Stygopyrgus variabilis Perez & Saenz, 2025

Stygopyrgus variabilis Perez & Saenz, 2025 sp. nov.

Figs 4 A, B View Figure 4 , 5 A, E View Figure 5 , 6 A View Figure 6 , 7 A – C View Figure 7

Type locality.

USA, Texas, Brewster County, Lower Canyons of the Rio Grande, hyporheic zone of Rio Grande below Las Palmas Spring # 5 , (29.76829, - 102.57352, WGS 84) GoogleMaps .

Material examined.

All sites are in Texas, USA. Holotype and Paratypes – Brewster County • Las Palmas Spring # 5 , drift net sample, collected by B. Schwartz, K. E. Perez, R. Winton, B. Hutchins, A. Cottrell, A. Sovie, A. Cressler, and B. Tobin, (29.76829, - 102.57352), 22 April 2022, ( ANSP 506748 About ANSP , ANSP 506749 About ANSP , ABC -000901 ) GoogleMaps .

Additional material examined.

– All sites are in Texas, USA. All sample collection through the joint effort of: B. Schwartz, K. E. Perez, R. Winton, B. Hutchins, A. Cottrell, A. Sovie, A. Cressler, and B. Tobin. Brewster County, Beaver Spring , drift net sample (29.77079, - 102.57088) 22 April 2022 ( ABC -000898 ) GoogleMaps ; • Las Palmas Spring # 1 , hyporheic sample (29.74829, - 102.6075), 19 April 2022, ( USNM) GoogleMaps ; • Son of Hot Springs , drift net sample (29.74846, - 102.54075), 21 April 2022, ( ABC -000900 ) GoogleMaps ; • Las Yeguas Spring , drift net sample (29.76233, - 102.57348), 22 April 2022, ( ABC -000903 ) GoogleMaps ; • Asa Jones Spring Complex , hyporheic sample (29.76833, - 102.56351), 20 April 2022, ( ABC -000902 ) GoogleMaps ; • Rio Grande below Las Palmas 5 and Las Yeguas Rapid , hyporheic sample (29.76916, - 102.57192), 20 April 2022, ( ABC -000904 ) GoogleMaps ; • Unnamed spring, downstream of Las Palmas 5 , drift net sample (29.76884, - 102.57198), 22 April 2022, ( ABC -000905 ) GoogleMaps ; • Spigot Spring , drift net sample (29.76719, - 102.56244), 23 April 2022, ( ABC -000906 ) GoogleMaps ; • Las Palmas 10 Spring , drift net sample (29.74693, - 102.54345), 23 April 2022, ( ABC -000899 ) GoogleMaps .

Diagnosis.

Minute shell, some individuals with prominent keeled appearance, with distinctive spiral and longitudinal lirae sculpture on teleoconch that extends to the sutures. Stygopyrgus variabilis differs from related species in the region by shell shape, and sculpture. Some populations (e. g. Son of Hot Springs, Rio Grande River, Brewster County) have a shorter, more ovate, and less heavily sculptured shell with fewer keeled individuals. A closely related species, Stygopyrgus gracilis sp. nov., has a turriform shape with transverse growth lines and faint spiral raised lines that are not present in Stygopyrgus variabilis . Stygopyrgus bartonensis has similar sculpture that is finer, without keels, and with spiral and longitudinal striations rather than lirae. Penis of Stygopyrgus variabilis lacks the papillae or apocrine glands of other Stygopyrgus and Texapyrgus species. Cusps of central radula tooth more sharply pointed than other Stygopyrgus species.

Description.

Shell minute, translucent to opaque, heavily sculptured, elongate ovate-conic with 1–3 prominent keel outlines per whorl on some individuals, some individuals with only a single weak keel most prominent on the body whorl or none (Fig. 4 A, B View Figure 4 , Suppl. material 1: fig. 1 J – R). First whorl of protoconch is elevated, separated from subsequent whorls (Fig. 5 A View Figure 5 ). Protoconch surface heavily sculptured by punctum that form irregularly shaped pits or wrinkles. Teleoconch sculpture includes irregularly spaced raised lateral lirae and more elevated longitudinal lirae dissected by prominent spiral keels. Aperture ovate, slightly pulled away from body whorl, only lightly touching body whorl at parietal corner. Lip reflected on basal and umbilical portions in larger individuals. Outer lip straight, simple, umbilicus open but partially obscured by reflected lip. Keels (when present) extend to the edge of the outer apertural lip. Operculum ovate, extremely thin, translucent, light brown with darker brown region at nucleus, submarginal nucleus, distinct oval muscle attachment, slightly raised on inner surface. Average shell measurements for adults (n = 20) from Las Palmas Springs 1 & 5 and Beaver Springs: shell height = 1.35 mm (SD = 0.08), shell width = 0.68 mm (SD = 0.04), aperture height = 0.45 mm (SD = 0.03), aperture width = 0.40 mm (SD = 0.03), number of whorls = 4.75 (SD = 0.20).

Body unpigmented. Snout nontapered, slight distal lobation. Foot short, rounded, without lateral wings. Cephalic tentacle tapered, rounded, unpigmented, no visible cilia. Mantle tissue unpigmented. No visible eyes or pigmented patches at base of eyestalks. Ctenidium with about 12–15 triangular lobes. Rectum ends near edge of mantle on right side of head, intestine straight and uncoiled with elongate oval fecal pellets. Esophagus enters stomach below, stomach speckled with scattered dark flecks of pigment. Penis large, tapering, attached behind right eye, with expanded base and irregularly ridged along the proximal 2 / 3, narrows before the “ arrowhead ” like terminal portion. The tip tapers with two rounded lobes near the distal end. No apocrine glands or papillae observed. In preserved specimen, penis has distinct curve forming a “ fish-hook ” shape.

Central radular tooth trapezoidal with rounded dorsal edge (Fig. 7 A View Figure 7 ) deeply curved; lateral margin thickened, lateral cusps 4–5 on each side; central cusp about 1 / 3 longer than adjacent cusps but similar in shape with an elongate oval shape, tapering at the end and at the base, one pair of basal cusps pointed, with small buttress, broadly conical, not needle-like, basal tongue v-shaped, medium deep basal socket. Face of lateral tooth rectangular, tooth curved, narrowing upon reaching the outer wing; outer wing tapering; central cusp slightly longer than lateral cusps, 5–6 cusps outer and 4–5 cusps inner direction, decreasing in size distally (Fig. 7 B View Figure 7 ). Inner marginal teeth with broad outer wing, 23–24 cusps visible, longer in length toward the central cusps, outermost cusps shorter, pointed, wide at base, cutting edge extends less than 25 % of the length of the tooth, lateral wing present. Outer marginal teeth broad and slightly curved at end, with 9–11 cusps. Cusps along inner edge longer; tooth face tapering to outer wing (Fig. 7 B, C View Figure 7 ).

Taxonomic remarks.

Both mitochondrial and nuclear phylogenies place this new species close to Stygopyrgus bartonensis with 11.1 % p-distance and with 10.1 % p-distance from Stygopyrgus gracilis . However, the branch support is only moderate for placement in Stygopyrgus . In addition, these species share a minute, elongately conic shell outline, pitted or malleated protoconch sculpture, and teleoconch sculpture including regular, raised spiral lines and collabral growth lines. These sculptural features are larger and more pronounced in Stygopyrgus variabilis . Stygopyrgus variabilis share aspects of penial morphology with S. bartonensis with a relatively slender penis tapering with a muscular base but lacks papillae. This combination of DNA and morphological features seems to situate this species squarely in the genus Stygopyrgus , but sufficiently merit species status. Beaver Spring specimens were 100 % keeled individuals, with both male and female individuals keeled. Other populations such as the type locality, Rio Grande at Las Palmas Spring 1, ~ 5 % are unkeeled, 20 % have one keel on the lower whorls only.

Etymology.

The name variabilis was chosen to reflect the highly variable nature of shell sculpture of this species from heavily keeled to nearly smooth.

Ecology and habitat.

The Lower Canyons Reach of the Rio Grande Wild and Scenic River is a remote section of the Rio Grande / Rio Bravo del Norte that forms the international border between the US and Mexico. This section of the Rio Grande is downstream from Big Bend National Park in a region of deep canyons through Chihuahuan Desert habitat, accessible only by multiple-day canoe excursion. Flow in the river includes some minor input from upstream, depending on the season, but is mostly maintained by groundwater input from numerous karst springs (some are thermal) discharging from the Edwards-Trinity Aquifer System ( Smith Trevizo 2004; Brauch 2012). The Las Palmas spring group discharges from the Glen Rose and Del Carmen Formations ( Brauch 2012) between river miles 739.6–735.8. Stygopyrgus variabilis was found in springs or hyporheic samples with water chemistry values ranging from: 25.79–32.26 ° C, pH from 7.22–8.78, conductivity from 471.7–986.6 μS / cm, and dissolved oxygen from 3.7–10.8 mg / L.

The springs and hyporheic zone of the Lower Canyons of the Rio Grande are a region of high invertebrate diversity, including both spring-associated and groundwater-associated (stygobiotic) taxa. Species found with Stygopyrgus variabilis included nematodes and freshwater annelida, crustaceans including ostracods, cyclopoid and harpaticoid copepods, Texanobathynella sp. ( Parabathynellidae ), Ingolfiellida, the amphipods Paraholsingerius sp. , Paramexiweckelia sp. , Parabogidiella sp. , Simplexia sp. , and Seborgia sp. , the isopods Speocirolana hardeni , Texicerberus amistad , Texicerberus schotteae , Thermosphaeroma subequalum , and Tethysbaena texana ( Thermosbaenacea ). Apart from Thermosphaeroma subequalum (associated with thermal freshwater springs) and the ostracods and copepods of unknown ecology, the crustaceans present are considered stygobionts. Other molluscs present included Physidae and Sphaeridae .