Oregon forestsnail (Allogona townsendiana) COSEWIC assessment and status report 2013: chapter 6

COSEWIC Status Report on the Oregon Forestsnail Allogona townsendiana in Canada - 2013

Wildlife Species Description and Significance

Name and Classification

Scientific Name:
Allogona townsendiana (I. Lea, 1838)

Classification:

Kingdom
Animalia
Phylum
Mollusca
Class
Gastropoda
Subclass
Pulmonata
Order
Stylommatophora
Suborder
Sigmurethra
Family
Polygyridae
Subfamily
Polygyrinae
Tribe
Allogonini
Genus
Allogona
Subgenus
Dysmedoma
Species
Allogona townsendiana

Synonyms:
The most recent compilation of the complete synonymy is in Pilsbry (1940). Subsequently, Branson (1977) lists Allogona townsendiana brunnea Vanatta, 1924 and Forsyth (2004) lists Allogona townsendiana frustrationis Pilsbry, 1940 as additional synonyms.

Subspecies:
No subspecies of A. townsendiana are recognized (Forsyth 2004; Forsyth pers. comm. 2011).

English Name:
Oregon Forestsnail

French Name:
Escargot-forestier de Townsend

Type Localities:
Helix townsendiana – near junction of Willamette and Columbia rivers, Washington; A. t. brunnea – Columbia River near Kelso, Washington; A. t. frustrationis North Head, Washington.

Taxonomic Background and Similarities:
Oregon Forestsnail (Allogona townsendiana) belongs to the family Polygyridae, a large and diverse group of North American terrestrial snails (Pilsbry 1940; Emberton 1994, 1995). The species has been treated as a member of the genus Allogona since Pilsbry (1939). The genus Allogona is currently believed (Pilsbry 1940; Emberton 1995; Turgeon et al. 1998) to contain four species: Selway Forestsnail, A. lombardii A.G. Smith 1943; Broad-banded Forestsnail, A. profunda (Say, 1821); Idaho Forestsnail, A. ptychophora (A.D. Brown, 1870); and Oregon Forestsnail. All three western species -- A. lombardii, A. ptychophora and A. townsendiana -- belong to the anatomically distinct subgenus Dysmedoma (Pilsbry 1939, 1940; Emberton 1995).

In Canada, Allogona is represented by three species: Oregon Forestsnail, Idaho Forestsnail and Broad-banded Forestsnail. Some British Columbia (BC) shells attributed to Selway Forestsnail by Smith (1943) and thought doubtful by La Rocque (1953) are likely Idaho Forestsnail (Forsyth 1999a).

Morphological Description

Oregon Forestsnail (Figures 1 and 2) is a large land snail (adult shell diameter, 20-35 mm; 5¼ - 6 whorls) with a slightly flattened and globular shell (Forsyth 2004).The shell varies from straw-yellow, amber to light reddish brown and has fine, wavy spiral striae; irregular, light-coloured, wrinkle-like axial riblets and an overall irregular dimpled sculpture (Forsyth 2004). The outer organic protein layer (periostracum) lacks hair-like structures present in some other polygyrids. The periostracum and spiral striae erode away in some individuals, leaving the shells grey-white. The lip of the aperture (opening) of the shell is thick and outwardly flared. Unlike some related species, there is no denticle inside the aperture.


Figure 1. Oregon Forestsnail (Allogona townsendiana) adult, June 11, 2010, Colony Farm Metro Vancouver Regional Park.

Photo of an adult Oregon Forestsnail, side view, crawling along a fallen tree.

Photograph by Jennifer Heron.

 


Figure 2. Oregon Forestsnail (Allogona townsendiana) underside of the shell showing the white apertural lip, June 11, 2010, Colony Farm Regional Park.

Photo of the underside of the shell of the Oregon Forestsnail (see long description below).

Photograph by Jennifer Heron.

Description of Figure 2

Photo of the underside of the shell of the Oregon Forestsnail, showing the lip of the aperture, which is white, thick and outwardly flared. The rest of the shell is reddish brown.


Steensma et al. (2009) studied and described Oregon Forestsnail eggs. Eggs are round, globose, opaque and greyish-white, slightly flattened and with a grainy texture and are laid singly or in clusters. Egg clutches laid in captivity average 34 eggs with each egg 2.5 - 4.0 mm in diameter. On average, first eggs laid in captivity were generally larger than last eggs in the clutch.

Juvenile Oregon Forestsnails have thinner, translucent shells and are not eroded. They have a thin unflared lip. Newly hatched juveniles are 2 mm diameter, and reach 3 - 3.5 mm after six weeks (Steensma et al. 2009). At 4 mm, juveniles were able to form an epiphragm (plug of dried mucus) in dry weather and by two months old were 5 mm diameter (Steensma et al. 2009).

Further morphological descriptions of Oregon Forestsnail are given by Pilsbry (1940), Kozloff (1976), and Forsyth (2004). Oregon Forestsnail is relatively easily distinguished from other land snails within its BC range; most confusion is with immature individuals. Comparisons with other similar land snails within the same geographic range as Oregon Forestsnail are detailed in Forsyth (2004).

Population Spatial Structure and Variability

In Canada, the spatial structure and variability of Oregon Forestsnail populations has not been studied. Similarly, no genetic studies have occurred on the species.

Designatable Units

Oregon Forestsnail has one designatable unit within Canada. No subspecies are recognized. The species occurs entirely in the COSEWIC (2011) Pacific National Ecological Area and there is no information on population genetic structure among sites. There also are no data on discreteness or evolutionary significance among populations.

Special Significance

Relatively few large, native land snails live in the coastal forests of BC, and Oregon Forestsnail is a valuable component of the biodiversity within these ecosystems. Land snails sequester calcium and other minerals from the environment, required for the development of their shells.

Oregon Forestsnail is used as an interpretive tool by conservation organizations to represent the importance of rare and endangered species within the lowland wet riparian broadleaf forests remaining throughout the Lower Fraser Valley. Conservation organizations such as the Fraser Valley Conservancy (MacMillan pers. comm. 2012) and South Coast Conservation Program (Robbins pers. comm. 2012; Welstead pers. comm. 2012) use Oregon Forestsnail when informing private landowners about stewardship opportunities.

Aboriginal Traditional Knowledge on this species is unknown as there is no information on Oregon Forestsnail and its importance to First Nations. However, there is some literature on the cultural significance of Stinging Nettle (Urtica dioica), which appears to be a habitat associate with Oregon Forestsnail (see Habitat Requirements and Interspecific Interactions).

Oregon Forestsnail, as with other herbivorous land snails, perform important ecological functions in forest ecosystems as decomposers and consumers of live and decaying plant matter (see Mason 1970; Richter 1979, 1980a,b; Gervais et al. 1998).

Some species may also function as dispersal agents for plant seeds (Gervais et al. 1998), fungal spores (Burke et al. 1999) and lichen (Boch et al. 2011). Burke (1999) suggested that Puget Oregonian (Cryptomastix devia) might aid in the dispersal of fungal spores, including mycorrhizal fungi that form tree-root associations, which promote healthy tree growth. Oregon Forestsnail is similar in size to Puget Oregonian and would likely play a similar role. The significance of the Oregon Forestsnail in such processes is unknown but may be considerable given the species’ relatively large size and local abundance in suitable moist habitats.

Oregon Forestsnail can be observed in high abundance at some sites (BC Conservation Data Centre 2013) and may provide a significant food source for other invertebrates, birds and/or small mammals. In particular, the concentration of calcium within shells is likely a significant source for other invertebrates.

In Canada, Oregon Forestsnail occurs at the northern limits of its range, and it may therefore hold distinctive adaptations and be of special scientific and conservation interest (Scudder 1989).

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