Hidden Snails of Texas and Mexico: Three New Groundwater Species Emerge from Deep Aquifers

Beneath the sun-baked karst of western Texas and northern Mexico, an entire world of blind, unpigmented snails has been quietly evolving in the dark. A new study published in the journal Subterranean Biology has formally described three species of groundwater snails that are new to science, while reshuffling the family tree of several others that have puzzled malacologists for more than a century. The work, led by researchers examining the freshwater gastropod family Cochliopidae, dramatically expands what is known about the hidden biodiversity of the Edwards-Trinity Aquifer System, one of North America’s most important but least visible ecosystems.

The Cochliopidae are a family of minute to small freshwater and estuarine snails comprising more than 250 species across 48 genera, distributed across the Nearctic, Neotropical, and Afrotropical realms. In the karst and hyporheic habitats of western Texas and northern Mexico alone, 11 genera and 27 described species of these phreatic snails have been documented. The Edwards-Trinity Aquifer System, or ETAS, is a complex transboundary aquifer extending from Texas into the Mexican state of Coahuila, where discrete groundwater watersheds sit within geologically contiguous limestone formations. The snails living inside these aquifers display the classic suite of subterranean adaptations: extremely small bodies, a complete lack of pigment, no eyes, and simplified or reduced organs such as missing ctenidia, the gills used by their surface-dwelling relatives.

What makes these snails especially fascinating is the paradox of their shells. While their soft tissues follow a conservative, morphologically static evolutionary pattern, their shells range wildly in form, from flat and depressed to tall and tower-shaped, from tightly coiled to half-coiled or even uncoiled, and adorned with ribs, keels, crosshatched fine lines, or rows of beads. Because no well-resolved phylogeny of the family existed, scientists could not determine whether these striking shell features had evolved once or repeatedly as separate lineages colonized underground habitats. The new study tackles that uncertainty head-on by combining newly generated mitochondrial and nuclear DNA sequences with detailed examination of shell shape, shell sculpture, and soft anatomy.

Collecting animals that live inside solid rock is no small feat. The research team sampled phreatic habitats by suspending 100-micron mesh drift nets over spring orifices and flowing wells for one to fourteen days, letting the current deliver snails to them. In some wells, they deployed an ingeniously simple bottle trap baited with pistachios. For the hyporheic zone, the saturated gravels beneath and alongside rivers where groundwater and surface water mix, they used a Bou-Rouch pump, installing a temporary well to a depth of 30 to 50 centimeters and pumping roughly 12 liters of water through a fine mesh net. Specimens were preserved in 95 percent alcohol, refrigerated, and later picked from detritus under magnification, with water chemistry recorded in the field using a multiparameter sonde.

The genetic work involved amplifying the mitochondrial cytochrome c oxidase subunit I gene, or COI, and the nuclear large subunit ribosomal DNA, then sequencing and aligning them alongside existing GenBank data. The team ran careful quality checks, including tests of substitution saturation in COI, which revealed that the third codon position was saturated while the first and second positions showed little saturation. Bayesian phylogenetic analyses were conducted in BEAST, with evolutionary models selected for each partition of the data. Across the COI alignment, pairwise genetic distances averaged 3.8 percent among species and 17.2 percent among genera, providing a useful yardstick for judging whether snail populations represent distinct species.

The first of the three new species, Stygopyrgus variabilis, was discovered in the Lower Canyons of the Rio Grande in Brewster County, Texas, a remote stretch of the river accessible only by multi-day canoe trips through Chihuahuan Desert canyons. The species lives up to its name: some populations are entirely composed of prominently keeled individuals, while others contain mostly smooth-shelled snails, a level of variability that would have confounded any classification based on shells alone. The minute, translucent shells measure about 1.35 millimeters in height and bear distinctive spiral and longitudinal lirae. Genetic distances of roughly 10 to 11 percent from its closest relatives, combined with differences in penial morphology and radular structure, confirmed its status as a separate species.

The second new species, Stygopyrgus gracilis, extends the known range of the genus dramatically westward. It was found in the hyporheic gravels of Fresno Creek in Big Bend Ranch State Park, in Presidio County, where a bedrock exposure in the streambed forces subsurface flows to the surface. Its nearly smooth, turriform shell contrasts with the heavily sculptured shells of its relatives, and it can be distinguished by a slender penis bearing a single blunt papilla. The discovery is significant because it shows that groundwater snail diversity does not stop at the boundaries of the Edwards-Trinity Aquifer System itself; adjacent karstic groundwater habitats can harbor important elements of the same evolutionary radiation.

Perhaps the most taxonomically dramatic result concerns a snail first collected in 1906. Tryonia diaboli was described from dry, bleached shells found in drift debris along the Devils River, and then vanished from the scientific literature for more than a century. When the team sequenced DNA from smooth individuals matching the old description and from highly sculptured individuals resembling Texapyrgus longleyi, a species described from the same river reach in 1991, they found something remarkable: the two morphological forms were not reciprocally monophyletic. They belong to a single species, and that species is not a member of Tryonia at all. The researchers synonymized the two names and reassigned the species as Texapyrgus diaboli, a new combination that finally gives this century-old enigma a proper home.

A similar reevaluation befell Phreatodrobia coronae, a snail from the Devils River and San Felipe Springs whose DNA consistently placed it outside the genus Phreatodrobia in both mitochondrial and nuclear analyses. Lacking strong enough phylogenetic support to confidently erect a new genus, the authors took the conservative route of transferring the species to Balconorbis, creating the new combination Balconorbis coronae. The third new species, Phreatodrobia embossa, was found in the hyporheic gravels of Caroline Springs at The Nature Conservancy’s Independence Creek Preserve in Terrell County. Its glassy shell carries a unique sculpture of regularly spaced rows of raised nodules, resembling embossed designs on leather, and its genetic distance from true Phreatodrobia species, 17.5 percent, is close to the average distance between genera in the family, hinting that future work may warrant yet another reassignment.

Beyond the novelty of new names, the study carries a sober conservation message. Many of these snails are known from single spring systems or short river reaches, and several already carry threat ranks ranging from Vulnerable to Endangered, with at least two Mexican species considered probably or possibly extinct as springs dry up. Groundwater pumping, development, and aridification threaten the fragile aquifers and hyporheic zones these animals inhabit. The authors emphasize that bridging the knowledge gap will require integrating research across international, geological, and ecological boundaries, particularly in the poorly sampled Mexican portion of the aquifer system. As DNA sequencing techniques improve and sampling expands into unexplored karst, the hidden snail fauna of North America’s aquifers may prove far richer, and far more imperiled, than anyone imagined.

Subject of Research: Taxonomy and phylogenetics of groundwater snails in karst and hyporheic habitats of western Texas and northern Mexico

Article Title: New and revised groundwater snails ( Mollusca , Caenogastropoda , Cochliopidae ) from karst and associated hyporheic habitats in western Texas and northern Mexico

Article References: Perez, K. E., Saenz, V., Guerrero, Y., Gonzalez, L., Guerrero, E., Diaz, P., Hutchins, B. T., & Schwartz, B. 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, 119-151. https://doi.org/10.3897/subtbiol.50.138174

Image Credits: AI Generated

DOI: 10.3897/subtbiol.50.138174

Keywords: Cochliopidae, groundwater snails, Edwards-Trinity Aquifer System, karst, hyporheic zone, new species, Texas, Coahuila, phylogenetics, stygobionts, taxonomy, biodiversity