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COSEWIC Assessment and Update Status Report on the Hotwater Physa (Physella Wrighti) in Canada

Species Information

Name and classification

Scientific name:

Physella wrighti Te and Clarke, 1985

English common name:

Hotwater Physa

French common name:

la physe d’eau chaude

The recognized authority for the classification of aquatic molluscs in the United States and Canada is Turgeon et al. (1998), who classify this species as follows:

Phylum:

Mollusca

Class:

Gastropoda

Subclass:

Pulmonata

Order:

Basommatophora

Family:

Physidae

Genus:

Physella

Species:

Physella wrighti

The Turgeon et al. (1998) list follows Te’s (1978) classification of the family Physidae, which transferred many species from Physa to Physella. However, some authors (e.g., Dillon 2000, Wethington and Guralnick 2004, Wethington and Lydeard 2007) have continued to use the genus namePhysa for the entire group.

The validity of P. wrighti as a distinct species is debated within the scientific community. Te and Clarke (1985) used morphological, anatomical, cladistic and phenetic analyses to describe the species and concluded that “P. wrighti is certainlya primitive species not closely related to any other species in northwestern North America and it is virtually impossible that it could have evolved as such a distinct species since the end of the Pleistocene”. They suggested that P. wrighti might have lived at its type locality for 100,000 years. However, Taylor (2003) synonymized P. wrighti with the related Physella gyrina based on morphology. P. gyrina is a generalist, commonly occurring in almost all perennial aquatic habitats, temporary pools and swamps and is the most widely distributed physid in Canada. Molecular studies are often used to define species relationships but the results forP. wrighti have been inconclusive. Interpretation of genetic data has not determined if P. wrighti is a unique species or if it is a geographically distinct population derived from P. gyrina (see Genetic Description).

Morphological description

Members of the subclass Pulmonata (pulmonates) have a richly vascularized pulmonary cavity formed by the mantle to extract oxygen from air or water. Pulmonates are thin-shelled and lack an operculum, which is the plate-like structure that seals the aperture when the animal is withdrawn into its shell. Snails of the order Basommatophora have their eyes at the base of the tentacles. The family Physidae is characterized by sinistral (left-handed) coiling of the shell (Figure 1). The shell of P. wrighti is light horn or yellowish horn in color and the surface is somewhat shiny but not glossy (Te and Clarke 1985). Average shell length of P. wrighti is about 5 mm, and the maximum length recorded is 9.1 mm (Lee and Ackerman 1999). Species of physids are particularly difficult to distinguish based on shell characteristics. A detailed morphological description is available in Te and Clarke (1985) but P. wrighti is most readily distinguished by its presence at the type locality in Liard River Hotsprings Provincial Park (LRHPP).

Figure 1. Physella wrighti, the Hotwater Physa. Illustration by Trent Hoover; used with permission.

Figure 1. Physella wrighti, the Hotwater Physa. Illustration by Trent Hoover; used with permission.

Both Te (1978) and Taylor (2003) used the terminal male reproductive system (the penial complex) to classify physids. Te (1978) found P. wrighti (as OTU82) to have penial type-bc, indicating morphology to be intermediate between type-b (P. gyrina group; bipartite glandular penial sheath) and type-c (P. acuta group; unipartite non-glandular penial sheath), but Wethington and Lydeard (2007) classify P. wrighti as having a type-b penis morphology. Taylor (2003) synonymizes P. wrighti with P. gyrina, and describes this group as having a bipartite penial sheath. Wethington and Guralnick (2004) and Wethington and Lydeard (2007) examined the penial morphology of P. wrighti and also placed it in the P. gyrina group.

Genetic description

Remigio et al. (2001) used the CO1 and 16S regions of mitochondrial DNA (mtDNA) to analyze the relationships of P. wrighti, P. johnsoni (the Banff Springs Snail, which is endemic to warm springs within Banff National Park) and P. gyrina (the Tadpole Physa, which is broadly distributed in North America). Uncorrected p distances separating P. wrighti from P. johnsoni were 0.6–1.6% at 16S, 1.4-1.9% at CO1; P. wrighti from P. gyrina 0.6–1.4% at 16S, 1.2–1.7% at CO1; and P. johnsoni from P. gyrina 0.0–0.4% at 16S, 0.5–1.2% at CO1.

Using the phylogenetic species concept to interpret the above data, Remigio et al. (2001) concluded that P. wrighti and P. johnsoni are distinct species that are closely related to P. gyrina, and that the low sequence divergence at the two mtDNA loci indicated recent colonization of these hot water environments, probably at the end of the last Pleistocene glaciation. Wethington and Guralnick (2004) used these same genetic data, but expanded the breadth of populations analysed by adding data from two additional hot spring endemics, P. wolfiana (from Hot Springs, CA) and P. aurea (from Hot spring at Bath, VA). They concluded that all the hotspring physids in their study represent one biological species and stated that the genetic distance between members of this group is generally not greater than 6%. Similarly, analyses using two mitochondrial DNA sequences (16S and CO1) and a comparison of penis morphology, (Wethington and Lydeard 2007) support the recognition of a single clade containing two species of physids with type-b penial morphology. Within this clade they lump seven different taxa, including P. wrighti and P. johnsoni under P. gyrina. However, given the recent recolonization proposed by Remigio et al. (2001), it is not surprising that mtDNA did not reflect greater divergence. Remigio et al. (2001) and Zanatta (pers. comm. 2007) recommend that species relationships might be clarified by analysis of other genes with faster evolutionary rates (e.g., the ND1 gene).

There are no known barriers to gene flow within the single population of P. wrighti, but the fine-scale genetic population structure has yet to be investigated. Zanatta (pers. comm. 2007) recommends a fine-scale population genetic analysis using microsatellite DNA, which are rapidly mutating markers useful for working with recently diverged or closely related taxa. Such an analysis would confirm if snails from the Alpha Pool, Alpha Stream and Beta Pool form one locality.

Designatable units

No designatable units below the species level exist. If P. wrighti were synonymized with P. gyrina, the Liard River Hotsprings Provincial Park (LRHPP) population would then be available for consideration as a designatable unit of P. gyrina due to its biogeographical distinctness. These hot water 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).