Banff Springs snail (Physella johnsoni) COSEWIC assessment and status report: chapter 5
Update
COSEWIC Status Report
on the
Banff Springs Snail
Physella johnsoni
in Canada
2008
Species Information
The recognized authority for the classification of aquatic molluscs in the United States and Canada is Turgeon et al. (1998). The current accepted classification of this species is as follows:
While Turgeon et al. (1998) assigned the English common name “striate physa” to this species, Banff Springs Snail is used by the jurisdictional authority, Parks Canada Agency, and by COSEWIC as it is more descriptive of the snail’s distribution and habitat. The original scientific name, Physa johnsoni, was used by Clarke (1973, 1981) in both his monograph and book on molluscs of Canada. Te’s (1978) revision of the family resulted in renaming most of the Physa as Physella, although others (Dillon 2000; Wethington and Guralnick 2004) still use Physa. There are also differing opinions on whether Physella johnsoni is a synonym of a related species, P. gyrina, a generalist and the most widely distributed physid in Canada (see Morphological and Genetic descriptions).
All members of the Physidae are sinistral (coiled to the left). When holding the shell with the spire (the point or apex) up, the aperture (opening where the snail’s body protrudes) will be on the left. All other cone-shaped North American freshwater snail families are dextral, with the aperture on the right.
The original description given by Clench (1926) and reproduced in Clarke (1973) is as follows:
“Shell sinistral, small, globose, thin. Color dark reddish horn, sometimes faintly striated. Whorls 4½ to 5, convex and well rounded, nuclear whorl darker in color. Spire rather short, terminating in an acute apex. Aperture well rounded, flaring slightly at the base. Palatal lip very thin, rarely labiate. Parietal lip of a thin deposit only on body whorl. Columella rather narrow, not twisted, inclined toward the left and not abruptly terminating in the body whorl but gradually continuing the general contour. Suture very well impressed, slightly indented. Sculpture of very fine growth lines but no cross striae. The loss of the periostracum on some of the most prominent growth lines gives it the appearance of striations as noted above. Varicose bands rare and most noticeable when seen from within the aperture.”
Te (1978) describes the shell as small, elongate-ovate in shape, with heavy and uneven growth lines (Figure 1). While the maximum published shell length is 8.8 mm (Clarke 1973) living animals with shells up to 11 mm in length have been observed with the animals ranging in colour from light-brown to black (Lepitzki unpubl. data). Lepitzki (1998) measured shells of live P. johnsoni (n=157), Physella gyrina (n=168) and another physid, most likely Physa megalochlamys (n=80), using an ocular micrometer at 10x magnification. The specimens had been collected from Banff National Park in 1997 for allozyme and mtDNA analyses. The P. johnsoni had significantly (P<0.05 for pairwise comparisons) larger shell width to length (F2402=63.795, P<0.001) and spire to length (F2402=6.759, P=0.001) ratios than the other two species. The maximum P. johnsoni shell length measured was 9.2 mm.
Figure 1: Line drawing and photograph of Physella johnsoni, the Banff Springs Snail
Drawing credit: D.A.W. Lepitzki, photograph credit: B.M. Lepitzki
In addition to shell morphology, penial morphology also is used in the classification of physids. Te (1978) found that the two P. johnsoni he dissected had Var-1 and Var-2 of the type-c penial complex, which placed it within the Physaacuta group. A type-c penial complex is characterized by the presence of a preputial gland and a single-parted non-glandular penial sheath. In contrast, Wethington and Guralnick (2004) placed P. johnsoni within the P. gyrina group, or Te’s (1978) type-b penial complex, characterized by a preputial gland and a two-parted penial sheath having both a glandular and non-glandular component. Taylor (2003) also places P. johnsoni within the P. gyrina group, having a bipartite penial sheath. However, Taylor did not personally study or verify morphologically any P. johnsoni or any P. gyrina specimens, so his rationale for placing P. johnsoni within the P. gyrina group is uncertain. Lepitzki (unpubl.) dissected two additional P. johnsoni and they had type-b, P. gyrina-group penial morphology (Figure 2).
Figure 2: Photograph of penial morphology of Physella johnsoni, the Banff Springs Snail
Collected from Lower Middle Spring, Banff National Park, June or July 2002 and maintained in a thermal water flow-through aquarium at the C&BNHS until the snail’s natural death on 8 February 2003. The bi-partite penial sheath is easily distinguished with non-glandular and glandular portions. Each ocular unit is 100 microns. (Photograph credit: D.A.W. Lepitzki)
Allozyme and mtDNA studies have been done on the Banff Springs Snail. Hebert (1997), using the specimens collected and measured by Lepitzki (1998), screened 20 individuals from each of five subpopulations for allozyme variation at 12 loci: Apk, Fum, Got-m, Got-s, Gpi, Idh, Ldh, Mdh-m, Mdh-s, Mpi, Pgm-1, and Pgm-2. He states in his unpublished report, to which Remigio et al. (2001) refer, that no allozyme variation was detected at four of the loci: Apk, Got-m, Mpi, and Pgm-2. Levels of intraspecific polymorphism also were low with no variation detected in P. megalochlamys. Variation in P. gyrina and P. johnsoni was restricted to the Idh locus. Despite limited polymorphism, marked genetic divergence was apparent among the taxa. Physa megalochlamys was easily distinguished from the two Physella by its diagnostic Gpi1.32 and Fum1.16 alleles. Populations of P. johnsoni and P. gyrina showed allelic substitutions at Got-s and at both Mdh loci. Hebert (1997) concluded that “genetic distance analysis confirmed that conspecific populations showed close genetic affinity, while there was marked divergence among the 3 taxa”. His unpublished genetic distance figure is reproduced as Figure 3. He also states that each of the species examined showed limited polymorphism but that this was not unexpected as these gastropods are hermaphrodites, capable of self-fertilization. Populations of each species showed limited genetic divergence but there was marked divergence among species and “diagnostic allozyme markers separate each of the species, permitting their reliable discrimination.” He concluded that his results “makes it very unlikely that P. johnsoni is simply a thermal spring ecotype of P. gyrina” and that the “results support the need to treat P. johnsoni as an endemic species with a substantial history of evolutionary divergence from P. gyrina.”
Figure 3: UPGMA of Nei’s unbiased genetic distances among three species of physid snails from Banff National Park, as shown by allozyme analyses
The snails from the Basin, Lomid (=Lower Middle), Upper C&B, Cave, and Lower C&B were P. johnsoni. The Physa sp. 1 and Physa sp. 2 were most likely Physa megalochlamys and were collected from the Hay Meadow and Rat Hole Ponds, between 2nd and 3rd Vermilion Lakes. Physella gyrina 1 through 5 were collected from: 1 = C&B marsh; 2 = Vermilion Cool Springs; 3 = 5-mile Pond; 4 = Herbert Lake; and 5 = Muleshoe Lake. Reproduced with permission from Hebert (1997).
The same snail collection was then subjected to mtDNA analyses. Remigio et al. (2001) sequenced the 16S and COI mitochondrial genes from a single snail from each of the populations (two P. johnsoni were sequenced from one subpopulation, Cave Spring, and found to have identical sequences so only one was used in the analyses). They found that the genetic distinctiveness between P. gyrina and P. johnsoni was apparent only in the parsimony analysis of the combined 16S and COI data and individuals from the Cave Spring always clustered away from the other samples of P. johnsoni.
Wethington and Guralnick (2004) then used the sequences of Remigio et al. (2001) available through GenBank. The scope of their study expanded beyond that of Remigio et al. (2001) by including other physids collected from thermal springs and cave environments throughout the United States. Two additional specimens of P. johnsoni collected from the type locality by Lepitzki in August 2001 also were sequenced and dissected for penial morphology. Their dissections and mtDNA analyses clearly placed P. johnsoni within the P. gyrina group. They concluded that P. johnsoni, because it did not form a distinct monophyletic group, was indistinguishable from other members of the western U.S. within the P. gyrina group. The outlier was from the Cave Spring subpopulation. A P. gyrina individual from the type locality in Iowa also was included in the P. johnsoni group along with the two additional specimens collected from the type locality by Lepitzki. Sampling is acknowledged as an explanation of P. johnsoni not forming a monophyletic group (n=8 P. johnsoni: 6 from Remigio et al. 2001 and 2 additional in Wethington and Guralnick 2004). The three other physids in the P. gyrina group collected from thermal springs all formed monophyletic groups but only two individuals of each species were sequenced. If more individuals of other species or more subpopulations of each species were sequenced, they might not be monophyletic either.
Another explanation for the small level of genetic differentiation between P. gyrina and P. johnsoni is the young age of P. johnsoni. Remigio et al. (2001) concluded that P. gyrina and P. johnsoni were distinct species that had only separated since the most recent ice age retreated, approximately 10 000 years ago. Newer evidence, from 14C age dating, suggests that the thermal spring habitat of P. johnsoni is even younger and only around 3200 to 5300 years old (Grasby et al. 2003). This short evolutionary history may offer an explanation why Wethington and Guralnick (2004) concluded that P. johnsoni is “indistinguishable” from P. gyrina and dispute the conclusions of Hebert (1997) and Remigio et al. (2001) that P. johnsoni is a valid species.
The latest phylogeny proposed by Wethington and Lydeard (2007), analysing two mitochondrial DNA sequences (16S and CO1) and comparing penis morphology, supports the recognition of physids with type-b penial morphology as a clade consisting of two species, and lumps seven different taxa, including P. wrighti and P. johnsoni, under P. gyrina.
No designatable units below the species level exist. If P. johnsoni were synonymized with P. gyrina, the Banff Springs population would be available for consideration as a designatable unit of P. gyrina due to its biogeographical distinctness and its biological adaptations to the harsh thermal spring environment. These hotwater specialist snails may represent unique and important ecological and/or evolutionary units (i.e., DUs under SARA) that still warrant protection. This can be further investigated using molecular markers that evolve more quickly than mtDNA markers (i.e., microsatellite DNA).
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