Microphreatus saltillensis Czaja & Cardoza-Martínez, 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 : 415-419

Microphreatus saltillensis Czaja & Cardoza-Martínez sp. nov.

The Chorros cavesnail

Figures 3A–F

Type locality.A groundwater fed spring (Manantial) inside the recreation park “Los Chorros”, Sierra de Zapalinamé , municipality of Arteaga , ca. 8 km southwest of the town of Arteaga, near the city of Saltillo, Coahuila, Mexico (25°22′54.39′′N, 100°47′17.38′′W, 1,914 m a.s.l) ( Fig. 2A) .

Type material. Holotype ( Fig. 3B–D), UJMC 628 , from type locality, leg. Gabriel Fernando Cardoza-Martínez, 30/xi/2018 . Paratypes, UJMC 629-633 b, from the same lot, 11 specimens .

Material Examined. Holotype and Paratypes – Groundwater fed spring (Manantial) inside the recreation park “Los Chorros”, Sierra de Zapalinamé , collected by Gabriel Fernando Cardoza-Martínez, 15 July 2024, and site 2 (25°22′50.01′′N, 100°47′19.10′′W), a narrow trickle of 40–50 cm wide, are located approximately 150 meters downstream, collected by Ricardo Marco Ávila Hernández, 05 December 2024, both sites ca. 8 km southwest of the town of Arteaga , Coahuila, Mexico .

Etymology. The species is named for its occurrence near the city of Saltillo, Coahuila, Mexico.

Diagnosis. Shells very small, less than 0.8 mm in diameter (width), with up to 2.3 tubular whorls, near-planispiral to low-trochoid, protoconch extends the length of a whorl, sculptured with wrinkled pits, teleoconch whorls with slightly undulated 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, aperture near-circular, inner lip often adnate to body whorl, peristome flared, umbilicus wide.

Description. Shell minute, ranging from 0.68 mm to 0.79 mm in diameter and 0.31 mm to 0.41 mm in height, white, near planispiral or (mostly) depressed-trochoid in form ( Fig. 3D), deeply impressed sutures, the spire is mostly raised up, protoconch of 1.25 whorls, sculptured with wrinkled pits ( Fig. 3G, H); diameter of protoconch about 280μm, diameter of nucleus about 90μm (holotype), the body whorl prominent, teleoconch whorls with slightly zigzag-shaped spiral lines which cross thin, and irregularly distributed collabral lines ( Fig. 5A, B), both lines start following the distinct transition to teleoconch ( Fig. 3H), distance between the spiral lines in average 15μm, lamelliform costae (like by M. coronae comb. nov.) absent, but always with about 13–20 faintly visible collabral riblets or varices ( Figs. 3E, F, 4A, B, 5C); aperture near-circular and prosocline, inner lip adnate to body whorl above, peristome slightly flared to trumpet-like, umbilicus deep and wide ( Figs. 3C, E, 4B, D).

Shell measurements (mean ± standard deviation in parentheses; n = 13): SH 0.36

(0.03) mm, SW 0.73 (0.03) mm, AH 0.30 (0.02) mm, AW 0.29 (0.03) mm, WN 2.3

(0.00), paratypes from the type locality.

Measurements of Holotype. SH 0.37 mm, SW 0.74 mm, AH 0.32 mm, AW 0.31 mm, WN 2.3.

Habitat. Like in many other sites with subterranean gastropods, no living individuals from the site Los Chorros could be obtained. However, the collected snails apparently died recently because the shells do not reveal any signs of external weathering. They were washed out from their subterranean habitat and deposited directly at the water outlet.

The new species lives in subterranean habitat (stygobitic). The water chemistry values at site 1 were as follows: temperature = 19.8 °C, pH = 7.00, conductivity = 1228 μS/cm, dissolved oxygen = 7.1 mg /L.

Distribution. Known from the type locality and one site in the immediate vicinity within the recreation park Los Chorros. The new species lives in subterranean habitat and appears to be endemic to the spring.

Conservation assessment. The NatureServe rank calculated for Microphreatus saltillensis sp. nov. is Critically Imperiled (N1).

Remarks. The new species can be distinguished from species of related subterranean genera like Balconorbis , Coahuilix or Phreatodrobia only using SEM images, as the fine shell wall and protoconch structures are not observable in light microscopic images ( Fig. 3B–D). Microphreatus saltillensis sp. nov. is distinguished from M. coronae by considerably smaller shells (SW 0.68–0.79 mm versus 1.04–1.17 mm), lack of lamelliform costae ( Fig. 3B, C), raised, but never uncoiled protoconch ( Fig. 3F–H) and significantly weaker spiral lines on the teleoconch ( Fig. 4A–D).

The size of a snail is usually determined by the height of the shell, or, in the case of planispiral shells, by its width (diameter). Microphreatus saltillensis sp. nov., with a diameter of 0.73 mm (on average), is therefore probably the world’s smallest described freshwater snail, only slightly smaller than Hadziella forneri Quiñonero-Salgado, López-Soriano & Rolán, 2023 ( Hydrobiidae ) from Spain, which is 0.77 mm wide ( Quiñonero-Salgado et al., 2023).

Other sympatric snails from site 1 belong to the genera Physella , Ferrissia , Chorrobius , Pyrgulopsis , Gyraulus and Galba .