Balconorbis sabinasense Czaja, Cardoza-Martinez & 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 : 94-95

Balconorbis sabinasense Czaja, Cardoza-Martinez & Estrada-Rodriguez sp. n. Figures 12-18

Type locality.

MEXICO, Coahuila state, Don Martín Basin, Sabinas River, Las Adjuntas (Rancho San Carlos): 101°11'41"W, 27°53'33"N, 354 m a.s.l. (Fig. 1).

Type material.

Holotype (Figs 12-14), UJMC 410, from type locality, leg. Cecilio Arreola Chapa, 30/xi/2018. Paratypes, UJMC 411, CNMO 7901, from the same lot, 11 specimens.

Etymology.

Derived from the name of Sabinas River, Coahuila, where the type locality is situated.

Referred material.

COAHUILA. Don Martín Basin, Sabinas River, UJMC 412, Ejido Santa María, Y. A. Sanchez-Montañez, 03/vii/2018. UJMC 413, Ejido Paso del Coyote, Cecilio Arreola Chapa, 30/xi/2018. UJMC 414, Sabinas (Agua Prieta), Y. A. Sanchez-Montañez, 08/vii/2018. UJMC 415, La Vega, Y. A. Sanchez-Montañez, 08/vii/2018.

Diagnosis.

Shell minute, less than 2 mm in diameter (width), planispiral, apertural plane not or only slightly inclined, with 3¾ or less whorls, bodywhorl keeled, with spiral lines and axial growth lines, protoconch smooth.

Differential diagnosis.

According to Hershler and Longley (1986), the characteristic spiral structure (Figs 16-18) of the shells distinguish the genus Balconorbis clearly from other subterranean littoridinines. Shells of the new species differs from B. uvaldensis by larger shells (1.51-1.81 mm by B. sabinasense sp. n. versus 0.90-1.22 mm by B. uvaldensis ) with weaker suture, more whorls, smooth to slightly pitted protoconch without spiral lines, and characteristic keels on the bodywhorl. Similar planispiral shells of the genus Phreatodrobia ( P. micra Hershler & Longley, 1986, P. plana Hershler & Longley, 1986 and P. rotunda Hershler & Longley, 1986) are considerable smaller, lack the keels on the bodywhorl and do not have the characteristic regularly-spaced spiral lines and axial growth lines.

Description.

Shell minute, planispiral, width 1.51-1.81 mm, height 0.42-0.61 mm, with 3¼–3¾ whorls; protoconch smooth to slightly pitted (Fig. 16), hidden in ventral view, has 1¼ whorls, first teleoconch whorl with strong and elevated axial growth lines which cross the spiral lines producing a square pattern (Figs 17, 18), about 80 elevated spiral lines are present on the bodywhorl, bodywhorl with one or two keels, one keel usually stronger; aperture rounded to ovate, apertural plane only slightly inclined from the columellar axis (Fig. 13); inner peristome is fused to the penultimate whorl, outer lip strong and advanced relative to the remaining peristome; shells transparent or whitish.

Shell measurements

(mean ± standard deviation in parentheses; n = 11): SH 0.53 (0.07) mm, SW 1.68 (0.10) mm, AH 0.59 (0.05) mm, AW 0.49 (0.08) mm, WN 3.5 (0.19), aperture plane inclination relative to shell axis = 22°-31°. Paratypes from the type locality.

Measurements of holotype.

SH 0.55 mm, SW 1.58 mm, AH 0.59 mm, AW 0.58 mm, HBW 0.50 mm, WN 3½, aperture plane inclination relative to shell axis = 25°.

Habitat.

Similar to Phreatomascogos gregoi sp. n.

Distribution. Known from the type locality and various sites among the Sabinas River between the localities Ejido Santa María and La Vega. Within the type locality and most other sites the new species occurs sympatrically to Phreatomascogos gregoi sp. n.

Remarks.

Almost all the members of Cochliopidae family with planispiral shells are subterranean forms. Freshwater gastropods with similar shell characteristics can be found only in the genus Balconorbis which is monotypic with B. uvaldensis in the Edwards (Balcones Fault Zone) Aquifer in Uvalde County, Texas, United State. Our new species resembles B. uvaldensis but differs from this by having larger shells, smooth protoconch, one more whorl and a bodywhorl with one or two more or less strong keels and by much closer, compressed set of the whorls resulting in weaker suture and larger columellar peristome. The later feature is unique in the genus Balconorbis and could indicate the possible position of the new species in a new geographically isolated genus. These conchological differences confirm that the new species is morphologically diagnosable and distinct to shells of the snails from Texas. It is the first record of the genus Balconorbis in Mexico.