Phreatodrobia spica Perez & Alvear, 2020

Alvear, Dominique, Diaz, Peter H., Gibson, J. Randy, Jones, Mary & Perez, Kathryn E., 2020, An unusually sculptured new species of Phreatodrobia Hershler & Longley (Mollusca: Caenogastropoda: Cochliopidae) from central Texas, Zootaxa 4810 (1), pp. 143-152 : 148-151

Phreatodrobia spica Perez & Alvear , n. sp.

Fig. 3 A–L; Fig. 4 A–D

Diagnosis. A trochiform cavesnail with irregular, wide-based spikes and spiral lirae over the entire teleoconch, and having a large, ovate aperture with a reflected lip.

Types. Holotype ANSP 1446 -A480669, paratypes ANSP 1448 -A480670–1449-A480668 all from Comal Springs , Comal County, Texas.

Type locality. Spring orifices feeding Spring Run 3, Comal Springs , Comal County, Texas .

Etymology. We use the specific epithet “spica” from the Latin for “spine” describing the shell sculpture having many narrowly tapering pustules or spines.

Remarks. We propose the descriptive common name the “spiky cavesnail”.

Description. Trochiform shell with rounded whorls and nearly gibbous body whorl. Protoconch (1.5 whorls) irregularly pitted ( Fig. 4B, D ). First non-embryonic whorl marked by raised, irregular spiral lirae and beginning of sculpture, including spikes and wide-based, pointed pustules. Spiral lirae extend to body whorl weakly, but irregular spikes and pustules increase in density and size extending to the lip with occasional sparse areas. Pustulose sculpture is highly unusual in hydrobioids ( Hershler & Ponder 1998) and is a notable unique feature of P. spica . Spiral lirae more visible when shell is dry. Recognizable growth lines on body whorl and wrinkles following the direction of body whorl. Impressed suture, with growth lines continuing to suture edge; spikes do not continue to suture.

Aperture usually slightly touching body whorl, although sometimes completely detached in adults. Ovate aperture with reflected lip, tilting anteriorly. Variable degree of lip reflection with some individuals having a completely reflected and somewhat thickened lip and others only reflected on the inner lip ( Figures 3 , 4 ), lip does not reflect in juveniles. Some large individuals have a groove followed by a collabral ridge or varix behind aperture. Shell umbilicate, with reflected lip partially obscuring umbilicus in adults.

Shell colorless to light yellow. Juveniles translucent, less translucent with maturity but never fully opaque. Operculum amber, thin, slightly convex, ovate, paucispiral, with submarginal nucleus and thickening on ventral surface coming to a distinct point. Columellar edge of operculum nearly straight but operculum rounded elsewhere coming to a slightly elongated upper edge.

Eyes not visible. Tissues unpigmented. Penis approximately length of snout (approximate due to variable tissue contraction with alcohol preservation) with lobes along the inner curvature, not coiled as in P. nugax . Intestine fills much of the pallial cavity with an open coiled, S-shaped bend, larger but similar to that figured for P. conica ( Hershler & Longley 1986b, Fig. 13E). Ctenidium stretches about halfway across the pallial cavity with the osphradium opposite midway. Osphradium occupies about 20% of pallial cavity length. Intestine sometimes filled with oval, orange-colored fecal pellets.

Twenty-one measured adult individuals from the type locality had an average shell height of 1.19 mm (range: 1.37– 0.74 mm), average shell width of 0.98 mm (range: 1.24– 0.49 mm), average aperture height 0.67 mm (range: 0.90– 0.40 mm), average aperture width 0.59 mm (range: 0.91– 0.36 mm), average number of all whorls 3.17 (range: 3.50–2.50). The shell is nearly as wide as it is tall with the average width ~83% of the average height. The aperture is very large, comprising 60% of the total shell width and 56% of height.

Habitat and Distribution. Phreatodrobia spica is known from samples taken at three localities along the southern edge of the Edwards and Edwards-Trinity aquifers ( Figure 1 ). The type locality is the waters emerging from spring orifices feeding into Spring Run #3, Comal Springs (Comal County), the largest springs in Texas ( Brune 1981) and home to numerous endangered aquatic species. The northernmost site where this species has been observed is Tahuaya Springs (Bell County). And finally, the westernmost record is Finegan Springs (Val Verde County) issuing from the east bank of the Devils River. Subterranean fauna associated with samples collected at each site include numerous species of isopods ( Lirceolus spp.), amphipods ( Stygobromus spp.), ostracods, mites ( Almuerzothyas sp.), and interstitial crustaceans ( Texanobathynella bowmani and Hobbsinella edwardensis ).

Phreatodrobia spica appears to be uncommon, although it is found across a wide distribution for a stygobitic snail, which tend to be narrow-range endemics. This distribution is similar to those of P. micra and P. nugax which also are found across the Edwards-Trinity Aquifer System. We examined the samples from a recent article, ( Alvear et al. 2020) that presented new records for phreatic snails from 80+ localities and did not find P. spica in additional localities. Less than 100 individuals of this species were encountered in thousands of hours of sampling with only a single individual at Finegan Springs, a site with very dense sampling.

Comparison with other Phreatodrobia from the region. The prominent sculptural features of P. spica usually readily distinguish it from other known phreatic snails of Texas. These include slender, sharp, and needle-like spikes, and tapered, blister-like pustules, however, all shells can probably be worn smooth over time. Therefore, the combination of shell shape and sculptural features are required to distinguish P. spica shells from the known phreatic snails of the region.

Phreatodrobia spica has a trochiform shell, slightly taller than wide, with a large, rounded aperture. Species with a very depressed shell (wider than tall) such as Balconorbis uvaldensis , P. plana , P. rotunda , and P. micra are readily distinguished. Texapyrgus longleyi , Tryonia diaboli , and Stygopyrgus bartonensis are distinguished by their elongate or conical shells (much taller than wide). Phreatoceras taylori has an uncoiled (trumpet shaped) shell. Phreatodrobia coronae has a distinctive horn-like protoconch. Phreatodrobia imitata and P. punctata are similar in shape, but with a much smaller body whorl compared to P. spica as well as distinctive sculpture and aperture features.

Two species of Phreatodrobia have potential for misidentification with P. spica . As currently circumscribed, P. nugax has a highly variable shell; very tall (elevated) P. nugax could be difficult to identify based on shell shape alone and would require reference to shell sculpture. Phreatodrobia conica has a shape similar to P. spica but with a minimally reflected lip and narrower body whorl.

Figures

FIGURE 1. Map of Texas with collection localities for Phreatodrobia spica n. sp. and additional Phreatodrobia spp. presented in phylogeny. Karstic regions highlighted in gray are drawn from US Karst Map dataset (Weary & Doctor 2014). Counties are also illustrated. Inset map of North America with the study area indicated, created by Benjamin Hutchins.

FIGURE 3. Shells of Phreatodrobia spica n. sp. A–C. holotype, Comal Springs, ANSP 1446-A480669, height=1.32 mm, width=0.79 mm. D–E. Comal Springs, ANSP 1449-A480668, height=1.26 mm. F. Tahuaya Springs, ANSP 1423-A480671, height=1.43 mm. G–H. Finegan Springs, height = 1.35 mm. I–J. Comal Springs individual, operculum 0.72 mm top to bottom, operculum exterior. J. operculum ventral. K–L. Comal Springs, bodies ~1.5 mm long. K. Body ventral. L. Body dorsal. A–C are in the same scale. D–H are in the same scale.

FIGURE 4. SEM of shells of Phreatodrobia spica n. sp. A, B. Comal Springs, ANSP 1446-A480669. C. D. Comal Springs, ANSP 1446-A480669.