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Recovery Strategy for Hotwater Physa

Species information and description

1. Background

Hotwater Physa, Physella wrighti, is a freshwater snail globally endemic to Liard River Hotsprings located in Liard River Hotsprings Provincial Park, BC (Figure 1).  The snail was first collected in 1973 and taxonomically described in 1985 (Te and Clarke 1985). Over 80 hotsprings are known throughout BC, and there have been few recent surveys of the invertebrate fauna at these hotsprings (e.g. Lee and Ackerman 1998; Salter 2001 and 2003; Remigio et al. 2001; Wethington and Guralnick 2004).


Figure 1.  Global location of Hotwater Physa (Salter 2003).

1.1 Species Assessment Information

The Status Report and Assessment Summary for Hotwater Physa is available from the Committee on the Status for Endangered Wildlife in Canada (COSEWIC) Secretariat (www.cosewic.gc.ca).

Common name: Hotwater Physa

Scientific name: Physella wrighti

COSEWIC Status: Endangered April 1998.

SARA Status: Endangered June 2003.

Last Examination and Change: May 2000.  No change.

Canadian Occurrence: British Columbia (BC).

Reason for designation: Small endemic population with narrow ecological requirements occurring in an extremely restricted area subject to threats resulting from human use of hot springs pools.  Probability of extinction is high.

Status history: Last assessment based on existing status report.

1.2 Description

Hotwater Physa, Physella wrighti (Te and Clarke 1985), is a freshwater snail classified in the gastropod family Physidae (generally referred to as physids) (Pulmonata: Physidae). Hotwater Physa has a very small blackish/grey shell ranging from 3.25 – 9.1 mm. The shell is sinistrally (i.e., opening to the left) coiled with an ear-shaped aperture, an outer lip callus along its rim, a curved perimeter and a narrow elongate-ovate shape (Figure 2) (Lee and Ackerman 1998).

Figure 2a. Hotwater Physa, Physella wrighti.

Figure 2a. Hotwater Physa, Physella wrighti.

Illustration by Trent Hoover, used with permission.

 Hotwater Physa on emergent vegetation : Close viewHotwater Physa on emergent vegetation

Figure 2b and c. Hotwater Physa on emergent vegetation. Photos by Mike Rowe, used with permission.

The Hotwater Physa is currently acknowledged by the American Fisheries Society as a unique species. Te and Clarke (1985) used morphological, anatomical, cladistic and phenetic analyses to describe the species and concluded that “P. wrighti is certainly a primitive species not closely related to any other species in northwestern North American and it is virtually impossible that it could have evolved as such a distinct species since the end of the Pleistocene”, and suggested that Hotwater Physamay have been at its type locality for 100,000 years.  

Molecular systematic relationships have been completed between P. wrighti and Physella johnsoni, an endemic snail to Banff Hotsprings, Alberta.  P. johnsoni is listed as Endangered by COSEWIC.  The relationship between both P. wrighti and P. johnsoni has been examined to determine the origins of these two species and their inter-specific relationships.  Remigio et al.’s (2001) molecular data supports continued recognition of Hotwater Physa as an endemic species but suggests the species probably arose from a population isolated during the most recent glacial retreat. Further, both species are endemic to their respective locations and that P. wrighti was likely the source ancestral population from which P. johnsoni was derived (Remigio et al. 2001).

More recently, molecular evidence has been used to assess species age and relationships and this has yielded contradictory results. Wethington (2002) found fault with Remigio et al.’s (2001) study design, and concluded from molecular data of physid hotspring endemics that there is not a monophyletic hot spring physid group and that members of the closely related Physa gyrina species group includes the putative Hotwater Physa. This group can invade or survive introduction to hot water environments and can develop distinctive shells in as few as five generations (Wethington and Guralnick 2004) and so are not distinct species but habituated populations. Taylor (2003) also regards Hotwater Physa to be a synonym of P. gyrina. An authoritative decision will probably not be available until the American Fisheries Society updates the mollusc species list that currently includes Hotwater Physa as a distinct species (Turgeon et al. 1998). 

Physids are very common throughout North America and are very difficult to identify, it is possible that other species may occur within Liard River Hotsprings Provincial Park. Additional Physella species endemic to hotsprings complexes in BC and western Canada have yet to be fully studied. A snail of the family Physidae is present in Deer River hotspring, which is approximately 50 km from Liard River Hotsprings Provincial Park. Initial examinations by biologists determined this snail to be Physella virginia, but a genetic comparison of its relationship to P.wrighti has not been completed.

1.3 Populations and distribution

The Hotwater Physa is known globally from one location at Liard River Hotsprings complex in the Liard River Hotsprings Provincial Park, BC.  The snail was initially reported from one breeding population along a 34 metre stretch of Alpha Stream, at the outlet of Alpha Pool (Figure 3) (Lee and Ackerman 1998).  This outlet is also the type locality for the species (Te and Clarke 1985; Lee and Ackerman 1998). 

The snail has since been observed to occupy additional habitats within the hotsprings complex, including Alpha and Beta Pools and the entire length of Alpha Stream (Figure 3) (Salter 2001, 2003). The area with the highest recorded density is downstream from the weir that impounds water flowing from Alpha Pool, specifically in a section from two metres below the weir at the head of the stream to 34 metres downstream (Salter, 2003). In 1997, the highest concentrations of snails were observed approximately 20 metres downstream from the weir (Lee and Ackerman 1998).Alpha Pool and Beta Pool have distinct sources and the two outflows do not connect. In previous surveys (Lee and Ackerman 1998) there was no observation of snails in Beta Pool. The recent population expansion in to Beta Pool may be due to previous populations being overlooked or to recent passive dispersal by park users or wildlife.

Researchers believe the distribution of the Hotwater Physa within Liard River Hotsprings complex varies spatially and temporally and the species likely occupies additional areas within the hotsprings complex (Salter, pers. comm.).  It is unlikely that new occurrences found within the Liard River Hotsprings complex are another species, although there is a remote possibility that specimens of P. gyrina are indirectly or unknowingly brought into Liard River Hotsprings Provincial Park through wildlife or transfer by people.  P. wrighti from P. gyrina can not be distinguished by the human eye.  Currently, the only visual means of distinguishing these two species is through dissections by a trained individual. 

The population trend has not been measured or tracked for Hotwater Physa.  Some population estimates of Hotwater Physa in upper Alpha Stream have been calculated. On October 1, 1997, the conservative population estimate within Alpha Stream was 979 – 1735 individuals (Lee and Ackerman 1998). In August 2000 and January 2001 the population in Alpha Stream was estimated at 5185-7000 individuals (Salter 2001, 2003). There has been no evidence of a decline in the population (Salter, pers. comm.). It is possible the snails congregate based on a combination of temperature, abiotic factors and biological cycles. As population estimates and species range both have increased over time, this population appears to be stable or expanding. However, it is premature to calculate population estimates based on one or two-day sampling efforts taken years apart.

The distribution of the Hotwater Physa within the Liard River hotsprings complex has not been thoroughly studied. Salter (2003) noted larger numbers of snails in the cooler perimeter of Alpha Pool, Alpha Stream and Beta Pool. The temperature in Alpha Pool and Beta Pool has been recorded daily by data loggers since July 2004 (Rowe, pers. comm.). Alpha Pool was artificially created through the installation of a dam and a weir. The dam backs up water directly from the hotwater vent (source) and creates a pool < 1.5 metre depth at a temperature varying between 40° - 44.5° Celsius, with temperatures > 45° Celsius at the source in July 2004 (Rowe, pers. comm.). A weir installed approximately 5 metres below the dam also captures the flow of a small cool water creek reducing the water temperature in this secondary pool to 38° Celsius in July 2004. This mixed source water tumbles over the weir to form Alpha Stream. This mixing of two water sources has affected the temperature distribution at the micro-habitat level in the area immediately downstream of the weir, but it is not certain how this may have influenced Hotwater Physa distribution. 

It is not known if snails congregate based on food sources alone, or more likely a combination of temperature, abiotic factors and biological cycles.  The cyclical nature of Hotwater Physa populations is unstudied. Researchers believe this population has remained stable since its first recording in 1973 (Lee, pers. comm., Salter pers. comm.), although this statement is an educated guess and some of the threats (Section 1.5) have since increased. 

 Distribution of the population