Idiopyrgus minor, Salvador & Silva & Bichuette, 2022

Salvador, Rodrigo B., Silva, Fernanda S. & Bichuette, Maria E., 2022, Phylogenetic position of the relict South American genus Idiopyrgus Pilsbry, 1911 (Gastropoda, Truncatelloidea), with the description of two new cave species, Zoosystematics and Evolution 98 (2), pp. 365-375 : 365

Idiopyrgus minor sp. nov.

Fig. 2I-K

Type material.

MZSP 158238 (holotype), LES 0027980 (2 paratypes, from Gruna do Engrunado).

Type locality.

Brazil, Bahia state, Feira da Mata municipality, Gruna da Pingueira II cave.

Distribution.

Known only from two caves in Feira da Mata municipality.

Etymology.

From Latin, meaning ‘little’.

Diagnosis.

Smaller shell than congeners; spire not as elongated and wider than most congeners; aperture proportionately larger in relation to preceding whorl.

Description.

Shell minute, truncatelloid, of translucent white color. Protoconch not too prominent. Shell smooth except for fine growth lines. Suture deep, rendering whorls lightly shouldered in holotype (Fig. 2I, J ). Whorl profile convex. Whorls increasing regularly in size with growth. Aperture large in proportion to body whorl; oval to circular, with a more acuminate end towards the upper lip insertion. Peristome complete; reflexed but not thickened. Shell rimate. Operculum translucent, of same color as the shell. Soft body white.

Measurements.

holotype: 5 whorls, H = 3.5 mm, D = 1.7 mm; paratype: H = 3.7 mm, D = 1.7 mm.

Genetic data.

GenBank reg. nrs: COI, ON720330; 16S, ON720564; ITS2+28S, ON720562.

Remarks.

Idiopyrgus minor sp. nov. can be distinguished from its congeners by its small size. It also has a less elongated and wider spire than most of its congeners, the exceptions being I. pilsbryi and I. walkeri (Fig. 1 ). Nevertheless, it can also be easily diagnosed from them: I. minor sp. nov. is much smaller and has a narrower shell than I. walkeri , and has a larger and wider aperture than I. pilsbryi . Finally, there is a reasonably large genetic distance separating I. minor sp. nov. from I. adamanteus sp. nov. (Fig. 3 ), comparable to the distance between the species of Truncatellidae and Stenothyridae present on the tree and larger than the distances between species of Pomatiopsidae .

This species is considered troglobitic, given its occurrence in a single cave of the Serra do Ramalho karst area. The specimens were collected in pools resulting from infiltration water and seem to be restricted to this habitat. Besides the translucent shell and lack of body pigmentation, the smaller body size of I. minor sp. nov. can also be interpreted as a possible troglomorphism, given that miniaturization is common in cave organisms, including snails ( Christiansen 2012; Salvador et al. 2022).

The Serra do Ramalho region has been historically exploited for agriculture, charcoal production, and mining, from small to large scales (Gallão and Bichuette 2018).

Phylogenetics and biogeography

The phylogenetic analysis included a total of 51 species (counting the outgroup) and all family level taxa were well-supported (posterior probability, PP = 1 in all cases except Falsicingulidae , for which PP = 0.98). As such, even though there is an unsolved polytomy in the tree (Fig. 3 ; in all likelihood a result of the exclusion of northern hemisphere families not immediately related to our question), the membership of each species to a particular family can be readily and reliably assessed.

In that regard, it can be seen that Idiopyrgus does not belong to Pomatiopsidae ; rather, it is grouped together with Tomichia and Coxiella . These three genera together form the family Tomichiidae , which recovers the results of Wilke et al. (2013) and fully establishes this family as a Gondwanan clade. The most basal branch in Tomichiidae appears to be the Australian Coxiella , with the South African Tomichia and the South American Idiopyrgus spp. forming a well-supported monophyletic clade (PP = 0.97; Fig. 3 ). This represents a typical Gondwanan scenario, with the Australian branch being the most basal one. Furthermore, it can be surmised that all three genera are relicts in their respective continents, given their present low diversity and restricted distributions ( Wilke 2019).

Nevertheless, the diversity of the Brazilian branch of this family might be underestimated. Specimens of I. souleyetianus have been reported from several localities in Brazil, including the aphotic zones of caves (e.g., Thiengo et al. 2005; Salvador et al. 2017; Salvador and Simone 2021). Shell morphology and the apparent isolation of some populations in cave systems indicate, however, that " I. souleyetianus " from the literature is likely a species complex (see also the discussion below). The two new species described herein from caves also point towards this conclusion. Future morphological and molecular studies might thus shed light on the real diversity of this group in Brazil.

It is also noteworthy that Tomichiidae is absent from Zealandia, especially considering that the family Tateidae is likewise a Gondwanan clade and has representatives in South America, Australia and New Zealand (Fig. 3 ; Zielske et al. 2016; Ponder 2019). Given the more basal position of Tateidae in relation to Tomichiidae , it can be hypothesized that it represents an earlier branch of Truncatelloidea and that Tomichiidae therefore arose after Zealandia had split from Gondwana (circa 80 Ma).

Taxonomy

Based on the study of type material and further specimens available to us, it was possible to reassess those species of Idiopyrgus that are presently considered synonyms (as per Simone 2006) of I. souleyetianus . This assessment was done based on conchological characters of the type specimens, topotypes, and additional voucher specimens, as no additional DNA-grade material could be acquired. Therefore, we are aware that our proposed arrangement may change in the future when more material becomes available.

I. souleyetianus (Fig. 1A, B ) is here restricted to populations from Espírito Santo state (Southeast Atlantic hydrographic region), which includes the type locality, and Bahia state ( São Francisco hydrographic region; specimens reported by Salvador et al. 2017) (Fig. 4 ). The specimens from northern Minas Gerais ( São Francisco hydrographic region; Fig. 4 ) assigned by Salvador and Simone (2021) to I. souleyetianus display much smaller shells, with fewer whorls and a weak axial teleoconch sculpture. As such, they might represent a still unrecognized species. Likewise, the records from Goiás state assigned to I. souleyetianus by Thiengo et al. (2005: not illustrated) stem from an area belonging to yet another hydrographic region (Tocantins-Araguaia; Fig. 4 ) and should be reassessed; they are treated here as Idiopyrgus sp.

Idiopyrgus pilsbryi (Fig. 1C, D ) has a much smaller shell than I. souleyetianus , having circa 2/3 of the latter’s shell length. The whorls of I. souleyetianus are taller than those of I. pilsbryi , and it has a taller spire with more whorls. I. pilsbryi inhabits Rio Grande do Norte state, ca. 1,500 km north of the type locality of I. souleyetianus , in a completely different climate and biome. It is geographically isolated from I. souleyetianus , whose northernmost occurrence is in central Bahia state (Fig. 4 ), and in a different hydrographic region (Eastern Northeast Atlantic region). As such, we reinstate I. pilsbryi as an accepted species.

Idiopyrgus walkeri was described from Minas Gerais state and its type specimen could not be located in the ANSP collection during the present study. By its description and published illustrations ( Pilsbry 1924, reproduced here as Fig. 1E ), it has a more conical shell and wider and more convex whorls than I. souleyetianus (being of similar size to it), alongside a shorter spire with fewer whorls. It inhabits the São Francisco hydrographic region (Fig. 4 ), but on a different area from where I. souleyetanus is known. In absence of type material, I. walkeri is here considered potentially distinct from I. souleyetanus , but it is regarded as a taxon inquirendum until the types come to light or topotypes are collected. Three specimens collected from caves in Coribe municipality, southern Bahia state (Fig. 4 ), have wider shells and could represent specimens of I. walkeri .

Idiopyrgus rudolphi (Fig. 1F, G ) is the most immediately diagnosable species, as its shell has a straighter spire profile, almost subulinid-like, with whorls only slightly convex in profile. This feature was deemed sufficient by Haas (1938) to describe the genus Hydracme to house this species. Considering the morphological variation known in the shell shape of truncatelloids, we prefer to follow the most conservative approach for now and to maintain Hydracme as a synonym of Idiopyrgus until molecular studies on I. rudolphi can be conducted and its position in the group’s phylogeny assessed. I. rudolphi is known only from its type locality in Pernambuco state ( São Francisco hydrographic region), seemingly isolated from I. souleyetianus to the south and I. pilsbryi to the north (Fig. 4 ).

The type material of Idiopyrgus brasiliensis , despite being databased in the MZSP collection, could not be located during the present study. According to the species description and published illustrations ( Rey 1959, reproduced here as Fig. 1H ), this species is very similar in shell shape and size to I. souleyetianus , but it has a taller body whorl, a more vertically positioned aperture, and a less reflected lip. It is also geographically isolated from other Idiopyrgus spp. (Fig. 3 ). Therefore, we reinstate it as an accepted species. It inhabits Mato Grosso do Sul state (Paraguay hydrographic region) and, given its occurrence in Corumbá municipality ( Rey 1959) close to the border, it should likely be found in Bolivia and perhaps Paraguay as well. The record from Mato Grosso state ( Simone 2006; Birckolz et al. 2016) is in all likelihood a misinterpretation of the records of Rey (1959), since back then the state of Mato Grosso still contained the area that later became Mato Grosso do Sul state. The genus Aquidauania , which was erected to house I. brasiliensis (erroneously described as belonging to Oncomelania Gredler, 1881, an Asian genus; Rey 1959), is therefore a synonym of Idiopyrgus ¸ as already indicated by Simone (2006).

The two new species described herein, I. adamanteus sp. nov. and I. minor sp. nov., are from Bahia state, belonging to the Middle São Francisco hydrographic region (Fig. 4 ). Idiopyrgus adamanteus sp. nov. is apparently isolated from I. souleyetianus in the south and I. rudolphi in the north (Fig. 4 ). The distribution of I. minor sp. nov., however, overlaps with I. souleyetianus and potentially with I. walkeri as well (Fig. 4 ). It is thus tempting to consider I. souleyetianus and/or I. walkeri as widespread species whose isolated populations in caves might have given rise to new species such as I. minor sp. nov.

Figures

Figure 1. Shells of Idiopyrgus spp. A, B. I. souleyetianus Pilsbry, 1911, holotype ANSP 100534; C, D. I. pilsbryi Baker, 1914, lectotype ANSP 130700; E. I. walkeri Pilsbry, 1924, reproduced from Pilsbry (1924: p. 6, fig. 9); F, G. I. rudolphi (Hass, 1938), holotype SMF 24365; H. I. brasiliensis (Rey, 1959), reproduced from Rey (1959: p. 145, fig. 2). Scale bars: 1 mm.

Figure 2. Shells of the two new species of Idiopyrgus. A-H. Idiopyrgus adamanteus sp. nov.; A, B. Holotype MZSP 158237; C. Paratype LES 0027975; D. Paratype LES 0027975; E. LES 0027975 (from Gruta da Machadinha); F. LES 0027978 (from Toca de Candeias); G. LES 0027977 (from Toca de Candeias); H. LES 0027978 (from Toca de Candeias); I-K. Idiopyrgus minor sp. nov.; I, J. Holotype MZSP 158238; K. Paratype LES 0027980 (from Gruna do Engrunado). Scale bars: 1 mm

Figure 3. Bayesian inference phylogenetic tree based on CO 1, 16 S, ITS 2 and 28 S, highlighting the family Tomichiidae and the position of Idiopyrgus. The posterior probabilities are shown on the nodes; scale bar is substitutions per site.

Figure 4. Map of Brazil showing state borders, hydrographic regions, and occurrence of Idiopyrgus spp. Black squares represent the known records of each species, while the red circles represent specimens of uncertain identification (potentially representing new species) here labeled Idiopyrgus sp.; the red square indicates the area where the specimen used in the phylogeny was collected.