Pachnodus snail (Pachnodus velutinus)

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Pachnodus velutinus on leaf
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Pachnodus snail fact file

Pachnodus snail description

KingdomAnimalia
PhylumMollusca
ClassGastropoda
OrderStylommatophora (2)
FamilyCerastuidae
GenusPachnodus (1)

The now extinct Pachnodus velutinus was one of a number of land-snail species endemic to the Seychelles islands. A diminutive species, the shell of Pachnodus velutinus was less than 1.5 centimetres wide, with five whorls, and coloured a matt, uniform beige, while, in contrast, the body was black (1). Although the shell was always very thin and poorly calcified, some specimens lacked a calcified shell layer altogether, possessing only a thin outer layer, the periostracum, made of hardened protein (3).

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Pachnodus snail biology

Although little is known about the biology of Pachnodus velutinus, one interesting and important feature was its inability to undergo aestivation (3), a period of dormancy that occurs when conditions are hot and dry, analogous to hibernation in cold environments (5). Most other species of the genus Pachnodus are capable of forming a mucous seal between the aperture of their shell and the substrate they are resting upon, thereby preventing moisture loss and allowing the snails to survive when humidity levels drop below 70 percent. However, Pachnodus velutinus lacked this ability, and could only survive at humidities of over 80 percent, below which it would quickly dry up and die (3).

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Pachnodus snail range

Pachnodus velutinus was endemic to the island of Mahé in the Seychelles. Surveys in 1972 indicated that it occupied a limited range of around two square kilometres within Morne Seychellois National Park in the north of island. However, over the period 1972 to 1994, this species’ range rapidly contracted. By 1987, Pachnodus velutinus was only found in the forests of the Congo Rouge Mountain, and by 1994, it had disappeared altogether (3).

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Pachnodus snail habitat

A predominantly tree-dwelling species, Pachnodus velutinus inhabited mid to high-altitude mountain forest. As its range contracted, the remaining populations persisted in the high-altitude, mist forest zone (3), an extremely humid, biodiverse region of mossy forest (4).

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Pachnodus snail status

Classified as Extinct (EX) on the IUCN Red List (2).

IUCN Red List species status – Extinct

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Pachnodus snail threats

As a result of breeding between Pachnodus velutinus and the closely related snail, Pachnodus niger, a hybrid species was created, Pachnodus niger x velutinus. This species, still in existence, has features of both parent species, but, most importantly, it has the strong, well-calcified shell of Pachnodus niger, allowing it to undergo aestivation. Both Pachnodus velutinus and the hybrid co-existed on Mahé as regionally separate populations until the 1960s, when clearance of mid-altitude forest (3), and possibly global climate change (6), may have caused the local climate within the ranges of both species to become significantly less humid (3) (6). Since it was able to aestivate, the hybrid species was better adapted to survive in the mid-altitude regions than Pachnodus velutinus and, as a result, from 1972 to 1987 it rapidly expanded its range. As the hybrid encroached on the range of Pachnodus velutinus the two snail species bred together, producing hybrid offspring, and thereby causing the range of pure Pachnodus velutinus individuals to contract. By 1987, the only surviving pure populations of Pachnodus velutinus were in the dampest, most climatically stable part of its former range, the high-altitude Congo Rouge mountain region. At this point the range of Pachnodus velutinus had become so small that random mating with the hybrid snails gradually bred this species out of existence (3).

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Pachnodus snail conservation

The extinction of Pachnodus velutinus may represent an important example of the diverse impacts that can result from climate change in local ecosystems. Despite reforestation of degraded areas in Morne Seychellois National Park (4), the effects of the change in forest composition (3), and perhaps global climate change (6), have already contributed to the loss of a unique component of Seychelles’ biodiversity. This is the first example of hybrid superiority causing the extinction of a parent species, and is also one of the few examples where an extinction process has been followed all the way to its conclusion (3). Hopefully, the study of this event will provide useful information for the prevention of future extinctions.

View information on this species at the UNEP World Conservation Monitoring Centre.
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Find out more

To learn more about conservation and biology in the Seychelles see:

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Authentication

Authenticated (10/04/2009) by Dr. Justin Gerlach. Scientific Co-ordinator, The Nature Protection Trust of Seychelles.
http://islandbiodiversity.com

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Glossary

Calcified
Hardened by the deposition of calcium carbonate.
Endemic
A species or taxonomic group that is only found in one particular country or geographic area.
Hybrid
The offspring produced by parents of two different species or subspecies.
Hybridising
Cross-breeding between two different species or subspecies.
Whorls
The spirals or convolutions in the shell of a snail.
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References

  1. Gerlach, J. (1999) Snails of the genus Pachnodus (Mollusca; Gastropoda; Enidae): their origins and evolution. Journal of Biogeography, 26: 251 - 255.
  2. IUCN Red List (April, 2009)
    http://www.iucnredlist.org
  3. Gerlach, J. (1996) Patterns of hybridization and extinction in a tropical land snail. Phelsuma, 4: 27 - 38.
  4. UNEP-WCMC World Database on Protected Areas (October, 2008)
    http://www.unep-wcmc.org/wdpa/
  5. Allaby, M. (1991) The Concise Oxford Dictionary of Zoology. Oxford University Press, Oxford.
  6. Gerlach, J. (01/01/0001 00:00:00) Climate change as a cause of population decline and extinction. The Open Conservation Journal,.
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Image credit

Pachnodus velutinus on leaf  
Pachnodus velutinus on leaf

© Dr. Justin Gerlach

Dr. Justin Gerlach
Nature Protection Trust of Seychelles
Seychelles
gerlachs@btinternet.com
http://islandbiodiversity.com

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