Phreatomascogos gregoi Czaja & Estrada-Rodriguez

Czaja, Alexander, Cardoza-Martinez, Gabriel Fernando, Meza-Sanchez, Iris Gabriela, Estrada-Rodriguez, Jose Luis, Saenz-Mata, Jorge, Becerra-Lopez, Jorge Luis, Romero-Mendez, Ulises, Estrada-Arellano, Josue Raymundo, Garza-Martinez, Miguel Angel & Paulin, Jose Antonio Davila, 2019, New genus, two new species and new records of subterranean freshwater snails (Caenogastropoda; Cochliopidae and Lithoglyphidae) from Coahuila and Durango, Northern Mexico, Subterranean Biology 29, pp. 89-102 : 92-93

Phreatomascogos gregoi Czaja & Estrada-Rodriguez sp. n. Figures 2-11

Type locality.

MEXICO, Coahuila state, Don Martín Basin, Álamos River, Ejido Paso del Coyote (101°15'19"W, 27°56'45"N, 369 m a.s.l.) (Fig. 1).

Types.

Holotype (Figs 2, 3), UJMC 400, from type locality, leg. Y. A. Sanchez-Montañez, 11/vii/2018. Paratypes, UJMC 401, CNMO 7900, from same lot, 16 dry shells.

Etymology.

Named after Dr. Jozef Grego, a well renowned Slovakian malacologist and specialist of subterranean snails.

Referred material.

COAHUILA. Don Martín Basin, Sabinas/ Álamos River, UJMC 402, Ejido Nacimiento de los Mascogos II, Y. A. Sanchez-Montañez, 03/vii/2018. UJMC 403, Ejido Santa María, Y. A. Sanchez-Montañez, 03/vii/2018. UJMC 404, Ejido Sauceda del Naranjo, Y. A. Sanchez-Montañez, 08/vii/2018. UJMC 405, Las Adjuntas (Rancho San Carlos), Y. A. Sanchez-Montañez, 08/vii/2018. UJMC 406, Sabinas (Agua Prieta), Y. A. Sanchez-Montañez, 08/vii/2018. UJMC 407, La Vega, Y. A. Sanchez-Montañez, 08/vii/2018.

Diagnosis.

The colorless, translucent to whitish shells are very small, valvatiform to low trochoid, umbilicate, with 3¾ or fewer rounded whorls; aperture near-circular, adnate to the bodywhorl; operculum paucispiral, strongly campanulate; teleoconch with bodywhorl which frequently has one or two, in some specimens winged, keels; umbilicus almost completely covered by a basal keel-like structure.

Description.

Shell small, valvatiform, varying in shape from (mostly) flat-trochoid to (rarely) low conical, height 0.65-0.99 mm (1 trochoid shell of 1.42 mm considerably higher, Fig. 9), width 1.22-1.54 mm; umbilicus almost completely covered by a basal keel of the bodywhorl (Fig. 8); protoconch smooth (Fig. 7); teleoconch with 3¼ or fewer rounded whorls with less prominent axial growth lines (Fig. 6), the border between protoconch and teleoconch approximately after 1.5 whorls, not well distinct (Fig. 7); teleoconch whorls occasionally nearly smooth, frequently with one or two, in some specimens winged, spiral keels (carinae) on the last two whorls, keels on the central and basal part of the bodywhorl, central keel usually more prominent (Figs 2, 4, 5), whorls strongly convex with deep sutures; the aperture is ovate and in almost all specimens angled adapically (Figs 2, 4), inclined 20-30° to the coiling axis; peristome thin, inner lip partly fused to the penultimate whorl; operculum (from five shells obtained by shell cleaning) extremely thin, 0.48-0.51 mm in diameter, near-circular, campanulate (Fig. 11), placed deeply behind the aperture, strongly, light amber colored, paucispiral with sub-central nucleus, nuclear region darker, with 2.5 whorls (Fig. 10).

Shell measurements

(mean ± standard deviation in parentheses; n = 16): SH 0.84 (0.09) mm, SW 1.36 (0.08) mm, AH 0.57 (0.07) mm, AW 0.55 (0.06) mm, WN 3.05 (0.19) whorls; HBW 0.72 (0.10) mm. Paratypes from the type locality.

Measurements of Holotype.

SH 0.86 mm; SW 1.38 mm; AH 0.52 mm; AW 0.56 mm, WN 3.00 whorls; HBW 0.73 mm.

Habitat.

Stygobiotic. Shells of Phreatomascogos gregoi n. sp. were collected on the shore of the river in sandy and clayed sediments with many gravels on the bottom. We suppose, the new species likely inhabits the interstitial waters within the water saturated underground gravel layer of the hyporheic zone (see below). Although the habitat of Phreatomascogos lays within one of the eight federal protected zones APRN (=Mexican Protected Natural Resource Areas), their habitat may be threatened (especially the sites Ejido Sauceda del Naranjo and Ejido San Juan de Sabinas) by local coal mining and agriculture.

Distribution.

Phreatomascogos gregoi sp. n. lives likely sympatrically with B. sabinasensis sp. n. (see below) in same subterranean, interstitial habitat. The species appears to be endemic to the Sabinas/ Álamos River, Don Martín Basin, between the upper basin and Venustiano Carranza dam.

Remarks.

The typical strong keels, the almost completely covered umbilicus structure and the strongly campanulate operculum are the most evident characteristics which differentiated the shell morphology of Phreatomascogos gregoi sp. n. from the shells of all other described subterranean gastropods. A North American subterranean species conchologically most closely related to our material is Phreatodrobia nugax Hershler & Longley, 1986 from South-Central Texas. This species shows similarities in size and general shell shape and in some details of the operculum structure (low conical shape). Nevertheless, they clearly differ by lack of the three mentioned shell features of Phreatomascogos gregoi sp. n. We considered especially the extreme campanulate operculum and the covered umbilicus of Phreatomascogos gregoi sp. n. as important apomorphic features that justify the erection of a new genus most likely within the Lithoglyphinae family. The mentioned conchological similarities between both Phreatomascogos from Coahuila and Phreatodrobia from Texas is the main reason of assigning the new genus to the family Lithoglyphinae (and not to the Cochliopidae ). However, the resemblance in the shell shape could be also result of an evolutional convergence. There should be also considered the below mentioned biogeographic considerations which indicate a phyletic affinity of both genera gathering within the same family. The Edwards-Trinity-Aquifer region, that hosts all eight Phreatodrobia species, is a transboundary aquifer Zone which shares the water resources of the states of Texas and Coahuila. The Sabinas River karst region, where Phreatomascogos gregoi sp. n. occurs, belong to this zone and is hydrogeologically the southwestern extension of this huge aquifer ( Boghici 2004; Sanchez et al. 2018). Biogeographically, both regions in Texas and Coahuila form a unit (province) where many subterranean forms radiated from a common epigean (?) ancestor (see also Hershler and Holsinger 1990). However, anatomic and molecular genetic data are needed to prove the hypothetical close relationship between both Lithoglyphidae genera.