Phreatodrobia bulla Perez & Castaneda, 2023

Perez, Kathryn E., Guerrero, Yamileth, Castaneda, Roel, Diaz, Peter H., Gibson, Randy, Schwartz, Benjamin & Hutchins, Benjamin T., 2023, Two new phreatic snails (Mollusca, Caenogastropoda, Cochliopidae) from the Edwards and Edwards-Trinity aquifers, Texas, Subterranean Biology 47, pp. 1-27 : 1

Phreatodrobia bulla Perez & Castaneda sp. nov.

Figs 7 , 8

Phreatodrobia cf imitata Perez et al., 2020, pp. 7.

Phreatodrobia conica Gibson et al., 2021b, pp. 33.

Type locality.

USA, Texas, Bell County, Hidden Springs (30.9382, -97.6044).

Material examined.

All sites are in Texas, USA. Holotype and Paratypes - Bell County, Hidden Springs, collected by Peter Diaz (30.9382, -97.6044), 27 October 2021 (ANSP 494658, 494660) .

Additional material examined.

- Bell County, Salado Springs Complex , Anderson Spring (30.9441, -97.5347); Stagecoach Inn Cave , Salado (30.9432, -97.5375), 1 May 2020, P. Diaz (ABC 005618); Copperhead Spring Cave, Ft. Cavazos (confidential location); Bent Oak Spring (30.8916, -97.7092), 17 August 2022 (ABC 005616); Gault Archaeological Site Spring (30.8916, -97.7095), 8 June 2019 (ABC 005615); Robertson Springs, Creek Springs (30.9445, -97.5413); Solana Ranch Spring (30.8997, -97.6390), 25 March 2020 (ABC 005620), P. Diaz; Spicewood Creek Pipe Spring (confidential location); Spring 23-398, Ft. Cavazos (confidential location); Camp Tahuaya, Tahuaya Spring Pool (31.0087, -97.5093). - Williamson County, PC Spring (30.4818, -97.7419), 23 March 2023 (ABC 005617) .

Diagnosis.

Shell translucent, conical, with nearly smooth teleoconch, dome-shaped protoconch with wrinkles. Aperture round to slightly ovate, usually separated from body whorl in adults. Mantle tissue white, unpigmented. Sharply pointed median cusp of central radular teeth with small basal cusp. Penis long, equal width most of the length, tapering sharply near tip, loosely to tightly coiled, length 2-3 times length of snout.

Description.

Shell translucent, usually pale tan, conical with 3.5-4.5 well rounded whorls (Fig. 7A-D ). Shell height ranges from 1.1-2.39 mm. Average shell measurements (n = 14 adult individuals): height = 1.86 mm (SD = 0.5), width = 1.22 mm (SD = 0.3), aperture height = 0.78 mm (SD = 0.2), aperture width = 0.77 mm (SD = 0.2), number of whorls = 4.0 (SD = 0.5). Sutures deeply impressed giving whorls a very rounded aspect. Body whorl wider than others, which taper steeply to a dome-shaped embryonic whorl. Spire with distinctive “bubble” or dome shape. Dome-like embryonic whorl sculptured with irregular granules and wrinkles (Fig. 7E-G ), teleoconch smooth, without visible sculpture, except under high-magnification. Regular growth lines visible on recent shells. In most individuals, aperture fully detached from previous whorl (appressed only at top of aperture in some smaller individuals). Aperture ovate, with simple, prosocline, reflected lip that flares at base. Umbilicus present.

Operculum round to broadly ovate, concave, amber in color, deeply concave, with narrow band of thinner material on outer margin, tapering to a point but without nuclear peg (Fig. 7H ). Opercular growth lines vague, simple. Nucleus slightly eccentric, central, paucispiral. Muscle attachment scar distinct and thickened toward edges, with undifferentiated edges. Attachment region callus thin.

Body visible through translucent shell. No eyes present. Ctenidium composed of triangular filaments, approximately as broad as high, stretching from posterior portion of pallial cavity nearly to mantle edge. Osphradium oval shaped, elongate, positioned opposite posterior portion of ctenidium, occupying ~25% of pallial cavity. Pallial portion of intestine with loops in posterior portion of pallial cavity similar to P. imitata . Fecal pellets in the coiled intestine usually clearly visible through the shell, bright orange, oval-elongate in shape (Fig. 7I ). Rectum ends just before mantle edge.

Snout narrow, longer than wide, deeply lobate distally, with folds along sides. Tentacles tapered, with scattered granules or pigmentation at base, length equal to snout. No eye visible. Foot rounded anteriorly, with lateral wings. Penis base well behind right tentacle, slightly wider and with deeper folds near base, tapering quickly to a consistent length until sharply tapering at tip. Slight folds continue until midway along penis length. Penis long, loosely to tightly coiled, curved and 2-3 times longer than snout (Fig. 7J ).

Central radular tooth with deeply indented dorsal edge; central cusp longer than adjacent cusps; lateral cusps 5-6 on each side, evenly decreasing in width towards tip, sharply pointed; basal cusps small, triangular; basal socket deep, v-shaped. Lateral tooth rectangular, with a longer central cusp and 4 (inner) - 7 (outer) cusps on either side. Some laterals with wide deposit down mid-line. Base of lateral tooth with triangular, well excavated ventral process, tapering to wing. Inner marginal teeth with ~25-30 cusps, similar in length, decreasing slightly in outermost cusps. Tooth surface tapering towards outer wing with narrow neck before flaring smoothly towards the base. Outer marginal teeth rounded, spoon-shaped, wide at top, smoothly curving, with 12-20 small cusps, tapering slightly to short neck.

Etymology.

We use the specific epithet " Phreatodrobia bulla " from the latin for “bubble”, referring to the rounded appearance of each whorl, particularly the rounded spire. We propose the common name “Brown’s cave snail" in honor of Mr. Tim Brown, a Bell County native and former Commissioner who has worked extensively to promote conservation of archaeological and groundwater resources in the region.

Ecology.

This new species is part of a diverse aquifer community. Relative abundance varies across the range of the species. At Creek Springs (part of the Robertson Springs Complex), as many as 200 snails can be captured over a couple of days of drift net collection, whereas at PC Springs, similar sampling effort yields only one or a few specimens. Phreatodrobia bulla sp. nov. is often collected with other phreatic snails: P. nugax , P. micra (Pilsbry & Ferriss, 1906), and Phreatoceras taylori ( Diaz and Warren 2018). Depending on site, P. bulla sp. nov. may also occur with several crustaceans Lirceolus sp., Stygobromus bakeri Gibson et al. 2021, Parabogidiella americana Holsinger, 1980, and undescribed Bathynellacea and Microcerberidae . They also occur with the federally threatened Salado salamander ( E. chisholmensis Chippindale, Price, Wiens & Hillis, 2000) and Jollyville salamander ( E. tonkawae Chippindale, Price, Wiens & Hillis, 2000).

Habitat.

All known localities for Phreatodrobia bulla sp. nov. are springs or hyporheic samples taken near springs discharging from the northern segment of the karstic Edwards Aquifer (north of the Colorado River). The northern segment lies adjacent to, but is disconnected from, the Barton Springs segment of the Edwards Aquifer, with the Colorado River a topographic low that forms the boundary between the two segments. Faults, erosion, and other geologic and geomorphic factors in the northern segment have resulted in groundwater basins that are relatively smaller and more dissected than in the Barton Springs and San Antonio segments to the south ( Jones 2003). More and smaller springs in the region, combined with relatively intensive sampling at many of those springs, are likely factors contributing to the higher number of known occurrences for this species, and it is likely that additional sampling in Bell, Williamson, and northern Travis Counties will result in more localities, particularly in the 45km gap between PC Spring (the southernmost location) and Kings Garden Spring. In particular, very little hyporheic sampling has been performed along streams and rivers in the region and this has been a productive method for sampling groundwater snails in other parts of Texas. Nevertheless, for the same reasons discussed for V. lillianae gen. et. sp. nov., it is unlikely that additional work will substantially expand the known range of P. bulla sp. nov.

Currently, P. bulla sp. nov., is known from 12 sites across a range of approximately 680 km2. Occurrence at multiples sites provides some security against catastrophic events (redundancy, sensu Shaffer and Stein 2000). Nevertheless, Bell and Williamson Counties are among the fastest growing counties in Texas, resulting in substantial pressure on groundwater resources. Eleven of the 12 locations occur within the Clearwater Underground Conservation District, which is tasked with developing and implementing a groundwater management plan for the Edwards and Trinity aquifers in Bell County ( Clearwater Underground Water Conservation District 2020). The desired future condition adopted by the conservation district, which provides a basis for some permitting and regulation of groundwater extraction, is preservation of a minimum acceptable springflow of 1.66 cfs at the Salado Springs complex (which includes Anderson and Creek springs) during hydraulic conditions equal to the 1950s drought of record. That is approximately 10% of average flows during the 1980s ( Brune 1995). Currently, several spring orifices in the region go dry during drought periods ( Diaz et al. 2015), illustrating that groundwater availability is the central conservation concern for P. bulla sp. nov. The sites where this species has been encountered are restricted to springs and spring-run hyporheic habitats, with sampling of wells or caverns in Bell and Williamson counties needed to determine its’ full extent. Without quantifying the severity and scope of threats, P. bulla sp. nov. is ranked as imperiled (G2S2) using NatureServe methodology.

Taxonomic remarks.

Intraspecific and interspecific sequence divergence averaged 2.45% and 10.73%, respectively, in our dataset of Texas phreatic snails. Phreatodrobia bulla has an average sequence divergence of 10.34% with the other members of Phreatodrobia and Phreatoceras , and 6% divergence with its sister Phreatoceras taylori . Interspecific variability in COI has been examined in several groups of subterranean hydrobioid gastropods inhabiting karstic environments. In Belgrandiella A. J. Wagner, 1928 “species” ( Hydrobiidae ) COI divergence ranged from 5.2-9.9% ( Jaszczyńska et al. 2022). An analysis of Bythinella Moquin-Tandon, 1856 ( Bythinellidae Locard, 1893) from a karstic region of France, which included epigean and cave species, found that maximum species-level divergence was 1.5% ( Bichain et al. 2008). In Kerkia Radoman, 1978 (West Balkans), a group of snails that resembles Phreatodrobia in habitat and morphology, interspecific genetic divergence ranged from 4.2%-14.7% ( Hofman et al. 2022) and similar values were found in Balkanica Georgiev, 2011 and related lineages ( Hydrobiidae , 7.8%-11.8%) in Bulgaria ( Osikowski et al. 2017). Thus, gene flow seems to vary by group and may be relatively low within some taxa or high, possibly facilitated by movement through routes such as the hyporheic or phreatic rhizosphere ( Haase et al. 2021). While there is not a molecular ruler denoting species-level distinction among subterranean species, Phreatodrobia bulla has sequence divergence comparable to other species of Phreatodrobia and greater than average species level divergence relative to most subterranean gastropods.

Lacking circumscription, Phreatodrobia bulla has been previously identified in recent literature ( Alvear et al. 2020a; Gibson et al. 2021b) as P. conica or P. cf imitata as it resembles these species in some aspects of shell morphology, the basis for those identifications. When we consider internal anatomy or DNA, these species are diagnosably different from P. bulla . The shell of Phreatodrobia conica is described ( Hershler and Longley 1986b) as having a simple aperture and a varix (ridge behind the aperture marking previous aperture position) near the end of the body whorl. It also has a distinctive teleoconch sculpture with numerous ridges. Its internal anatomy is distinguished by the lack of a ctenidium and a square-shaped central radular tooth. Phreatodrobia bulla in contrast has a flared and reflected aperture in adults with no sign of a varix in any material examined. The teleoconch sculpture is smooth without ridges and with a few collabral growth lines near the aperture, however, these are not elevated as described in P. conica . Finally, P. bulla has a robust ctenidium and the usual V-shaped central radular tooth both in contrast to what is described for P. conica. Access to the type locality of P. conica has not been possible during this study, preventing collection of tissues for DNA data collection. Even in the absence of DNA data, however, the anatomical distinctions between these species are sufficient to describe P. bulla as distinct from P. conica .

Phreatodrobia imitata and P. bulla share the same general shell shape and highly flared aperture (Fig. 9 ). However, the shells are readily distinguished. Phreatodrobia imitata has a translucent or clear shell which is heavily sculptured shell with collabral costae (ribs) and spiral lines (running opposite the ribs) while the teleoconch of P. bulla is unsculptured. Even though these sculptural features appear to consistently distinguish P. imitata and P. bulla , sculptural characters alone are insufficient to distinguish these species as ribs are polymorphic among Phreatodrobia and other freshwater snails. There are more pronounced differences found in the radula and DNA. The central radular tooth of P. imitata has a very narrow central cusp, 6-7 cusps on either side, and it lacks a basal cusp. The central radular tooth of P. bulla has a wider central cusp, ~ 5 cusps on either side, and it possesses a distinct basal cusp. We obtained P. imitata individuals for DNA analysis from Aldridge Well near the type locality ( Verstræten Well), and both COI and LSU phylogenies have strong support for placement of P. imitata as the sister lineage to other species of Phreatodrobia , not part of the P. bulla clade.

Previous classification efforts have not determined the placement of Phreatodrobia and Phreatoceras within a subfamily of Cochliopidae ( Hershler and Thompson 1992; Liu et al. 2001). They are both found in the Edwards and Edwards-Trinity Aquifers and share features such as a minute, colorless, translucent shell and pitted protoconch microsculpture ( Hershler and Longley 1986b; Hershler and Longley 1986a). Phreatoceras is diagnosed primarily by its unique, uncoiled, horn-like shell (see Fig. 3 ), and some features that are shared with various Phreatodrobia species such as a smooth teleoconch, loss of ctenidium, long central cusp of the central radular tooth. In the COI phylogeny, P. bulla is found sister to Phreatoceras taylori from the same springs in Bell County, Texas, but with weak support, and both are embedded within a clade of Phreatodrobia species. When nine anatomical characteristics of the two genera are compared (Table 1 ), P. bulla shares one distinctive characteristic with Phreatoceras , four characteristics with both genera, and five characteristics with Phreatodrobia . As this proposed new species shares more morphological characteristics with Phreatodrobia , is found by the DNA data within a clade of Phreatodrobia and does not have the distinctive trumpet shaped shell of Phreatoceras , we place it in Phreatodrobia .

We propose two potential explanations for the sister relationship of P. bulla and Phreatoceras taylori . In this study, we have not sampled the type locality of Phreatoceras , so it is possible that snails with a trumpet-shaped shell that we sampled in Bell County are not Phreatoceras taylori sensu stricto, described from Real County, Texas, ~250 km distant. Alternatively, Phreatoceras may be better considered a morphologically divergent member of Phreatodrobia rather than a separate genus. We have observed other species of Phreatodrobia , such as P. nugax , with a loosely coiled or partially uncoiled shell in some individuals, lending some observational support to this possibility. Examination and sequencing of Phreatoceras taylori from the type locality, is needed to resolve its relationship to Phreatodrobia .

Figures

Figure 3. Highest likelihood trees resulting from maximum-likelihood analysis of COI alignment. Ultra-fast bootstrap values> 95 % are shown with black branches, 50 - 94 % with gray branches and <50 with dashed gray branches. Terminals are labeled with Genbank voucher numbers, species, and sampling locality for our focal taxa. Type localities are indicated with an *. The new species and select relatives are figured.

Figure 7. Shells and anatomical features of Phreatodrobia bulla sp. nov. All sites are in Texas A-C holotype, Hidden Springs, Bell County, ANSP 494658 D SEM of individual from Hidden Springs, Bell County ANSP 494660 E-G SEM of embryonic shell sculpture H SEM of operculum, inner surface I dorsal view of body J ventral view of penis K dorsal view of penis. Scale bars: 100 µm (A-E); 100 µm (J, K).

Figure 8. SEMs of radula of Phreatodrobia bulla sp. nov. Hidden Springs, Bell County A central and marginal radular teeth B portion of radular ribbon C lateral teeth. Scale bars: 2 µm.

Figure 9. Shells of the species described here compared to shells of similar species in the region. All localities are in Texas A Vitropyrgus lillianae gen. et sp. nov. Comal Springs, Comal County B Stygopyrgus bartonensis, Barton Springs, Travis County C Phreatodrobia imitata, Verstraeten Well, Bexar County D Pyrgophorus spinosus juvenile, San Marcos River, near Martindale, Guadalupe / Caldwell County Line E Phreatodrobia bulla sp. nov. PC Spring, Williamson County.