Abstract
BackgroundSpecies of anopheline mosquitoes are largely distributed over emerged lands around the world and, within the tropics, few areas are without these insects, which are vectors of malaria parasites. Among the exceptions is the Seychelles archipelago in the western Indian Ocean. However, in the Aldabra island group, located in the extreme western portion of the archipelago, Anopheles gambiae s.l. was introduced, leading to massive proliferation and then elimination, with the most recent autochthonous malaria cases recorded in 1931.MethodsIn order to re-examine the absence of anopheline mosquitoes in the Seychelles, an entomological field survey was conducted in December 2008 at 17 sites on four granitic islands, including Mahé and Praslin, and ten sites on coralline atolls in the extreme west, including Aldabra.ResultsNo evidence of larval or adult anophelines was found at the surveyed sites, which supports their absence in the Seychelles.ConclusionsIn the granitic islands of the Seychelles, the climate is favourable for anophelines. However, these islands are protected by their remoteness and prevailing seasonal winds. In addition, stagnant freshwater, required in anopheline larval development, is relatively uncommon on the granitic islands because of the steep slopes. In the southwestern atolls (Aldabra and Providence-Farquhar groups), the presence of a long dry season of up to nine months and the total absence of permanent natural freshwater prevents the breeding of anophelines and their successful colonization. The Seychelles does not have any native land mammals and like in other parts of the world (Antarctica, Iceland, New Caledonia, Central Pacific islands) their absence is associated with the lack of anophelines. This suggests an obligatory relationship for anophelines to feed on terrestrial mammals, without alternative for blood-feeding sources, such as bats, birds and reptiles.
Highlights
Species of anopheline mosquitoes are largely distributed over emerged lands around the world and, within the tropics, few areas are without these insects, which are vectors of malaria parasites
In the first two zones, there are no mosquitoes, and in the third category there are no anophelines, culicine mosquitoes abound. In this list of islands without anophelines, the case of the Seychelles is puzzling for at least two reasons: (i) the Seychelles is located in the southwestern Indian Ocean where most existing islands, if not all, present autochthonous or introduced anophelines (African east coast, Madagascar, the Comoros archipelago, La Réunion, Mauritius); (ii) subequatorial islands in the Seychelles have climatic conditions suitable for anophelines
Any attempt to explain the absence of anophelines in the Seychelles archipelago is inherently speculative. The resolution of this question is an interesting and important biogeographic challenge linked with a number of theoretical and applied biological aspects associated with invasive species, the debate of adaptation vs. evolution and the aspect of elimination-eradication of natural populations by man
Summary
Species of anopheline mosquitoes are largely distributed over emerged lands around the world and, within the tropics, few areas are without these insects, which are vectors of malaria parasites. Malaria parasites of mammals, including humans, are exclusively transmitted by mosquitoes belonging to the genus Anopheles These anophelines are observed virtually worldwide, the distribution of a given Anopheles species varies from being highly localized to sub-continental. In the first two zones, there are no mosquitoes, and in the third category there are no anophelines (sub-family Anophelinae), culicine mosquitoes (sub-family Culicinae) abound In this list of islands without anophelines, the case of the Seychelles is puzzling for at least two reasons: (i) the Seychelles is located in the southwestern Indian Ocean where most existing islands, if not all, present autochthonous or introduced anophelines (African east coast, Madagascar, the Comoros archipelago, La Réunion, Mauritius); (ii) subequatorial islands in the Seychelles have climatic conditions suitable for anophelines. It is difficult to explain the absence of anophelines in the Seychelles [3,4]
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