°53.9′W, 18.9.1997 (11 spms); Aukena: SE of Pointe Mata Kuiti, little humid cliff, sandy ground, 23°08.1′S, 134°55.0′W, altitude 5 m, 18.9.1997 (6 spms); Akamaru: coastal fallow land near former village, 23°10.7′S, 134°54.7′W, altitude 2 m, 19.9.1997 (15 spms); Akamaru: small deposit of drift material on sandy ground, 23°10.6′S, 134°54.9′W, 19.9.1997 (59 spms).

BPBM: Mangareva: North part of Rikitea, 6 ft. (Bot. 187), 7.6.1934, coll. Kondo (BPBM 141671a: 10 spms, BPBM 141672: 1 spm); Mangareva: North end of Rikitea, inland 6 ft., 2 ft., flat (Bot. 189), 8.6.1934, coll. Kondo & Cooke, Jr. (BPBM 141701a: 22 spms); Mangareva: Northeast of Vaituatai Bay, inland 2–6 ft., 1–3 ft., flat (Bot. 197), 9.6.1934, coll. Anderson & Cooke, Jr. (BPBM 139028a: 26 spms, BPBM 139029c: 1 spm); Mangareva: [no other locality data], 3.2.1933, ex coll. Fulton (BPBM 115423: 12 spms); Aukena: west end of island (Bot. 82), 28.5.1934, coll. Anderson & Cooke (BPBM 138680: 6 spms); Aukena: inland 100 ft., on trail near gap, 6 ft., flat (Bot. 88), 28.5.1934, coll. Anderson & Cooke (BPBM 138741a: 8 spms, BPBM 138742a: 2 spms, BPBM 138745d: 5 spms, BPBM 138785a: 15 spms); Aukena: 1st cove East of gap (Bot. 102), 28.5.1934, coll. Anderson (BPBM 138786a: 7 spms, BPBM 138788: 1 spm); Aukena: 2nd cove East of gap (Bot. 103), 28.5.1934, coll. Anderson (BPBM 138812a: 7 spms); Aukena: North side (Bot. 104), 28.5.1934, coll. Anderson (BPBM 138824: 15 spms); Aukena: western end, hillside, flat, medium, on dead leaves under coconut trash (Bot. 79), 27.5.1934, coll. Wright & Kondo (BPBM 140849: 1 spm); Aukena: flat, inland 20 yards, elevation 6 ft., on dead leaves under coconut trash (=Bot. 79) (Bot. 82), 28.5.1934, coll. Anderson & Cooke, Jr. (BPBM 140855: 4 spms); Kamaka: 50–200 ft., Hillside, dry, under stones, on dead leaves (Bot. 109), 31.5.1934, coll. St. John (BPBM 141014: 1 spm, BPBM 141015: 3 spms); Agakauitai: Northwest side of island, back of beach, inland 100+ ft., hillside, medium, flat, on sandy soil (Bot. 195), 8.6.1934, coll. Anderson (BPBM 138923: 13 spms); Akamaru: north-west side, flat, medium, inland 20–200 ft.; elevation 6 ft., on cliffs, fossil (Bot. 97), 28.5.1934, coll. Anderson (BPBM 138846e: 2 spms); Akamaru: flat, elevation 3–5 ft., in wave cutting, fossil (Bot. 107), 30.5.1934, coll. Anderson & Cooke, Jr. (BPBM 138865a: 24 spms).

Museum and Institute of Zoology of the Polish Academy of Sciences, Warsaw, Poland: Gambier Inseln [Gambier Islands], ex Hidalgo, collection A. J. Wagner (MIZ 8758: 2 spms).

Museo Nacional de Ciencias Naturales, Madrid, Spain: 5 lots, all labelled Helicina pazi, Gambier Is, all live-taken with dried soft parts. Source unknown,, all ex coll. Azpeitia (15.05/36611: 110 spms, 15.05/47820: 29 spms, 15.05/47822: 11 spms, 15.05/47823: 7 spms); “historical collection, gift from Richardson” (15.05/47821: 122 spms).

Museum d’Histoire Naturelle de Bordeaux, France : 1 spm ex Hidalgo, in coll. Guestier [not seen].

Diagnostic features: Small, shiny, rather solid-shelled, yellowish or reddish-brownish species, aperture simple with only a slight ridge at the lower parietal side and a very shortly reflected outer lip.

Description: Shell (Figs. 3a, b, 17): Conical-globose with a well rounded periphery, rather thick, small and shiny. Fresh specimens whitish-yellowish to reddish-brownish without any pattern, colour more intensive towards the apex, slightly translucent. Calcareous shell surface rather smooth and shiny. Embryonic shell about 1 whorl and 540 μm in diameter; subsequent 3.0 (lectotype) whorls convex and evenly rounded at the periphery. Whorls regularly expanding, evenly descending and forming a straight spire with a blunt apex. Suture moderately impressed. Aperture oblique, straight and comparably small, last whorl usually inserting a little below the periphery. Aperture at the lower parietal side with a slight ridge along the line from the insertion of the body whorl towards the base becoming more strongly developed towards the base and inside at the base another weakly developed internal lamella starting from the columella and slightly diverging from the outer lip. Outer lip only slightly thickened, sharply and shortly reflected. Lower part of the lip with a straight transition to the short and flat columellar area. Basal callus whitish and more or less strongly developed, towards its centre often granulated.

Internal shell structures: Upper part of the internal septum expanding for about 0.4 whorls, upper attachment of the retractor muscles overlapping the last two whorls. See Fig. 17.

Operculum: Rather straight along the parietal side, S-line only slightly bent, outer surface rather shiny and granular (Fig. 12c).

Judging from the old dried material, the mantle surface of the live animals was mottled with dark spots.

Distribution: Nesiocina pazi was found in several places on Mangareva, Aukena, Akamaru, Agakauitai and Kamaka. In three of the four samples taken on Aukena and on Akamaru in 1997 it was the most abundant species of Helicinidae, and it is probable that the type material of N. pazi originates from one of these islands, where it comprises respectively 55 and 88 % of all helicinid specimens.

Remarks: Hidalgo (1913:1735) attributed the name Helicina pazi to himself. However, although Hidalgo requested from Crosse that the species, if new, should be named after Paz; Crosse is the author of the description and, under Article 50.1 of the Code, is thus the author of the name.

Nesiocina pazi very closely resembles Nesiocina hendersoni (Preece 1998) (described as Pleuropoma hendersoni) from Henderson Island of the Pitcairn group which was only known from two intact shells and a number of fragments. Preece (1998) pointed out that some juveniles of both species exhibit very strong spiral ribs on the early teleoconch. The older teleoconch shows a pattern of dense oblique diverging grooves. These two characteristics were also described as typical for the northern radiation of helicinid species of the genus Sturanya in New Caledonia (Richling 2009), obviously a common feature in helicinids. A direct comparison with two additional complete specimens of N. hendersoni from more recent excavations (Henderson Island, North Beach, Hen-6 [for exact location see Preece 1998], 75–90 cm Layer 9, coll. R. C. Preece, 1997, University Museum of Zoology Cambridge, Figs. 3d, 18) mainly confirms the differences pointed out in the original description. According to the drawing (Preece 1998: Fig. 7) the holotype has a more thickly developed basal callus. With an average height of 1.5–1.6 mm all specimens are significantly smaller than N. pazi. In shape they more closely resemble S. makaroaensis but as in N. pazi, the columella of N. hendersoni is straight and short instead of being strongly bent forwards. Furthermore N. hendersoni possesses the slight ridge at the lower parietal side of the aperture which is wanting in S. makaroaensis. For differences of N. pazi from S. makaroaensis see under the latter species.

Fig. 18
figure 18

Nesiocina hendersoni, Henderson Island, 75–90 cm Layer 9, 1997 excavation (station Hen-6), coll. R. C. Preece, University Museum of Zoology Cambridge: height: 1.5 mm; scale bar 1 mm

Sturanya Wagner 1905

Type species: Helicina plicatilis Mousson 1865, by subsequent designation of Kobelt (1905: 207).

Sturanya makaroaensis n. sp.

Type material: Holotype MNHN 24996, 2.2/2.8/2.5 mm (collected by Bouchet, 14 September 1997) (Figs. 3c, 19).

Fig. 19
figure 19

Sturanya makaroaensis n. sp., holotype, Gambier Islands: Makaroa, MNHN 24996: height: 2.2 mm, dashed line attachment area of retractor muscles; scale bar 1 mm

Paratypes: MNHN 24997, about 350 spms, some with dried animals inside, same collection data as holotype.

Type locality: French Polynesia, Gambier Islands: Makaroa, SE exposed coastal cliff with thickets of Beach Hibiscus (Hibiscus tiliaceus), 23°12.8′S, 134°58.4′W, altitude 15–20 m.

Material examined: MNHN (coll. Bouchet): Aukena: central isthmus, bare sandy ground, 23°07.6′S, 134°54.0′W, altitude 7–8 m, 18.9.1997 (8 spms); Akamaru: small deposit of drift material on sandy ground, 23°10.6′S, 134°54.9′W, 19.9.1997 (9 spms); Makaroa: SE exposed coastal cliff with thickets of Beach Hibiscus (Hibiscus tiliaceus), 23°12.8′S, 134°58.4′W, altitude 15–20 m, 14.9.1997 (about 351 spms).

BPBM: Aukena: inland 100 ft., on trail near gap, 6 ft., flat (Bot. 88), 28.5.1934, coll. Anderson & Cooke (BPBM 138745e: 1 spm); Makaroa: 30–100 ft., Hillside, medium, on cluffs & dead leaves (Bot. 110), 31.5.1934, coll. Anderson (BPBM 141031: 42 spms + opercula, BPBM 141032/141032a: 17 spms + 14 opercula, BPBM 141033: 37 spms, BPBM 141034: 9 spms, BPBM 141035: 3 spms); Akamaru: flat, elevation 3–5 ft., in wave cutting, fossil (Bot. 107), 30.5.1934, coll. Anderson & Cooke, Jr. (BPBM 138865b: 3 spms).

Diagnostic features: Small, hairy and rather thin-shelled, brownish species, aperture simple without any structures, outer lip almost not reflected.

Description: Shell (Figs. 3c, 19): Depressed-globose with a well rounded periphery, rather thin, small and shiny when periostracum is eroded. Fresh specimens uniformly light yellowish-brownish without any pattern, slightly translucent. Calcareous shell surface rather smooth and shiny, usually covered with a thin hairy brownish periostracum. Hairs rather short and in dense spiral rows. Embryonic shell about 1 whorl and 500 μm in diameter; subsequent 3.1 (holotype) whorls convex and evenly rounded at the periphery. Whorls regularly expanding, evenly descending and forming a straight, rather low spire with a blunt apex. Suture moderately impressed. Aperture oblique and slightly curved backwards, comparably small, last whorl usually inserting a little below the periphery. Inside the aperture no structures. Outer lip not thickened and almost not reflected. Lower part of the lip straight, but remarkably protruded and columella strongly bent forwards in its lower part (clearly visible in lateral view). Basal callus whitish and rather smooth, only towards the lower columella reddish.

Internal shell structures: Upper part of the internal septum expanding for about 0.4 whorls, upper attachment of the retractor muscles overlapping the last two whorls. See Fig. 19.

Operculum: Remarkably curved in its upper part of the parietal side, S-line strongly bent, upper edge nearly pointed, outer surface rather shiny and slightly granular (Fig. 12d).

Female reproductive system (Fig. 20): See appendix “2”: Anatomical investigations for generic assignment.

Fig. 20
figure 20

Pallial female reproductive system of Sturanya makaroaensis n. sp., Gambier Islands: Makaroa, MNHN 24997, ventral (left) and dorsal view (right); scale bar 0.5 mm

Distribution: In 1934 and 1997, living specimens of Sturanya makaroaensis were only found on Makaroa Island, where they were quite abundant. Faded specimens are also known from Aukena and Akamaru.

Remarks: Superficially, S. makaroaensis most closely resembles the more widely distributed N. pazi, but the latter lacks the hairy periostracum when adult, possesses a slightly more elevated shell and in S. makaroaensis the columella is strongly bent forwards instead of being straight and short. The resulting shapes of the opercula are clearly different (see Figs. 12c, d). The structure of the periostracum of S. makaroaensis is quite similar to that of Sturanya gallina (Gassies 1870) from the Loyalty Islands, New Caledonia (Richling 2009: 266–267, Figs. 12, 13).

Etymology: The name refers to the island of Makaroa where the species was still surviving in 1997.

Key to the Helicinidae of the Gambier Islands

  1. 1

    Aperture with parietal lamellae or a knob-like structure..........2

  2. Aperture without parietal lamellae or similar structures..........5

Species with parietal apertural structures

  1. 2

    With three parietal lamellae..........Nesiocina trilamellata n. sp.

  2. With fewer than three lamellae or simpler structures..........3

  3. 3

    With one rather long parietal lamella..........Nesiocina unilamellata n. sp.

  4. With two shorter lamellae or only a knob on the parietal side..........4

  5. 4

    With two short parietal lamellae..........Nesiocina gambierensis n. sp.

  6. Parietal side with knob-like structure, operculum with external apophyses..........Nesiocina superoperculata n. sp.

 

Species without parietal apertural structures

  1. 5

    Parietal side continuous..........6

  2. Parietal side of the aperture with an oblique ridge..........8

  3. 6

    Shell surface smooth or with spiral periostracal ridges, diameter greater than height..........Sturanya makaroaensis n. sp.

  4. Shell surface with axial folds or ribs..........7

  5. 7

    Axial ribs narrowly spaced, continuing down to the basal callus, shell highly elevated..........Nesiocina grohi n. sp.

  6. Axial ribs widely spaced, vanishing below the periphery, moderately elevated..........Nesiocina pauciplicata n. sp.

  7. 8

    Shell of greater diameter than height, apertural ridge only on parietal side..........Nesiocina pazi

  8. Shell of greater height than diameter, additional apertural ridge along the basal side..........9

  9. 9

    Suture only very slightly impressed, spire convex..........Nesiocina abdoui n. sp.

  10. Suture deeply impressed, whorls slightly shouldered, spire straight..........Nesiocina mangarevae n. sp.

Appendix 2: Generic affiliations of Gambier Islands helicinids

For comparative purposes, the anatomy of the female pallial reproductive system of the following species was investigated. Terminology follows Richling (2004, 2009):

  1. (1)

    Helicina discoidea Pease 1868, from Tahiti, collected on 1 May 2008 by O. Gargominy and B. Fontaine, and deposited in the Muséum national d’Histoire naturelle, Paris (MNHN).

  2. (2)

    Helicina parvula Pease 1868, from Atiu, Cook Islands, collected on 23 August 1929 by. Peter H. Buck, and deposited in the Bernice Pauahi Bishop Museum, Honolulu (BPBM; catalog number BPBM 94992). This species is conchologically very similar to Helicina pazi.

  3. (3)

    Helicina solidula G. B. Sowerby I 1839 (possibly a species complex), from Rurutu, Austral Islands, collected on 22 November 2003 by O. Gargominy and B. Fontaine, and deposited in the Muséum national d’Histoire naturelle, Paris (MNHN).

  4. (4)

    Helicina cf. villosa Anton 1838, from Raivavae, Austral Islands, collected on 24 November 2002 by O. Gargominy and B. Fontaine, and deposited in the Muséum national d’Histoire naturelle, Paris (MNHN).

Helicina discoidea is the type species of Discoidea, and had also previously been classified in Aphanoconia; H. parvula had previously been classified in Sturanya; H. solidula had been classified in Orobophana Wagner 1905 and H. villosa in Pleuropoma.

Anatomical investigations for generic assignment

Sturanya makaroaensis n. sp. (Fig. 20): Receptaculum seminis absent; V-organ regularly developed; pedicel proximally connected to the short descending limb of V-organ and a dorsal sperm sac; dorsal sperm sac large and prominent, shaped like an elongate, regular egg, stalk almost missing, surface smooth and undifferentiated, internal structures not visible; provaginal sac of moderate size, extending up to the reception chamber, broadly connected to the provaginal duct in its middle portion; provaginal duct opening at about 1/3 of the length of the pallial oviduct; distal appendage weakly discernible, entering pallial oviduct close to its base, extending up to 2/5 of its length and thence up to the provaginal opening, longitudinally orientated. These characters agree with those of Sturanya as described by Richling (2009).

Sturanya makaroaensis differs substantially from species of Discoidea (=Nesiocina, see below) and Pleuropoma in the presence of a dorsal sperm sac instead of a receptaculum seminis, indicating that it should be classified in Sturanya.

The genus Discoidea Wagner 1905 was established for other helicinids from the South Pacific, with Helicina discoidea Pease 1868, from the Society Islands (French Polynesia), as the type species. The name Discoidea Wagner 1905, however, is invalid because it is a junior homonym of Discoidea Agassiz 1834 (Echinodermata). Given our current state of knowledge, other supraspecific names for Pacific or Australasian helicinids (Wagner 1905; 1907–1911; Iredale 1941) cannot be regarded as valid names for the Remote Oceania taxa discussed here. We therefore propose Nesiocina as a replacement for Discoidea Wagner 1905, non Agassiz 1834 (see Appendix “1”).

Nesiocina discoidea (Fig. 21): Receptaculum seminis present, entering rather narrowly at the uppermost inner side of the descending limb of the V-organ, of small size and rounded, 1/3 of the diameter of the pedicel; pedicel proximally only connected with V-organ, clearly differentiated in the two specimen dissected, no dorsal sperm sac developed; provaginal sac slightly expanded and of moderate length, extending up to reception chamber or the middle of the pedicel, entering provaginal duct broadly in its middle part; provaginal duct opening at about 2/5 of the length of the pallial oviduct; distal appendage at the lower part of the pallial oviduct present, but its length could not be determined, possibly nearly extending up to the provaginal opening.

Fig. 21
figure 21

Pallial female reproductive system of Nesiocina discoidea, Tahiti, MNHN (right image is a different specimen, showing variation in the provaginal sac), length of distal appendage uncertain; scale bar 1 mm

Nesiocina parvula (Fig. 22): Receptaculum seminis present, entering relatively broadly at the uppermost inner side of the descending limb of the V-organ, of moderate size and rounded, a little more than half of the diameter of the pedicel; pedicel proximally only connected with V-organ, not clearly differentiated in the specimen dissected, no dorsal sperm sac developed; provaginal sac rather slender and short, extending up to the middle of the reception chamber, entering provaginal duct in its middle part; provaginal duct opening at about 2/5 of the length of the pallial oviduct; presence or absence of a distal appendage at the lower part of the pallial oviduct uncertain (not visible in the single specimen dissected).

Fig. 22
figure 22

Pallial female reproductive system of Nesiocina parvula, Cook Islands, BPBM 94992; scale bar 0.5 mm

Nesiocina solidula (Fig. 23) and N. cf. villosa (Fig. 24): Rather similar. Receptaculum seminis present, entering rather broadly at the uppermost inner side of the descending limb of the V-organ, in both species of moderate size and rounded—slightly larger in villosa, less or about half of the diameter of the pedicel; pedicel proximally only connected with V-organ, no dorsal sperm sac developed; provaginal sac prominently developed and rather broad, extending up to the pedicel, entering provaginal duct in its lower third close to its opening; provaginal duct opening at about 1/3 to 1/4 of the length of the pallial oviduct; distal appendage at the lower part of the pallial oviduct present in both species, but in solidula either badly preserved or very difficult to differentiate in its proximal portion, length slightly more than 1/3 of the pallial oviduct, in solidula apparently longer to at least 1/2 of the pallial oviduct, long and quite broad, at the margins slightly wavy, longitudinally orientated.

Fig. 23
figure 23

Pallial female reproductive system of Nesiocina solidula, French Polynesia, Rurutu, MNHN, length of distal appendage uncertain; scale bar 1 mm

Fig. 24
figure 24

Pallial female reproductive system of Nesiocina cf. villosa, French Polynesia, Raivavae, MNHN; scale bar 1 mm

The female reproductive system of the four species from the Society, Cook and Austral Islands is thus very similar, pointing to a close relationship. By comparison with the anatomically known Australasian and Pacific genera, it most closely resembles that of the Philippine genus Pleuropoma (Richling 2009: 262) with regard to the presence or absence of the different appendages (i.e. receptaculum seminis, provaginal sac, distal appendage). However, the South Pacific taxa differ from Pleuropoma in their more broadly connected receptaculum seminis, of significantly smaller size, and in the longitudinal (instead of transverse) orientation of the considerably longer distal appendage on the pallial oviduct. These differences allow differentiation of this group from Pleuropoma.

For lack of a more global and robust higher classification of Australasian and Pacific helicinids, it presently seems most practical to recognise Nesiocina as a genus, anatomically nearest to Pleuropoma. In addition to the type species (Nesiocina discoidea), we assign the three species mentioned above to Nesiocina.

These anatomical results (and unpublished data on additional species) reveal that this part of Polynesia is at least dominated, if not exclusively inhabited, by species of the genus Nesiocina, which extends beyond French Polynesia as far as Henderson Island (species of the solidula complex; Preece 1995; Fig. 4). The occurrence of a species of Sturanya in the Gambier Islands constitutes the only record so far of the genus in this part of the Pacific. Given this context, there are two equally likely hypotheses for the generic assignment of the extinct Gambier Islands species: (1) assignment to Sturanya because the only anatomically known species from the Gambier Islands belongs to that genus; or (2) assignment to Nesiocina because of the range of the genus and conchological similarity between at least N. parvula from the Cook Islands and the Gambier Islands N. pazi. Despite a superficial similarity between Sturanya makaroaensis and N. pazi, S. makaroaensis appears to differ significantly from the rest of the Gambier Island radiation of helicinids, and all Gambier Island helicinids other than S. makaroaensis are here tentatively assigned to Nesiocina.

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Richling, I., Bouchet, P. Extinct even before scientific recognition: a remarkable radiation of helicinid snails (Helicinidae) on the Gambier Islands, French Polynesia. Biodivers Conserv 22, 2433–2468 (2013). https://doi.org/10.1007/s10531-013-0496-2

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