Stylodonta unidentata 


Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Mollusca Gastropoda Stylommatophora Acavidae

Scientific Name: Stylodonta unidentata
Species Authority: (Holten, 1802)
Taxonomic Notes: Six subspecies of this snail are recongized:

Stylodonta unidentata unidentata (Holten, 1802)
Stylodonta unidentata sebertae Gerlach, 2003
Stylodonta unidentata pyramidata (Pfeiffer, 1841)
Stylodonta unidentata globata (Martens, 1869)
Stylodonta unidentata praslina Gerlach, 2003
Stylodonta unidentata parva Gerlach, 2003

Assessment Information [top]

Red List Category & Criteria: Vulnerable B2ab(iii) ver 3.1
Year Published: 2009
Date Assessed: 2006-01-11
Assessor(s): Gerlach, J.
Reviewer(s): Miller, R.M., Pollock, C.M. & Seddon, M.B.
While populations of this species are stable, the species has a limited range (AOO = 87 km²; it is found in 8 locations) and there is ongoing deterioration in habitat quality throughout this range due to invasion by introduced plants, especially Cinnamomum verum.

Geographic Range [top]

Range Description: This snail species is endemic to the Seychelles. It is found on the islands of Mahé, Therèse, Conception, Silhouette, Praslin, Curieuse, La Digue and Felicite; it has become extinct on the island of St. Anne. Its total area of occupancy (AOO) is 87 km². Most of the populations on the different islands represent different subspecies; the distribution and the area of occupancy (AOO) of each is as follows:

S. u. unidentata: North Mahé, St. Anne (extinct), Therèse and Conception; AOO = 22 km²
S. u. sebertae: West Mahé; AOO = 11 km²
S. u. pyramidata: South Mahé; AOO = 22 km²
S. u. globata: Silhouette; AOO = 19 km²
S. u. praslina: Praslin and Curieuse; AOO = 8 km²
S. u. parva: La Digue; AOO = 5 km²
Countries occurrence:
Seychelles (Seychelles (main island group))
Estimated area of occupancy (AOO) - km2: 87
Continuing decline in area of occupancy (AOO): No
Extreme fluctuations in area of occupancy (AOO): No
Estimated extent of occurrence (EOO) - km2:
Continuing decline in extent of occurrence (EOO): No
Extreme fluctuations in extent of occurrence (EOO): No
Number of Locations: 8
Continuing decline in number of locations: No
Extreme fluctuations in the number of locations: No
Lower elevation limit (metres): 10
Upper elevation limit (metres): 990
Range Map: Click here to open the map viewer and explore range.

Population [top]

Population: The total population size of this species has been estimated to be 7.2 million individuals, though some subspecies are much more abundant than others. The numbers of individuals of each subspecies is estimated to be:

S. u.unidentata: 145,000
S. u. sebertae: 1,000
S. u. pyramidata: 15,000
S. u. globata: 7 million
S. u. praslina: 700
S. u. parva: 50,000

This snail's populations are considered to be stable.
Current Population Trend: Stable
Additional data:
Number of mature individuals: 7211700 Continuing decline of mature individuals: No
Extreme fluctuations: No
Continuing decline in subpopulations: No
Extreme fluctuations in subpopulations: No All individuals in one subpopulation: No

Habitat and Ecology [top]

Habitat and Ecology: This snail is a terrestrial lowland forest species, found in leaf litter, and is one of the most important detritivores in the forest ecosystem. It does not lay eggs, but rather gives birth to live young.
Systems: Terrestrial
Continuing decline in area, extent and/or quality of habitat: Yes
Generation Length (years): 5

Threats [top]

Major Threat(s): It is threatened on Mahé and Praslin islands by habitat degradation caused by invasive exotic plants, especially Cinnamomum verum, and predation by the Common Tenrec (Tenrec ecaudatus), a small mammal that was introduced to the Seychelles. Small-scale farming and wood plantations have threatened this snail's habitat in the past.

Conservation Actions [top]

Conservation Actions: Some populations are protected in Morne Seychellois National Park (S. u. unidentata) and Praslin National Park (S. u. praslina). The other subspecies require the protection of Mt. Sebert on Mahé island (S. u. sebertae); the La Reserve/Brullee area on Mahé island (S. u. pyramidata); Silhouette island (S. u. globata); and the summit of La Digue (S. u. parva).

Citation: Gerlach, J. 2009. Stylodonta unidentata. The IUCN Red List of Threatened Species 2009: e.T168106A6450480. . Downloaded on 01 December 2015.
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