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Range of Pristiloma crateris Pilsbry , 1946 ( Gastropoda : Pulmonata : Pristilomatidae ) in the United States Pacific Northwest

Extensive holdings of Pristiloma snails in the Oregon State Arthropod Collection were evaluated and reidentified as necessary. The study confirmed the distinctness of Pristiloma crateris from other species and delineated a range in Pacific Northwest National Forests, primarily along the western and eastern slopes of the Cascade Range in Oregon.

Originally described as a subspecies of the far northern species Pristiloma arcticum (Lehnert, 1884), Pristiloma crateris Pilsbry, 1946, has been independently recognized as a full species (Roth 2011;Burke 2013). The criteria for this taxonomic ranking are reviewed herein. The taxon is considered a Special-Status Species by the USDA PNW Forest Service and USDI Oregon/Washington Bureau of Land Management Interagency Special Status/Sensitive Species Program, and therefore of conservation interest. Examination of 191 samples previously identified as belonging to the genus Pristiloma Ancey, 1887, from the Oregon State Arthropod Collection, Oregon State University, Corvallis (hereinafter, OSAC), resulted in the identification of 104 samples of P. crateris and delineation of its range in Pacific Northwest National Forests.
Specimens were collected in the course of Forest Service and Bureau of Land Management regular surveys to determine plants and animals present in the lands under these agencies' stewardship. For surveys not specifically targeting Pristiloma, the collecting method was visual search in likely areas, such as zones of forest floor litter or around the bases of plants in wet meadows. Duncan et al. (2003: 20, 21) provided these special instructions for surveys focusing on P. crateris: During surveys of suitable wet habitat, search the undersides of woody debris, among wet mosses, rushes and other low vegetation at the edges of wetlands, springs, seeps and streams and in perennially damp forest floor litter, especially where it has accumulated at the bases of shrubs and against logs. Pick up a small handful of litter, vegetation or moss and examine both sides of each leaf or frond. Use a 10-15× hand lens to examine any object that might be a snail. Examination of material in bright light helps to make animals active and easier to detect. Care should be taken to avoid wind, even gentle air currents, that could cause shells to blow away while examining them.
The collected and sorted material was identified preliminarily by various workers and catalogued into the OSAC. Number of specimens per sample ranged from one to seven.
By far the majority of samples consisted of only the shells of the snails. A few had the contained animal soft parts. This was not a serious limitation, because shell characters are adequate to tell apart the species in question, except in poorly preserved or fragmentary material. This has also been the conclusion of previous authors, such as Pilsbry (1946: key, p. 396) and Thomas E. Burke (written communication, 2010). A single specimen (UMA04-008b) had the entire animal preserved with its genitalia in extruded position. This was informative as discussed below.
Because the thin, fragile shells of Pristiloma are not likely to survive extensive transport, I consider the presence of empty shells at a site to be presumptive evidence of an extant population at that locality.
Arrows indicate areas of difference in shell profile, described in text. According to this distinction, all but one "arcticumtype" Pristiloma in the samples examined in this study are P. crateris. Small juveniles may have a whorl profile that is widest above the middle, but their relatively shallow base (and the fact that they co-occur with undoubted adult P. crateris in some samples) makes their identification as P. crateris certain. In the course of identifying the many lots of P. crateris in this study, I came to think of them as resembling a child-size coated aspirin tabletan image that may help workers recognize the species and distinguish it from, for example, P. lansingi.

NOTES ON GEOGRAPHIC DISTRIBUTION
Pilsbry's (ibid.: fig. 214) illustration of a paratype of P arcticum from Point Barrow also shows a large, bulbous, first embryonic whorl, a feature not seen in any samples in this study. Forsyth's (2002;2004: 92) figure of a specimen collected in montane forest in the Babine Range, Skeena Mountains, British Columbia shows the same feature. It is also present in specimens from Attu, Aleutian Islands, Alaska (Roth and Lindberg 1981;specimens in California Academy of Sciences). I have not examined specimens from mainland Alaska or British Columbia, but if consistent, the large first embryonic whorl should be a good identifying character. Pilsbry (1946:401) gave the range of P. a. arcticum as Point Barrow, Alaska (the type locality) to Pierce County, Washington (Paradise Park and Longmire). Sample WEN01-001 of this study, from Kittitas County, Washington, is from slightly northeast of those localities. It is tempting to suggest that WEN01 001 and Pilsbry's Pierce County specimens are conspecific, although I have not seen shells of the latter. WEN01 001 does not have a bulbous first embryonic whorl and in that respect differs from the more northern shells of P. arcticum. The only illustration and description of the genitalia of P. arcticum is that by Baker (1931: pl. 15, figs. 1-3; reproduced by Pilsbry 1946 as figs. 211(1-3)), based on Paradise Park specimens. The anatomy of topotypic, Point Barrow, specimens of P. arcticum is unknown. Therefore, the only published figures purporting to be of P. arcticum may actually depict P. crateris. It is worth noting that specimen UMA04-008b of this study, from Umatilla County, Oregon, has a penial papilla similar to that described and illustrated by Baker (1930: pl. 15, fig. 3) for "arcticum." All specimens herein identified as P. crateris are consistent in form and deployed within a relatively continuous, delimited range. This is consistent with their representing "a separately evolving metapopulation lineage, where a metapopulation is an inclusive 240×. The most informative orientation of the shell is with the aperture on the right and opening toward the viewer, as in the images of Figure 1 above. To achieve this stable viewing position I examined all specimens in a small dish of black paraffin (a jar lid would serve adequately) in which I had impressed a groove 5 mm long and about 0.5 wide. Shell specimens were placed in this groove and adjusted with an artist's camel-hair brush until positioned correctly.
In the most recent monograph of the genus (Pilsbry 1946), Pristiloma arcticum crateris was described in the following text: The shell is imperforate, depressed, with quite low, conoid spire and rounded periphery, median in position; pinkish buff, glossy. Sculpture of weak but subregular ripples of growth below the suture, soon disappearing, leaving the peripheral region and base smooth except for very weak lines of growth; very fine, close spirals are seen on the upper surface. The whorls are regularly and rather closely coiled, the last not unduly wider. The aperture is narrowly crescentic, the outer and basal margins of the lip thin, columellar margin slightly spreading, thickened within, reflected at the insertion in a small callus over the axis. Height 1.5, diameter 2.75 mm; 5½ whorls. H./d. index about 54.5. * * * It is very similar to P. arcticum, but the base is more flattened, producing a less deeply concave basal lip and somewhat different shape of the aperture, and there is a fraction of a whorl more (Pilsbry 1946: 402-403).
The description was accompanied by a line illustration of the shell (ibid.: fig. 215; Figure 1). The description focuses entirely on shell characters. No anatomical detail was provided. The purported subspecies was contrasted with P. a. arcticum, which was described as follows and illustrated by line drawing (ibid.: fig. 214): "Shell imperforate, globose-depressed, most minutely striate, uniform tawny-brown, glossy. Whorls 5½ to 6, convex, very narrow, the last somewhat convex at base. Aperture depressed, lunar; peristome simple, acute, the basal margin arcuate. Width 2 mm., alt. 1.5 mm." (E. Lehnert.) * * * It is a glossy shell, with the general shape of P. lansingi; growth-striae faint, spire low-conic, whorls 4¾, slowly and regularly increasing, and last not disproportionately wide as in P. johnsoni, but about as in P. lansingi. Aperture narrowly crescentic. … The width of the spire a little exceeds two-thirds the greatest diameter of the shell (ibid.: 401-402).
Allowing for differences in authors' word-choice, the main differences between these descriptions, borne out by the illustrations, duplicated below, lie in the profile of the whorl and aperture. In P. arcticum the widest part of the shell occurs high on the whorl. Below this point (called the "periphery" by Pilsbry and other authors) the whorl slopes steeply to the convex base (Figure 1, right). In P. crateris, the widest part of the shell occurs near the middle of the whorl ("median," in Pilsbry's description) and the slope of the shell below the periphery is more rounded, leading to a less deeply "dished" base (Figure 1, left).
Check List | www.biotaxa.org/cl Volume 11 | Number 2 | Article 1571 3 Roth | Pristiloma crateris in Pacific Northwest, USA population made up of a set of connected subpopulations and a lineage is a population extended through time or an ancestral-descendant series of time-limited (instantaneous) populations" as in the definition of a species by de Queiroz (2005).
At the present time, we have insufficient information to demonstrate intergradation between P. crateris of this study and P. arcticum from elsewhere (which would be suggestive of a subspecific relationship), or identity of the reproductive systems of undoubted P. arcticum from the North Slope of Alaska with those of specimens from the Pacific Northwest. In the absence of such additional evidence demonstrating a subspecific relationship, the proper course is to recognize two different species: Pristiloma arcticum and Pristiloma crateris.
Of 191 samples examined, 104 were identified as Pristiloma crateris; they are listed in Table 1 by their OSAC tracking numbers, county of occurrence in Oregon, UTM coordinates (on the nad27 datum), collectors' names, and dates of collection. Figure 2 is a map showing the localities plotted on a base map of Oregon state boundaries.
Prior to the present study, the range of P. crateris had been stated in the literature only in general terms (e.g., Burke 2013: 259: "P. crateris occurs in the Oregon Cascade  Mountains in mid-to higher elevation wet meadows and riparian areas.").
All sampling locations yielding P. crateris were contained within a bounding box extending from 45.4822° N, 122.9308° W (utm Zone 10: 505411e 5036524n) to 42.3120° N, 119.6161° W (utm Zone 10: 778892e 4689962n). The type locality of the species, "one mile south of Crater Lake, Klamath County, Oregon" (Pilsbry 1946: 403) is imprecise by today's standards but can be estimated as approximately 42.89° N, 122.13° W, within the perimeters of the range shown by this study, and near the northern end of the southernmost group of localities shown on Figure 2.