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Rhachistia aldabrae 

Scope: Global
Language: English
Status_ne_offStatus_dd_offStatus_lc_offStatus_nt_offStatus_vu_offStatus_en_offStatus_cr_onStatus_ew_offStatus_ex_off

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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Mollusca Gastropoda Stylommatophora Cerastidae

Scientific Name: Rhachistia aldabrae (Martens, 1898)
Common Name(s):
English Aldabra Banded Snail

Assessment Information [top]

Red List Category & Criteria: Critically Endangered B1ac(iv)+2ac(iv) ver 3.1
Year Published: 2018
Date Assessed: 2017-02-03
Assessor(s): Gerlach, J.
Reviewer(s): Seddon, M.B. & Neubert, E.
Justification:
This snail was locally abundant in the 1970s and, in spite of a rapid decline and difficulty finding individuals starting in the 1980s, it was still represented by shells in 2000. A single live individual was found in 1997. Despite extensive and repeated searches in all previously recorded and potentially inhabitable areas, no evidence of its continued survival was found until 2014. A small population is now known to exist in dense scrub on Malabar island. As extensive areas of suitable habitat are present and there is no evidence of an increase in rat predation it has been hypothesised that the population decline from the 1970s onwards was due to climate change (Gerlach 2007). Increasing frequency of dry years is believed to have led to neonatal mortality and complete recruitment failure in most areas, causing population senescence and decline.

The known extant population currently occurs in one location within a very small area, with an extent of occurrence (EOO) and area of occupancy (AOO) both less than 5 km². Its population trend is not known, but repeated and prolonged dry conditions are suspected to cause severe fluctuations in the population. This snail is therefore assessed as Critically Endangered (CR B1ac(iv)+2ac(iv)).
Previously published Red List assessments:

Geographic Range [top]

Range Description:This species is endemic to Aldabra atoll, in the Seychelles islands. Historically its area of occupancy (AOO) was estimated to be 150 km² but it is now restricted to less than 5 km².
Countries occurrence:
Native:
Seychelles (Aldabra)
Additional data:
Estimated area of occupancy (AOO) - km2:5Continuing decline in area of occupancy (AOO):Unknown
Estimated extent of occurrence (EOO) - km2:5
Continuing decline in extent of occurrence (EOO):Unknown
Number of Locations:1Continuing decline in number of locations:Unknown
Lower elevation limit (metres):1
Upper elevation limit (metres):5
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:This snail was widespread and abundant in the 1970s, though it was believed to have declined since then; the only change in this time was an increase in dry years. Fresh shells were found in the late 1980s and a single live animal was found in 1997. Abundant old shells were found as recently as 2000. Despite extensive searches, no evidence of its continued survival was between 1997 and 2014 when one adult and a small number of juveniles were located.
Current Population Trend:Unknown
Additional data:
Extreme fluctuations:Yes
All individuals in one subpopulation:Yes

Habitat and Ecology [top]

Habitat and Ecology:An arboreal species that was found on trees in open woodland, in the prolonged dry season this snail aestivated (remains in a dormant state) on trees.
Systems:Terrestrial

Threats [top]

Major Threat(s): This species is restricted to Aldabra atoll, all of which is less than 8 m above sea level and mostly 1-2 m above sea level. Sea level rise has therefore been a major threat throughout its range. A small number of rat-eaten shells have been found, indicating that low levels of rat predation are a minor threat. In the 1970s, the species was common and widespread, but it declined rapidly thereafter. Only isolated individuals were found in the 1980s, and one in 1997. The species was then considered to be extinct until its rediscovery in 2014.

Its pronounced population decline and range contraction after the 1970s has been attributed to an increase in the frequency of relatively dry years; it was thought that prolonged dry conditions led to mortality of juveniles and recruitment failure. There is no evidence of significant habitat loss in many of the areas where R. aldabrae populations have disappeared, and no evidence of increases in rat predation.

Conservation Actions [top]

Conservation Actions: This snail is present in the Aldabra Special Reserve. Its continued survival depends on the continued preservation of the habitat. No specific conservation measures have been identified and more information is needed on its ecology, population sizes and trends.

Citation: Gerlach, J. 2018. Rhachistia aldabrae. The IUCN Red List of Threatened Species 2018: e.T168122A67485998. . Downloaded on 22 August 2018.
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