Geostilbia aperta ( Swainson, 1840 )

Manganelli, Giuseppe, Benocci, Andrea, Barbato, Debora & Giusti, Folco, 2024, Five alien achatinid land snails (Gastropoda, Eupulmonata) first reported in greenhouses of Italian botanical gardens, ZooKeys 1208, pp. 99-132 : 99-132

Geostilbia aperta ( Swainson, 1840)

Macrospira aperta Swainson, 1840: 335, fig. 97 e, f. Type locality: no locality given; according to Smith (1892: 269), Saint Vincent, Lesser Antilles, West Indies. Type material: unknown. Note: Swainson attributed the new species to the reverend L. Guilding from St. Vincent, from whom he received the material used for the description. Probably this material was accompanied by a manuscript name which Swainson adopted for denoting the species.

Achatina gundlachi Pfeiffer, 1850: 80. Type locality: Cuba. Type material: unknown.

Geostilbia caledonica Crosse, 1867: 186–187, Pl. 7, fig. 4. Type locality: Nouméa, New Caledonia. Type material: 1 syntype in Crosse collection (MNHN-IM- 2000-4720) ( Breure et al. 2022 b).

Material examined.

Italy • 2 shells; Trento , Tropical greenhouse of the Science Museum of Trento ( MUSE); 46 ° 03 ' 45.16 " N, 11 ° 06 ' 50.08 " E; 01 Feb. 2022; D. Barbato, A. Benocci leg.; GMC 51189 .

Description.

Shell (Figs 33 , 34 ). Dextral, very small in size, imperforate, elongate, very slender, cylindro-conical, thin and fragile, pearly off-white, colourless, glossy and transparent when fresh, ~ 4 slightly convex whorls separated by rather deep sutures. Apex obtuse, rounded, and smooth. Last whorl ~ 2 / 3 of shell height. Aperture small, ~ 1 / 3 of shell height, ovate to pyriform, basally flared, slightly prosocline. Peristome interrupted, not thickened or reflected, with callous rim on parietum and columella; columella straight or slightly concave; outer margin slightly arched forward in the middle in lateral view. Protoconch smooth; teleoconch with very fine collabral lines and very fine spiral grooves particularly evident on last whorl. Shell dimensions: SH 2.8 mm; SD 1.0 mm; AH 1.1 mm; AW 0.6 mm.

Body and anatomy. Unknown.

Remarks.

The early taxonomy of this land snail revolves around three named species: Macrospira aperta Swainson, 1840 , Achatina gundlachi Pfeiffer, 1850 and Geostilbia caledonica Crosse, 1867 .

Commenting on the land mollusc species introduced to Saint Helena, Edgar Smith maintained that Achatina gundlachi was a junior synonym of Macrospira aperta based on examination of Guilding’s material from St. Vincent, West Indies, deposited in the British Museum ( Smith 1892) and that Geostilbia caledonica was also a junior synonym of Megaspira [sic] aperta ( Smith, 1895).

Pilsbry (1908), in his exhaustive revision of orthurethrous snails included in the second edition of the Manual of Conchology, partly rejected Smith’s conclusions, regarding Macrospira aperta as a species inadequately described and Geostilbia as a section of Cecilioides . He used Cecilioides gundlachi as the valid name for the species. However many years later, dealing with this group again, he adopted Cecilioides aperta as the valid name ( Pilsbry 1946).

No subsequent authors made any significant contribution for a better taxonomic framework of the species. They repeated what Pilsbry (1946) proposed, believing that a species of Geostilbia , sometimes considered a subgenus or a synonym of Cecilioides , could be found almost everywhere in the world, having spread from the Mesoamerican area (e. g., Robinson 1999). Indeed many consider it native to the West Indies, Neotropics or Caribbean basin ( Dundee 1974; Cowie 1997; Robinson 1999; Chase and Robinson 2001; Rosenberg and Muratov 2006; Thompson 2011; Miquel and Herrera 2014; Nurinsiyaha et al. 2016; Nurinsiyaha and Hausdorf 2019). Others report it to be native to southern Europe ( Cotton 1954) or North America north of Mexico ( Nekola 2014).

A species, sometimes named Geostilbia aperta (or Cecilioides aperta ), Geostilbia caledonica (or Cecilioides caledonica ) or Geostilbia gundlachi (or Cecilioides gundlachi ), is currently reported from North, Central and South America, the Caribbean and the Indo-Pacific Region from South-East Asia to Hawaii and Cook Islands (see Table 5 for details and references). It has also been reported from Saint Helena ( Smith 1892) based on a misidentification of Cecilioides acicula ( Crowley and Pain 1977) , and from Barbados, Curaçao and Galapagos, where it was no longer found in recent field surveys ( Chase and Robinson 2001; Miquel and Herrera 2014; Hovestadt and van Leeuwen 2017).

It is difficult to say anything new about this group of species, since there is no anatomical data. We can rely on shell characters, the most interesting of which are the spiral sculpture particularly evident in the last whorl, the non-truncated or slightly truncated columella and the basally flared aperture. Based on a close resemblance to Cecilioides , the species of Geostilbia have been placed in the family Ferussaciidae but this similarity could also be due to convergence: true Cecilioides have no microsculpture on the teleoconch, have a truncated columella and have no basally flared aperture. In their phylogenetic analysis of the achatinoideans Fontanilla et al. (2017) examined a Cecilioides species – Cecilioides gokweana ( Boettger, 1870) – which may actually be Geostilbia , if the material investigated, collected by DG Herbert, matches the description of this species given by him (cf. Herbert 2010: 127 for description and figure). If this is confirmed, Geostilbia does not belong to the Ferussaciidae but to a distinct group of Achatinidae . This of course does not resolve the relationships between Cecilioides and Geostilbia : Cecilioides may really belong to the Ferussaciidae or to the same or a different group of Achatinidae , which may include Geostilbia .

MolluscaBase lists eight Geostilbia species ( MolluscaBase eds 2024 c), but except for the widespread Geostilbia aperta and the south American Geostilbia blandiana Crosse, 1880 , all the others occur from Madagascar to Southeast Asia. There is great uncertainty about species-level taxonomy of Geostilbia . Some of the species listed by MolluscaBase may prove to be true Cecilioides based on the apparent absence of spiral microsculpture, truncated columella and not basally flared aperture: this could be true of Geostilbia philippinica von Möllendorff, 1890 and Geostilbia sheilae Groh, 2015 (see Groh 2015). On the other hand, some species reported as Cecilioides by MolluscaBase, such as the East African Cecilioides callipeplum ( Connolly, 1923) , for which clear spiral sculpture is reported by Verdcourt (1986), van Bruggen and van Goethem (2001), and van Bruggen (2008), or the South African Cecilioides gokweana , for which clear spiral sculpture is described by Herbert (2010), may prove to be true Geostilbia .

We assign two shells found in the litter of the tropical greenhouse of MUSE to this species (unfortunately no living specimen was found during our collecting). This is the first report of the species from Europe.

Figures

Figures 33, 34. Shells of Geostilbia aperta from the tropical greenhouse of the Science Museum of Trento (MUSE), D. Barbato & A. Benocci leg. 01 Feb. 2022.