PristilomaIdentifying the species of a genus of minute land snails

in the Pacific Northwest

by Barry Roth, Consultant745 Cole Street

San Francisco, CA 94117

Contents

1. Introduction

2. The snail shell

3. History and taxonomy of Pristiloma

4. Identifying Pristiloma species

5. Re-verification of OSAC specimens

6. Conclusions

1. Introduction

Goal: to review land snail specimens previously

identified as belonging to the genus Pristiloma Ancey, 1887, from the collections of the OSAC and to verify or re-identify them as needed. Of particular interest is the putative distinction between Pristiloma arcticum arcticum and what was known at the time as P. a. crateris.

Material: 191 samples previously identified as belonging to the

following taxa:

◦Pristiloma arcticum (Lehnert, 1884) (3 samples);◦P. arcticum arcticum (Lehnert, 1884) (19 samples);◦P. arcticum crateris Pilsbry, 1946 (98 samples); ◦P. arcticum n. ssp. (2 samples); ◦P. idahoense Pilsbry, 1902 (1 sample); ◦P. lansingi (Bland, 1875) (38 samples); ◦P. pilsbryi Vanatta, 1899 (1 sample); ◦P. sp. (20 samples); ◦P. stearnsii (Bland, 1875) (8 samples); ◦P. subrupicola (Dall in Packard, 1877) (1 sample).

Methods:

Specimens examined under stereoscopic microscope at magnifications of 60X, 120X, and occasionally 240X.

Orientation of the shell with the aperture on the right and opening toward the viewer:

For stable viewing position, specimens examined in a small dish of black paraffin in which was 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

Ref: Pilsbry (1946)

2. The snail shell

Practically everyone has an image in mind of what a snail looks like…

Ref: file photo

>30,000 species of terrestrial (land-dwelling) snails

In a variety of shapes and sizes

Ref: Kerney & Cameron (1979)

Dissected alimentary and reproductive systems of garden snail, Helix aspersa

Ref: Kerney & Cameron (1979)

Each rotation around the axis = 1 whorl:

Ref: Kerney & Cameron (1979)

Apical (top) view of snail shell

Ref: mod. after Kerney & Cameron (1979)

lip

Parts of a snail shell

Ref: Kerney & Cameron (1979)

umbilicus

Ref: Kerney & Cameron (1979)

umbilicus

umbilicus

Ref: Pilsbry (1946)

3. Pristiloma: History & taxonomy

Pristiloma

◦ A genus of the Northern Hemisphere.

◦ Member of family Pristilomatidae (but sometimes placed in family

Zonitidae).

◦ First species found and named in 1875.

◦ First recognized as a genus by Ancey, 1886.

◦ Approximately 16 species, most diverse in western North

America.

◦ Intensively studied by H. B. Baker (1931), followed by H. A. Pilsbry

(1946).

Pristiloma, subdivisions:

◦ H. B. Baker (1931):

◦ Subdivided genus into 3 subgenera and 5 “sections.”

◦ Based on characters of the reproductive system

(accessible only by dissection).

◦ Reproductive system characters not important in

identification of species.

◦ Species can be told apart by features of the shell.

4. Identifying Pristiloma species

First step:◦Does the shell have an umbilicus or not?◦PNW species that have an umbilicus:

P. chersinella P. wascoense

◦Species without an umbilicus: P. pilsbryi P. stearnsii P. lansingi P. idahoense P. crateris P. johnsoni (not present in project samples)

The two umbilicate species examined in project material:

P. chersinella – shell breadth 3-4 mm; range from southern Oregon (Klamath County) to southern Sierra Nevada, California.

P. wascoense – shell breadth about 2 mm; range southern Washington (Klickitat County and northern Oregon (Wasco County)

Pristiloma chersinella (a) and P. wascoense (b)

Ref: Pilsbry (1946)

Among project material, two species that lack an umbilicus are sculptured on the whorls of the spire with distinct radial grooves:

P. pilsbryi – with deep, rather coarse grooves, distinctly “bumpy” seen in top view; range from Pacific County, Washington to northwestern Oregon (Tillamook County).

P. stearnsii – with shallower, more crowded grooves; range from southern Alaska to southern Oregon (Curry County)

Pristiloma pilsbryiRef: Pilsbry (1946)

Pristiloma stearnsiiRef: Pilsbry (1946)

Among project material, three species that lack an umbilicus lack distinct radial grooves on the whorls of the spire:

P. idahoense – with low spire and deeply dished, bowl-shaped base; up to 6.5 whorls, narrow mouth, without an internal rib. Idaho.

P. lansingi – with low spire and deeply dished, bowl-shaped base; up to 5.5 whorls, with a toothy rib along the basal and outer margins of the lip at adulthood. British Columbia to northwestern California.

P. crateris – with widest part of the shell near middle of body whorl (“median”) and slope of the shell below the periphery more rounded, leading to a less deeply “dished” base. Western slope of Cascade Range from northern to southern Oregon, questionably in Washington.

Pristiloma idahoenseRef: Pilsbry (1946)

Pristiloma lansingiRef: Pilsbry (1946)

Pristiloma craterisRef: Pilsbry (1946)

5. Re-verification of Pristiloma specimens

A major question was the distinction, if any, between Pristiloma arcticum arcticum and P. a. crateris. (Following most prior literature, including Pilsbry’s 1946 monograph, the two were regarded as subspecies of one species, P. arcticum.

Of 19 samples previously identified as P. arcticum arcticum, only one sample (WEN01-001) was arguably identifiable as that taxon. The rest were either P. lansingi, P. crateris, or tentatively identifiable as juvenile P. stearnsii.

In the most recent monograph of the genus (Pilsbry, 1946), Pristiloma arcticum crateris was described as follows:

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; 5 ½ 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 purported subspecies was contrasted with P. a. arcticum, which was described as follows:

“Shell imperforate, globose-depressed, most minutely striate, uniform tawny-brown, glossy. Whorls 5 ½ 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 4 ¾, 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 (Pilsbry, 1946: 401-402).

Distinction between P arcticum and P. crateris

Pristiloma crateris Pristiloma arcticum

Ref: Pilsbry (1946)

Conclusions, 1: P. crateris is a readily recognizable species in the study area.

Most samples previously identified as P. arcticum crateris (96) were confirmed as crateris by this study; two others were re-identified as P. lansingi. Pristiloma crateris (the argument for considering it a species rather than a subspecies of P. arcticum is given below) is a readily recognizable species with a distinctive range within the study area.

Conclusions, 2: P. crateris is a species, not a subspecies

All specimens herein identified as 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 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).

Conclusions, 3

At the present time, we have insufficient information to demonstrate intergradation between crateris of this study and arcticum from elsewhere (which would be suggestive of a subspecific relationship), or identity of the reproductive systems of undoubted 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.

Only P. crateris is definitely represented in the material considered by this study. More material would be needed to address the question of P. arcticum occurring in Washington State.

References

Baker, H. B. 1931. Nearctic vitreine land snails. –Proceedings of the Academy of Natural Sciences Philadelphia 83: 85‑117, pls. 13-20.

de Queiroz, K. 2005. Different species problems and their resolution. BioEssays 27: 1263-1269.

Kerney, M. P. & R. A. D. Cameron 1979. A Field Guide to the Land Snails of Britain and North-West Europe. William Collins Sons: London. 288 pp.

Pilsbry, H. A. 1946. Land Mollusca of North America (north of Mexico). Academy of Natural Sciences of Philadelphia, Monograph 3 2(1): i-viii, 1-520, frontis.