Microphreatus Czaja & Estrada-Rodríguez, 2025

Czaja, Alexander, Cardoza-Martínez, Gabriel Fernando, Becerra-López, Jorge Luis, Estrada-Rodríguez, José Luis, Alonzo-Rojo, Fernando, Ávila-Rodríguez, Verónica & Valenzuela-García, Ana Alejandra, 2025, World’s smallest freshwater snail? A new genus and species of subterranean snail (Gastropoda, Cochliopidae) with extremely tiny shell from Los Chorros, Coahuila, northern Mexico, Zootaxa 5660 (3), pp. 413-425 : 414-415

Microphreatus Czaja & Estrada-Rodríguez gen. nov.

Type species. Microphreatus saltillensis sp. nov., by original designation.

Other species included as comb. nov. into the new genus is Balconorbis coronae ( Hershler, 1987) (see below).

Diagnosis. Shells very small, mostly less than 1 mm in diameter (width), with up to 2.5 tubular whorls and near-planispiral to low-trochoid in form, protoconch extends the length of a whorl (1.25 whorls), sculptured with wrinkled pits, teleoconch whorls with slightly zigzag-shaped spiral lines which cross thin, weakly, irregularly distributed collabral lines; both lines start following the transition to teleoconch, teleoconch with a few collabral ribs which varying from low riblets or varices (by M. saltillensis sp. nov.) to strong lamelliform costae (by M. coronae comb. nov.), aperture near-circular, inner lip often adnate to body whorl above, peristome flared (trumpet-like), umbilicus wide.

Differential diagnosis. Shells with similar characteristics to our material were only observed in the subterranean genus Balconorbis Hershler & Longley 1986 , which includes two subterranean species from United States and Mexico. Nevertheless, the both genera differ in shell morphology by a combination of morphological and structural shell wall characteristics with diagnostic importance. These differences are summarized in Table 1.

Remarks. In addition to the different general shell shape, size and number of whorls ( Figs. 4A–H), it is mainly the protoconch and teleoconch microstructure that allows the shells of both genera to be distinguished from each other. The protoconch of shells of Microphreatus are wrinkled with deep pits and without spiral lines ( Fig. 3G, H), while in Balconorbis it is smooth to slightly pitted and the spiral lines reach almost to the apex. In addition, the strong spiral and the rounded axial lines on Balconorbis shells produce a characteristic cancellate pattern ( Fig. 5E, F). Such sculpture is not developed on teleoconch of shells of both Microphreatus species, where the collabral lines are hardly developed or, if at all, only very irregularly distributed on the teleoconch ( Fig. 5A, B, E, F).

Both Microphreatus species have either well-developed costae ( M. coronae , Fig. 4C, D), or at least clearly visible low varices on the teleoconch ( M. saltillensis sp. nov., Fig. 5B, C, D). At these points the spiral lines appear interrupted and offset in their course ( Figs. 3E, F, 5A–D). Such shell sculpture in form of costae or varices does not occur on the teleoconch of shells of both Balconorbis species. Other diagnostic difference between both genera includes the raised (or even free and uncoiled) apex by shells of Microphreatus ( Fig. 4A, C), meanwhile both Balconorbis species always have planispiral shells with a sunken apex ( Fig. 4E–G).

Etymology. The name is derived from Greek word micro (μικρός) = small, and phreato = groundwater environment where the snails live.

Distribution. Disjunct, with one species living in Val Verde County, southern Texas, United States and other in Los Chorros, Coahuila, Mexico (see below).