NSW National Parks and Wildlife Service, July 2001
ISBN 0 731 6298 5
Mitchell's Rainforest Snail Thersites mitchellae is a large terrestrial snail found only in remnant lowland rainforest and swamp forest areas on the NSW far north coast. Its status has declined from common to extremely rare and it is now listed as an endangered species under Schedule 1 of the TSC Act. The aim of this recovery plan is to identify and protect the remaining habitat and extant populations of the species and to encourage community involvement in the conservation and recovery of the species.
The genus Thersites (Family Camaenidae Subfamily Camaeninae) is a group of large land snails inhabiting rainforests in north-east NSW and south-east Queensland (Bishop 1978). Three species are currently recognised: Thersites mitchellae (Cox, 1864) from lowland subtropical rainforest and swamp sclerophyll forest with a rainforest understorey between the Richmond and Tweed Rivers in north-east NSW, Thersites novaehollandiae (Gray, 1834) from temperate and subtropical rainforest from Barrington Tops to the NSW/Queensland border and Thersites richmondiana (Reeve, 1852) from upland temperate and subtropical rainforest between the McPherson Range on the NSW/Queensland border and the Conondale Range in south-east Queensland (Bishop 1978; Stanisic pers. comm.).
Mitchell's Rainforest Snail has a large shell up to 55 mm wide and 50 mm high, with a strongly elevated spire giving it a triangular profile, and a thickened, reflected lip. The shell is deep reddish chestnut to black in colour with two prominent yellow bands, and has a very fine microsculpture which gives it a satin appearance when held in bright light. The body colour is black with a thin lighter line on the dorsal midline.
The current range of Mitchell's Rainforest Snail remains approximately the same as its historical distribution, the coastal plain between the Richmond and Tweed Rivers. Within this range, however, land clearing has been extensive and Mitchell's Rainforest Snail is now restricted to remnant areas of habitat.
Recent sites are distributed along the coastal plain between Banora Point in the north and Lennox Head in the south (Fig. 1). The largest known population of Mitchell's Rainforest Snail and largest remaining single area of suitable habitat is in Stotts Island Nature Reserve in the Tweed River near Murwillumbah (Stanisic 2000). A complex of smaller populations and habitat fragments has also been identified in remnant areas around Cumbebin Wetland at Byron Bay (Fig. 1).
Mitchell's Rainforest Snail was recorded at a site at Wilsons River near Mullumbimby in 1980 (Stanisic 1999) (Fig. 1). The Wilsons River site (elevation 230 m AHD) is outside the normal range of the species and the other snail species present there, including Ngairea corticicola, Thersites richmondiana and Pedinogyra rotabilis, are indicative of an upland snail fauna assemblage rather than the lowland snail fauna assemblage usually associated with Mitchell's Rainforest Snail (Stanisic 1999). Mitchell's Rainforest Snail was not found during a targeted search of the Wilsons River site in 1999 and the original record there is suspected of being the result of accidental translocation (Stanisic 1999).
Judging from collections of the species last century, Mitchell's Rainforest Snail was previously common within its range but has since declined in abundance (Australian Museum 2001). The species has been listed as endangered (category ENC2a) on the IUCN Red List (International Union for the Conservation of Nature 1997).
Between 1955 and 1995 Mitchell's Rainforest Snail was recorded at only two sites, Stotts Island and Wilsons River (Stanisic 1998). Additional coastal plain sites supporting Mitchell's Rainforest Snail have been recorded since 1996, reflecting an increased level of survey as well as increased community awareness.
Many of the recent sites are considered marginal, having only a small area of habitat and populations represented by three or less known specimens of Mitchell's Rainforest Snail (Stanisic 1998). In contrast, the population status at Stotts Island is considered good, with approximately 120 hectares of lowland subtropical rainforest present (Floyd 1990) and an estimated Mitchell's Rainforest Snail population of several hundred snails (Stanisic 2000).
Mitchell's Rainforest Snail is restricted to lowland subtropical rainforest and swamp sclerophyll forest with a rainforest understorey, typically on alluvial soils with a basaltic influence. It is apparently absent from other rainforest types in the area, such as littoral rainforest (Stanisic 1998). This type of correlation with particular rainforest communities is common in many land snail species in eastern Australia (Stanisic 1994).
Lowland rainforest and swamp sclerophyll forest in the Tweed-Richmond area has been extensively cleared for agriculture and urban development and now occurs only in small remnants. Most of the sites where the species is known to survive are located on slightly elevated ground on the margins of coastal wetlands. This may be an artefact of land clearing patterns or indicate a real microhabitat preference. Breeding populations of Mitchell's Rainforest Snail can persist in narrow strips of remnant rainforest (Stanisic 1998), although the longer-term viability of populations at such sites is uncertain.
Little information is available on the ecology of Mitchell's Rainforest Snail. Its restriction to rainforest and swamp forest, a trait shared with 90% of eastern Australian land snail species (Stanisic 1994), suggests a dependence on high moisture levels, low fire frequency and a well developed leaf litter layer. Microhabitat data for Mitchell's Rainforest Snail includes records of live animals found by day sheltering under palm fronds on the ground, inside dead palm frond stems, under leaf litter at the base of trees, and under bark in fig Ficus trees (Stanisic 1998), and active at night on the surface of leaf litter (Stanisic 2000; Murphy pers. obs.). Stanisic (1998) concluded that Mitchell's Rainforest Snail is probably terrestrial and that juveniles are possibly arboreal. The related Thersites novaehollandiae is active at night, foraging on the rainforest floor and up to six metres high on tree trunks, and is most obviously active on warm, wet nights (Murphy pers. obs.).
Mitchell's Rainforest Snail is herbivorous and is thought to feed on leaf litter, fungus and lichen (Stanisic 2000). Breeding behaviour has only been observed once, with a clutch of 70 small, round, white eggs laid below the surface of leaf litter in November (Murphy pers. obs.).
Key habitat components for Mitchell's Rainforest Snail are a well-developed leaf litter layer (providing food, shelter and breeding sites) and an intact forest canopy (maintaining a moist microclimate and providing a source of leaf litter).
Mitchell's Rainforest Snail is listed as an endangered species on Schedule 1, Part 1 of the TSC Act. The species is not currently listed on the Commonwealth Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act). This plan proposes nomination of the species for listing under the Commonwealth EPBC Act.