Aldabra banded snail  (Rachistia aldabra)

This species is thought to be Extinct (2), but has not yet been classified by the IUCN.

This once abundant snail is thought to have fallen victim to the direct impacts of climate change (2) (3). The Aldabra banded snail belongs to one of the most successful groups of land snails, the Pulmonata, of which there are a great number of species inhabiting a wide range of habitats (4). These land snails all possess a coiled shell into which the body can retract (4). The shell of this species is an attractive pattern of orange and bluish-purple bands against a dark background (3). The Pulmonata, the name of which arises from the Latin word for lung, all have a mantle cavity that is modified into a lung, and breath through a small opening called the pneumostome (5).

The Aldabra banded snail is endemic to Aldabra atoll (2), which comprises a number of islands surrounding a shallow lagoon (6). It has been recorded from the islands of Picard, Malabar, Polymnie, Esprit and Grande Terre (2).

This snail has been recorded from a variety of habitats within Aldabra, including mixed scrub and areas of mixed vegetation (2).

Very little is known about the biology and ecology of this genus (2), let alone this rarely seen species. Closely related species are known to graze on algae, have high reproductive rates, and live for a relatively long time, around two to three years (2).

One feature that is known about the Aldabra banded snail is that during dry periods, it aestivates on the branches of shrubs (2). Aestivation is a period of dormancy, or inactivity, in which the snail withdraws into its shell and secretes a substance between the shell and plant, which helps the snail conserve vital water (3) (4) (7).

The last living specimen of the Aldabra banded snail was collected in 1997. Extensive searches for this species were conducted in many parts of Aldabra atoll in 2005 and 2006, but only shells were discovered, which were estimated to be five years old (2). Most of these remains therefore date the last living Aldabra banded snails to around 2000, meaning this snail is now most likely extinct (2).

With changes in habitat or food availability being ruled out as the cause of this extinction, (considerable changes have not been identified in either), the most likely cause is a change in climate (2), the most significant change on Aldabra atoll over the last 30 years (3). Throughout the 1980s and 1990s, there has been a decrease in rainfall, with increasingly prolonged dry periods (2).

Scientists have discovered that while adult land snails are well able to tolerate long periods of aestivation and dry conditions, juveniles suffer increased mortality and newly hatched snails do not survive aestivation (3). Thus, with an increase in long, dry periods, juvenile mortality would have risen, leaving an ageing population. As the old snails died naturally, the species would have slipped into extinction (2).

Now considered to be extinct, it is most likely too late for any conservation action for the Aldabra banded snail, but hopefully lessons can be learnt from the loss of this species. Its extinction highlights the complex challenges that may arise from climate change, and the wave of extinction that may hit biodiversity as climate change continues (2) (3).

For further information on climate change and conservation see:

• Conservation International:
  www.web.conservation.org/xp/CIWEB/programs/climatechange

• Algae: simple plants that lack roots, stems and leaves but contain the green pigment chlorophyll. Most occur in marine and freshwater habitats.
• Endemic: a species or taxonomic group that is only found in one particular country or geographic area.
• Genus: a category used in taxonomy, which is below ‘family’ and above ‘species’. A genus tends to contain species that have characteristics in common. The genus forms the first part of a ‘binomial’ Latin species name; the second part is the specific name.
• Mantle: in molluscs, a fold of skin that encloses a space known as the mantle cavity, which contains the gills. The mantle is responsible for the secretion of the shell.

1. ZipcodeZoo (June, 2008)
   http://zipcodezoo.com/Animals/r/rachistia_praetermissus.asp#Taxonomy
2. Gerlach, J. (2007) Short-term climate change and the extinction of the snail Rhachistia aldabrae (Gastropoda: Pulmonata). Biology Letters, 3 (5): 581 - 585.
3. Gerlach, J. (2008) Extinction of Aldabra Banded Snail May Be First Directly Linked to Climate Change.Hotspots E-News, Conservation International. Available at: http://maildogmanager.com/page.html?p=000001XDDtjKFP98UZkgchujPw4uhcMNdgVqw
4. Sturm, C.F., Pearce, T.A. and Valdés, A. (2006) The Mollusks: A Guide to their Study, Collection and Preservation.Universal-Publishers Inc, Boca Raton, Florida.
5. Barnes, R.S.K., Calow, P., Olive, P.J.W., Golding, D.W. and Spicer, J.I. (2001) The Invertebrates: A Synthesis.Blackwell Science Ltd, Oxford.
6. UNEP-WCMC Protected Areas Programme (November, 2008)
   http://www.unep-wcmc.org/sites/wh/pdf/Aldabra%20Atoll.pdf
7. Allaby, M. (1991) Dictionary of Zoology.Oxford University Press, Oxford.