Opeas hannense (Rang, 1831)

Helix hannensis Rang, 1831: 41–42, pl. 3, fig. 8. Type locality: Senegal, Cape Verde Peninsula, Hann village (“ village de Hann sur la presqu’île du Cap-Verd ”). Type material: unknown.

Material examined.

Italy • 47 shells and 1 spirit specimen; Trento, Tropical greenhouse of the Science Museum of Trento (MUSE); 46 ° 03 ' 45.16 " N, 11 ° 06 ' 50.08 " E; 4 Jan. 2019, 10 Feb. 2019, 04 May 2019; D. Barbato, G. Bolzonella leg.; GMC 51196 • 17 shells; same locality; 01 Feb. 2022; D. Barbato, A. Benocci leg.; GMC 51187 • 1 shell; same locality; 02 Feb. 2023; F. Rossi leg.; GMC 57345 • 6 shells; same locality; 09 Feb. 2023; D. Barbato, A. Benocci leg.; GMC 57352.

Description.

Shell (Figs 17–20). Dextral, very small in size, very minutely perforate, elongate, very slender, conical, rather robust, pearly off-white, glossy or waxy and sub-transparent when fresh, with 5–6 slightly convex whorls separated by moderately deep sutures. Apex obtuse, rounded, and smooth. Last whorl ~ ½ of shell height and less convex than preceding ones. Aperture small, ~ 1 / 3 of shell height, obliquely pyriform, slightly prosocline. Peristome interrupted, not thickened, only slightly reflected on columella, sometimes with subtle callous rim on parietum; columella straight; outer margin sinuous in lateral view (approximately inverse S-shaped). Protoconch smooth; teleoconch with weak irregular collabral growth lines. Shell dimensions: SH 4.0– 4.8 mm; SD 1.6–1.8 mm; AH 1.6–1.7 mm; AW 0.9–1.0 mm.

Female distal genitalia (Fig. 14). Free oviduct very short and wide. Bursa copulatrix sac-like, oval with short slender duct (as long as bursa copulatrix), initially not flared. Vagina very short and wide (as long as free oviduct).

Male distal genitalia (Fig. 14). Vas deferens almost uniform in diameter (very thin to thin along its entire length), entering penial complex at its proximal end. Penial complex apparently consisting only of penis. Penis short, almost uniformly cylindrical with thin short penial sheath enveloping its distal tract. Penial retractor muscle inserted on proximal end of penis.

Genital atrium (Fig. 14). Rather long.

Remarks.

The species was first named Helix clavulus by Férussac (1821: 52) based on specimens from Guadeloupe and then Helix goodalli by Miller (1822: 381) on specimens from near Bristol, England. Unfortunately Férussac’s name was not accompanied by a description, a definition, or an indication and so it is not available, whereas Miller’s name, extensively used until the early 20 th century (cf. Pilsbry 1906 b), turned out to be a junior homonym and was replaced by Bulimus pumilus established by Pfeiffer (1840) on specimens from Cuba (cf. Pilsbry 1910). Pilsbry (1906 a: 141–142) also discussed the hypothesis of Wollaston (1878: 510) that Helix goodalli was a junior synonym of Helix hannense established by Rang (1831) on specimens from the Cape Verde Peninsula, Senegal, observing: “ whether this course was well-founded is a question which must remain unsettled until specimens from Rang’s original locality can be compared. ” Consequently he never adopted Rang’s name for this species (cf. Pilsbry 1946: 181–182). The synonymy of the two species was reproposed by Groh (1983) based on study of the original descriptions and the literature, and has subsequently been adopted by most recent authors (e. g., von Proschwitz 1994; Cowie 1997, 2000; Chase and Robinson 2001; Bank and Menkhorst 2008; Gerber and Clark 2015; Horsák et al. 2020). Perplexity persists about the real identity of the species described by Rang. Only the dimensions, which are consistent with those of an Opeas species, support Groh’s interpretation. Otherwise the situation remains as described by Pilsbry more than a hundred years ago: type material of Rang’s species is unknown; no one has reported or studied material from the type locality, which when Rang visited it, was a small village, today englobed in the city of Dakar (where a green area, the Parc forestier et zoologique de Hann, still survives in Hann); finally Rang’s description and illustration are completely inadequate to establish the identity of the species he treated; his figure depicts a snail with shell having all the whorls quite round, whereas this species has the last whorl almost flat (incidentally Robinson (1999: table 1) considered Opeas hannense to be absent from Africa).

Opeas hannense is regarded as native to tropical America (Pilsbry 1946; Kerney and Cameron 1979; Deisler and Abbott 1984; Cowie 1997; Cowie et al. 2008; Brook et al. 2010; Miquel and Herrera 2014) where it is widespread in Central America and the West Indies. On the contrary Robinson (1999: Table 1) regarded it as native to East Asia. It has been introduced into South America, Atlantic islands, East Africa, West Indian Ocean islands, South-east Asia, and Pacific islands. It has also been reported from the mid temperate latitudes of the northern hemisphere where it only occurs in greenhouses and hothouses (see Table 2 for details and references). Since these reports are only based on shell identifications, it is not possible to exclude that some are misidentifications (e. g., Muratov 2010: fig. 28).

The genital anatomy of Opeas species was investigated by Baker (1945), Baker in Pilsbry (1946), and Gittenberger and van Bruggen (2013). Only three Opeas species have been studied: Opeas hannense (see Baker 1945: 86, as Opeas pumilum), Opeas pyrgula (Schmacker & Boettger, 1891) (see Baker 1945: 87; Baker in Pilsbry 1946: 183–184, figs 88 2, 3) and an unidentified species from Misali islet, Zanzibar (see Gittenberger and van Bruggen 2013: 251, fig. 9), but only the anatomy of Opeas pyrgula is adequately described. The distal genitalia of the only adult specimen that we have been able to study (Fig. 14) agrees with the general scheme described for these species and in particular with the features reported by Baker (1945) and Baker in Pilsbry (1946) for Opeas pumilum and Opeas pyrgula and with the description of Opeas pumilum given by Baker (1945). The major difference between the two species consists in the swelling between the base of the duct of the bursa copulatrix and the proximal vagina: well developed in Opeas pyrgula and much less enlarged in Opeas pumilum.

Thus, little continues to be known about the genital anatomy of this genus. We need to ascertain whether the proximal complex of the penis is really undivided, to understand the relationships between the vas deferens and the penial sheath and whether the different structure of the female distal genitalia is due to individual variation or to phylogenetic divergence.

Opeas hannense has only been found in the tropical greenhouse of MUSE, where it is uncommon. This is the first report from Italy.