Stygopyrgus gracilis Perez, Saenz & Gonzalez, 2025

Perez, Kathryn E., Saenz, Vanessa, Guerrero, Yamileth, Gonzalez, Lisa, Guerrero, Evan, Diaz, Pete, Hutchins, Benjamin T. & Schwartz, Benjamin F., 2025, New and revised groundwater snails (Mollusca, Caenogastropoda, Cochliopidae) from karst and associated hyporheic habitats in western Texas and northern Mexico, Subterranean Biology 50, pp. 119-151 : 119-151

Stygopyrgus gracilis Perez, Saenz & Gonzalez, 2025 sp. nov.

Figs 4 C , 5 B, F , 6 B , 7 B

Diagnosis.

Shell minute, elongate, turriform, nearly smooth on both protoconch and teleoconch with few fine spiral lines and longitudinal growth lines. Penis slender with single, blunt, elongate papillae on inner curve near mid-length and sharply tapering distal end.

Type locality.

USA, Texas, Presidio County, Big Bend Ranch State Park, Fresno Creek, hyporheic zone (29.74829, - 102.6075) .

Material examined.

Holotype and Paratypes – USA, Texas, Presidio County • Big Bend Ranch State Park, Fresno Creek , hyporheic sample, collected by K. E. Perez, A. Cottrell, L. Pustka, (29.30064, - 103.84596), 5 October 2021 ( ANSP 506744 , ANSP 506745 ) .

Additional material examined.

– USA, Texas, Presidio County • Big Bend Ranch State Park, Fresno Creek , hyporheic sample, collected by K. E. Perez, B. Schwartz, B. Hutchins (29.30064, - 103.84596), 10 December 2021 ( ABC -000919 ) .

Description.

Shell minute, transparent, thin, nearly smooth, turriform, lightly sculptured. Spire finely tapering. Sutures deeply incised, growing deeper towards the body whorl, until aperture is separated from body whorl. Fine transverse lines along occasional whorls of teleoconch. Fine collabral growth lines at regular intervals on last two whorls. Sutures contain irregular indentations increasing in quantity approaching apex. Body whorl and aperture separated from previous whorl in adults. First whorl of protoconch elevated. Aperture ovate, lip thin, slightly reflected at base of aperture in adults, and not thickened. Aperture tilted forward. Umbilicus a wide slit due to separation of aperture from body whorl. Average shell measurements for adults (n = 8): shell height = 1.32 mm (SD = 0.14), shell width = 0.52 mm (SD = 0.06), aperture height = 0.38 mm (SD = 0.04), aperture width = 0.30 mm (SD = 0.03), number of whorls = 5.34 (SD = 0.55).

Body unpigmented, snout rounded, slightly tapered, with rounded distal lobes. Tentacles short, squat, tapered with no eyes or eye patches visible. Foot short, anterior portion rounded, without lateral wings. Ctenidium, when present, a series of 10 + triangular lobes found through the pallial roof, not present in some individuals. Intestine curves in “ s ” shape through pallial cavity, with rectum ending near edge of mantle. Intestine lined with short oval fecal pellets. Operculum extremely thin, lightly pigmented amber, nucleus submarginal, edge rounded, distinct muscle attachment scar. No apparent thickened or raised portion on inner surface. Penis very long, thin, and tapering, hooked at the distal end in preserved specimens. Proximal half of the penis length with shallow folds, one papilla present about 2 / 3 of way towards distal end. Female reproductive anatomy not described due to a lack of female specimens.

Central radula tooth trapezoidal. Central cusp of central radular tooth oval with rounded edge (Fig. 7 D ); lateral cusps 5–6 on each side; central cusp about 1 / 3 longer than adjacent cusps with elongate oval shape, lateral cusps become less rounded and more pointed distally, tapering at the end. Single pair of basal cusps conical, pointed, singular basal cusps pointed, with small buttress. Basal tongue broadly v-shaped. Face of lateral tooth rectangular, narrowing upon reaching the outer wing; outer wing tapering; central cusp slightly longer than lateral cusps, 6–7 cusps outer and 5 cusps inner direction, decreasing in size distally. Inner marginal teeth with broad outer wing, no basal notch, 22–23 cusps visible, similar in length, inner cusps slightly longer, middle cusps slightly wider at base, last 2–3 cusps shorter than the rest. Outer marginal teeth broad and curved at end, cusps 9–11. Middle cusps longer, fingerlike (Fig. 7 E, F ).

Taxonomic remarks.

Both mitochondrial and nuclear gene phylogenies place this new species near Stygopyrgus bartonensis (9.6 % p-distance) and with 10.1 % p-distance from Stygopyrgus variabilis . However, the branch support is only moderate for placement in Stygopyrgus . In addition, these species share a minute shell, elongately conic shell outline, and teleoconch sculpture including raised spiral lines and collabral growth lines. These sculptural features are greatly reduced in Stygopyrgus gracilis . Stygopyrgus gracilis share aspects of penial morphology with S. bartonensis with a slender penis tapering but with a single instead of two papillae and a more elongate and sharply tapering distal end. This combination of DNA and morphological features seems to situate this species squarely in the genus Stygopyrgus but is sufficiently distinctive to merit species status.

Etymology.

The name “ gracilis ” was chosen to reflect the elongate, slender form of the shell.

Ecology and habitat.

Fresno Creek watershed contains a mostly dry gravelly desert stream channel with intermittent reaches of permanent water, flowing southward through Fresno Canyon on the west side of the Solitario in Big Bend Ranch State Park ( Rush 1960). Fresno Creek joins the Rio Grande / Rio Bravo ~ 2.3 km below the type locality. Upstream reaches of Fresno Creek receive small amounts of water from springs in the Chorro Canyon tributary, where water emerges from volcanic rock layers. The eastern side of the basin contains extensive, karstified limestones of the Solitario and Terlingua monocline. Small springs along Fresno Creek itself, and groundwater underflow discharging into the extensive gravels forming the creek bed, also contribute to flows. Stygopyrgus gracilis was found in hyporheic samples collected from gravel deposits in a short perennial reach where a bedrock exposure in the streambed forces subsurface flows in upstream gravels to reach the surface. Water chemistry values averaging 24.89 ° C, pH 7.95, conductivity of 1071 μS / cm, and dissolved oxygen of 5.37 mg / L.

Fresno Creek fauna included nematodes, freshwater mites, freshwater Annelida, and crustaceans (ostracods, cyclopoid and harpaticoid copepods). Insects included Dytiscidae and Dryopidae .

Figures

Figure 4. SEM of shells A shells of Stygopyrgus variabilis sp. nov., Las Palmas 5, Lower Canyons of the Rio Grande, Brewster Co., TX. ANSP 506748 B Las Palmas 1, Lower Canyons of the Rio Grande, Brewster Co., TX C shell of Stygopyrgus gracilis sp. nov., Fresno Creek, Fresno Canyon, Presidio County, TX. ANSP 506744 D Texapyrgus diaboli, comb. nov., lectotype of Tryonia diaboli, ANSP 91726, Drift of Devils River, Val Verde County, TX E Big Satan Canyon, Devils River, Val Verde County, TX F Phreatodrobia embossa sp. nov., Caroline Springs, Terrell County, TX. ANSP 506746. Scale bars: 500 µm (A – C, F); 300 µm (D, E).

Figure 5. SEM of protoconch sculpture A – D & teleoconch sculpture E – I A, E Stygopyrgus variabilis sp. nov. Individual from Las Palmas 1, Lower Canyons of the Rio Grande, Brewster County, TX B, F Stygopyrgus gracilis sp. nov. Individual from Fresno Canyon, Presidio County, TX C, G Texapyrgus diaboli comb. nov. Individual from Big Satan Canyon, Devils River, Val Verde County, TX D, H Phreatodrobia embossa sp. nov. Individual from Caroline Springs, Terrell County, TX I Balconorbis coronae comb. nov. Individual from Indian Springs Canyon Springs, Val Verde County, TX. Scale bars: 100 µm (A – E); 50 µm (E – I).

Figure 6. Dorsal view of the penis of the three new species A Stygopyrgus variabilis B Stygopyrgus gracilis C Phreatodrobia embossa. Yellow color is due to Bouin’s solution for dissection, preserved tissues are white. Scale bars: 100 µm.

Figure 7. SEM of radula (A – F) of new Stygopyrgus species A – C Stygopyrgus variabilis, Las Palmas 1, Lower Canyons of the Rio Grande, Brewster County, TX D – F Stygopyrgus gracilis, Fresno Canyon, Presidio County, TX. Scale bar radula: 1 µm.