This species was described and named by Pilsbry (1926) as Physa zionis, and this name was used by Jones (1940a) as well as Gregg (1940) and Ng and Barnes (1986). Chamberlin and Jones (1929) elevated the subgenus that Pilsbry (1926) had established solely for this species to generic status and thus arranged this species as Petrophysa zionis, a name also used by Chamberlin and Roscoe (1948).
Ng (1983), Ng and Barnes (1986), and Whipple (1987) called this species the Zion snail. Clarke (1991) referred to it as the Zion tadpole snail.
No subspecies have been proposed in this species; however, Pilsbry (1926) commented that "further collecting, keeping the shells of each colony separate, might possibly show that there are recognizable racial differences between snails of the more widely isolated colonies."
Status in Utah
This species is entirely endemic to 2 connected canyons, Zion Canyon and Orderville Canyon, along the North Fork of the Virgin River in Zion National Park, Washington County, Utah, a linear stretch of about 3.1 mi.
Ng and Barnes (1986) commented that this species "... has probably never existed in large numbers, and, in comparison to other snails, it may be considered rare." However, Clarke (1991), based on field work conducted in 1990, reported: "Estimated population sizes In [sic] Zion Canyon varied from about 200 snails (in one seep along the Gateway to the Narrows Trail) to about 250,000 snails in Dipper Seep and Cattail Seep (which are continuous and considered here as a single seep). A huge population of P. zionis, with about 5 to 10 million snails, was found along the south side of Orderville Canyon from its mouth to the third waterfall, a distance of about 0.8 miles." However, it should be noted that Clarke's estimates sometimes are inordinately high (see, for example, account for Oreohelix eurekensis).
Although some natural mortality factors are known (e.g., predation, see Ng 1983, Whipple 1987) and others have been speculated (freezing, floods, and rock slides, see Whipple 1987), it is doubtful that such factors represent important threats to the continued survival of this species.
Chamberlin and Jones (1929) wrote: "Mr. [A. M.] Woodbury reported that [the cliff seeps near the type locality and upstream] had been stripped of snails by collectors on previous occasions, but that in a few days migration from above had soon renewed the supply."
Clarke (1991) has discussed some potential threats that would jeopardize the existence of the species: "Since P. zionis is not yet listed as Endangered, there is no legal restriction against the construction of new walkways (which might entail blasting of the cliffs on which P. zionis lives), dewatering of the area east of the Virgin River and south of Orderville Canyon (P. zionis depends on seeps there for its survival), or other activities which might be planned to accomodate [sic] increasing numbers of visitors to Zion National Park."
Population trend in this species is not known; likely it is stable.
Inventory elsewhere in Zion National Park or in surrounding areas, where springs or seeps are present along canyon walls, may be of value.
Habitats Utilized in Utah
This species inhabits seeps and "hanging gardens", mainly on the vertical sandstone walls of the narrow canyons through which the North Fork of the Virgin River flows (Pilsbry 1926, Ng and Barnes 1986). These wet canyon walls are covered with algae (Pilsbry 1926), and the "hanging gardens" are composed of such plants as maidenhair ferns, cardinal flowers, and columbines (Whipple 1987). Gregg (1940) found several colonies of this species on "[w]et faces of cliffs" and one colony "on horizontal surfaces of large flat rocks at the base of the cliff as well as on the perpendicular surface of the cliff."