Oregon forestsnail (Allogona townsendiana) COSEWIC assessment and status report 2013: chapter 9

Biology

Life Cycle and Reproduction

A recent Oregon Forestsnail study assessed population size, reproductive timing and habitats, seasonal behaviours and juvenile activity over a four-year period at TWU-ESA in Langley (Steensma et al. 2009) and provides most of the information summarized below.

Seasonal patterns of Oregon Forestsnail activity was assessed by tracking 15 snails for two years by harmonic detection finder (see Steensma et al. 2009). In general, mating begins in February and lasts through early June. As the warmer and drier summer months approach, snails seek shelter deep within litter, under logs or the bark of coarse woody debris, or in similar shelter places within the deciduous forests where they predominantly live (see Habitat Requirements). This aestivation period lasts a few months and in mid- to late September snails will become active again for the wet fall months. Once the first frosts occur, Oregon Forestsnail enters hibernation until the following spring. Winter hibernation begins sometime between late October and lasts until late February, when temperatures are below 10.6°C (Steensma et al. 2009).

During hibernation Oregon Forestsnails bury themselves 2 - 7 cm within leaf litter, moss, soil or other forms of cover, form an epiphragm, and orient themselves with the aperture of the shell upwards (Steensma et al. 2009). Adult snails are not likely to move although five tracked adults moved (average distance 14 cm) during the hibernation period, and may have fed during this time. Juveniles have not been observed during hibernation months (Steensma et al. 2009; BC Conservation Data Centre 2013).

This species is most active during the wet spring months when mating takes place. Oregon Forestsnail is hermaphroditic; although self-fertilization is unknown it could decrease reproductive success as has been seen in other gastropods (Forsyth 2004). Oregon Forestsnail mating pairs (Figure 11) have been observed at three sites in BC. Snails are active from early February with the peak mating period from early March through early May (Steensma et al. 2009), and as late as June (Kus 2005). Mating has been observed to occur directly on or within proximity (< 3 m) of coarse woody debris (e.g., logs); Stinging Nettle also grew less than 1 m from mating pairs (Steensma et al. 2009). Snails are more active during mating season, move farther and are more likely to be communal; these behaviours increase vulnerability.


Figure 11. Mating individuals of Oregon Forestsnail (Allogona townsendiana) as observed at Trinity Western University Ecological Study area

Photo of two Oregon Forestsnails mating (see long description below).

Figure 2B reproduced with permission from Steensma et al. 2009.

Description of Figure 11

Photo of two Oregon Forestsnails mating on a fallen branch in the Trinity Western University Ecological Study area.


Oregon Forestsnail nesting and egg laying has been documented at three different sites in BC (Cemetery Hill, Nicomen Slough and TWU-ESA, see Table 1) (Steensma et al. 2009); Ovaska et al. (2001) also have a number of observations for other sites. Steensma et al. (2009) observed snails nesting from April 20 - June 20, peaking in mid-May. Oviposition occurs after adult snails dig or burrow into new or existing nesting holes. Fifty-three nests were surveyed over a two-year period with adult snails digging a 6 - 10 cm flask-shaped hole, the equivalent of their body size, with their foot. Most snails dug nests although snails were also observed nesting within pre-existing depressions in soil, moss and under coarse woody debris. Snails have also been observed laying eggs at the base of vegetation, such as Creeping Buttercup, and occasionally within the same burrow as another snail.

Juvenile snails hatched approximately 8 - 9 weeks after oviposition, with direct observation of hatching observed at two nests following 63 and 64 days of incubation (Steensma et al. 2009). Asynchronous hatching was observed although this is likely a factor of environmental variability and has also been observed in other land snails (Forsyth 2004). Juveniles began dispersing from the nest site within hours of hatching, found mostly on mixed soil, leaf litter or soil only. Following hatching, snail activity included climbing less than one metre on tall vegetation within close proximity to the nest although no relationship was found between climbing behaviour and temperature or humidity. Vegetation favoured by juvenile Oregon Forestsnail individuals included Stinging Nettle, Reed Canary Grass (Phalaris arundinacea), Indian Plum, and Himalayan Balsam (Impatiens glandulifera). Older juveniles (not hatchlings) were observed feeding on Stinging Nettle (Steensma et al. 2009).

Adults likely reach reproductive maturity by 2 years and have a life span of at least 5 (Steensma et al. 2009) to 8 years (COSEWIC 2002) although 5-8 years is likely an underestimation. Based on the probable age at first reproduction and life span, generation time is approximately 4 to 6 years; the average generation time estimate is 5 years.

Physiology and Adaptability

There is no information on the physiology or adaptability of Oregon Forestsnail in BC. The snail's association with Stinging Nettle (see Habitat Requirements and Interspecific Interactions) is not considered host plant specificity.

Dispersal and Migration

Oregon Forestsnail is sedentary and has a patchy distribution throughout the northern part of its global range. Oregon Forestsnail is not seasonally migratory nor does the species move from different habitat types during various stages of its life history although minor seasonal movements could occur between foraging, egg-laying, and hibernation areas (see below).

Studies of 21 tracked adult Oregon Forestsnail at TWU-ESA since 2005 showed, minimum convex polygon home ranges from 12.5 to 331.4 m2 (Lilley pers. comm. 2011; Steensma pers. comm. 2011). The home ranges were frequently long and narrow and most of the snails' movements appeared to be in one direction (Lilley pers. comm. 2011; Steensma pers. comm. 2011). This is consistent with other studies that show snails follow logs and other forms of woody debris, as well as their own mucus trails (Prior 1985).

Edworthy et al. (2012) recorded average adult Oregon Forestsnail movement of 2.5 m/month; snails generally remained in a core area of less than 15 m2. The maximum daily dispersal was 4.5 m and the maximum displacement over 3 years was 32.2 m. The urban/rural environment likely presents inhospitable and impassable barriers, including roads, yards and agricultural fields. The home range of a single individual ranged from 18.4 m2 to 404.4 m2 and often overlapped both forest and meadow habitat. No difference in home range size was found between snails that used primarily forest versus those that used primarily edge or meadow habitat, likely due to the presence of Stinging Nettle in edge and meadows.

Gastropods can use homing behaviour near their home site, foraging and returning to the same site. This homing behaviour ensures their return to suitable shelter and minimizes dehydration (Rollo and Wellington 1981; Prior 1985) and could indicate territorial defence (Rollo and Wellington 1981). Oregon Forestsnails leave a dilute mucus trail during movements and likely exhibit homing behaviour by following their own mucus trails or the trails of other gastropods.

Passive dispersal or transport aided by a carrier (e.g., mammals, birds) is unlikely. There is a possibility Oregon Forestsnail e.g., juveniles or adults could be transported during flooding events on pieces of large woody debris with a large, intact moss or litter layer.

Interspecific Interactions

Oregon Forestsnail is often observed consuming and/or found in close association with Stinging Nettle (BC Conservation Data Centre 2013). There is no direct research that links the nutritive values of Stinging Nettle with Oregon Forestsnail, yet research on other land snails shows a significant correlation. Home ranges of Oregon Forestsnail were significantly smaller in the presence of Stinging Nettle (Edworthy et al. 2012). Field observations of the European land snail (Cornu aspersum) showed Stinging Nettle is significantly correlated with snail presence, partially explained by higher protein, ash and calcium contents in the plant (Iglesias and Castillejo 1998).

Oregon Forestsnail are likely eaten by small mammals, birds, and other molluscs. The native snail Robust Lancetooth (Haplotrema vancouverense) has been observed feeding on both eggs and juveniles of Oregon Forestsnail in at least four instances (Steensma et al. 2009). Studies at TWU-ESA noted Oregon Forestsnail juvenile hatchlings (and adults) shared their habitat with other land snails, including Robust Lancetooth, Pacific Sideband (Monadenia fidelis) (native), Grovesnail (Cepea nemoralis) (introduced), Pygmy Oregonian (Cryptomastix devia) (native), and Northwest Hesperian (Vespericola columbianus) (native), as well as with various slugs including non-native Arion species.

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