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| dct:abstract
| - Asellids (Isopoda, Asellota, Aselloidea) are typically walkers (epibenthic), typically oculated, typically pigmented Crustaceans, typically dwellers of holarctic surface freshwater. Their populations isolated in dark habitats (ecotones between surface and underground water) grow richer and richer in specimens more and more deprived of pigment, then microphtalmous. Finally, their speciation has given a number of species entirely unpigmented and anophtalmous, highly adapted to life in underground waters: - Open space water of karstic aquifers (generally large species),- Interstitial water in loose deposits of suitable granulometry: sediments, alluvium, eluvium, sometimes colluvium, impregnated with phreatic water (generally smaller species). So, if a recent genus can include only pigmented-oculated species, a multispecific Asellid genus reaching its evolutive climax shows together: expansive and endemic surface species, microphtalmous more or less unpigmented stygophiles, and a number of stygobitic ones. At last, a declining genus is represented only by stygobitic species, the number of which decreases finally to one, before vanishing. It is well known that the original distribution of surface forms has been widely altered by all men's activities concerning freshwater habitats. Nevertheless, the distribution of underground species remains generally %22natural%22 and they behave like good witnesses for paleobiogeographical events. A curious species only known from an underground river of Western France (Bataille, dept. Deux-Sevres, hydrographic basin of the Charente River): Gallasellus heilyi (Legrand, 1956), required the creation of the monospecific genus Gallasellus Henry & Magniez, 1977. G. heilyi shows strong relationships with the endemic asellids of the NW quarter of the Iberian Peninsula (Gen. Synasellus Braga, 1944, Bragasellus Henry & Magniez, 1968) and also with North American asellids (gen. Caecidotea Packard, 1871), but not with the common Eurasian strains (Gen. Asellus E.L. Geoffroy 1762 or Proasellus Dudich, 1925). This relict form has been rediscovered in a small aquifer in the North part of Oleron Island (Dept. Charente-Maritime): a well in St-Denis d'Oleron. As Oleron Island represents the left side of the lower part of the paleo-valley of the Charente River, which emerged during the Quaternary low sea-levels, G. heilyi is obviously a remain of the ancient fauna of this small hydrographic basin. We find here a confirmation of the fact that the settlement of France and W Europe includes 3 steps: - 1. An ancient stock, from Atlantic origin and related to North American lineage, now represented by the multispecific genera Bragasellus and Synasellus of NW Iberian Peninsula and the mono-specific Gallasellus from The Charente River Basin. - 2. A more recent and highly multispecific stock with a peri-mediterranean distribution: Gen. Proasellus (the expansive surface species Proasellus meridianus (Racovitza, 1919) colonizing the Charente River). - 3. A very recent stock (Holocene and present), from Asiatic origin (Gen. Asellus, very expansive and eurytope) presently invading the Western part of France (for example in the Garonne River basin). There, its presence is so recent that it is represented by surface and some stygophilic populations only.Presently, the Charente River basin can be considered, as well as NW Iberian Peninsula, a sanctuary for the more ancient Asellid stock of Western Europe. The permanence of underground freshwater aquifers in a continental island represents a factor of conservation of stygofauna biodiversity. Nevertheless, as the expansive Asellus stock is gradually, but ineluctably invading Western Europe, the future stygophilic and stygobitic Asellus forms likely will compete and eliminate the relict Gallasellus and still prosperous Proasellus.
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| bibo:abstract
| - Asellids (Isopoda, Asellota, Aselloidea) are typically walkers (epibenthic), typically oculated, typically pigmented Crustaceans, typically dwellers of holarctic surface freshwater. Their populations isolated in dark habitats (ecotones between surface and underground water) grow richer and richer in specimens more and more deprived of pigment, then microphtalmous. Finally, their speciation has given a number of species entirely unpigmented and anophtalmous, highly adapted to life in underground waters: - Open space water of karstic aquifers (generally large species),- Interstitial water in loose deposits of suitable granulometry: sediments, alluvium, eluvium, sometimes colluvium, impregnated with phreatic water (generally smaller species). So, if a recent genus can include only pigmented-oculated species, a multispecific Asellid genus reaching its evolutive climax shows together: expansive and endemic surface species, microphtalmous more or less unpigmented stygophiles, and a number of stygobitic ones. At last, a declining genus is represented only by stygobitic species, the number of which decreases finally to one, before vanishing. It is well known that the original distribution of surface forms has been widely altered by all men's activities concerning freshwater habitats. Nevertheless, the distribution of underground species remains generally %22natural%22 and they behave like good witnesses for paleobiogeographical events. A curious species only known from an underground river of Western France (Bataille, dept. Deux-Sevres, hydrographic basin of the Charente River): Gallasellus heilyi (Legrand, 1956), required the creation of the monospecific genus Gallasellus Henry & Magniez, 1977. G. heilyi shows strong relationships with the endemic asellids of the NW quarter of the Iberian Peninsula (Gen. Synasellus Braga, 1944, Bragasellus Henry & Magniez, 1968) and also with North American asellids (gen. Caecidotea Packard, 1871), but not with the common Eurasian strains (Gen. Asellus E.L. Geoffroy 1762 or Proasellus Dudich, 1925). This relict form has been rediscovered in a small aquifer in the North part of Oleron Island (Dept. Charente-Maritime): a well in St-Denis d'Oleron. As Oleron Island represents the left side of the lower part of the paleo-valley of the Charente River, which emerged during the Quaternary low sea-levels, G. heilyi is obviously a remain of the ancient fauna of this small hydrographic basin. We find here a confirmation of the fact that the settlement of France and W Europe includes 3 steps: - 1. An ancient stock, from Atlantic origin and related to North American lineage, now represented by the multispecific genera Bragasellus and Synasellus of NW Iberian Peninsula and the mono-specific Gallasellus from The Charente River Basin. - 2. A more recent and highly multispecific stock with a peri-mediterranean distribution: Gen. Proasellus (the expansive surface species Proasellus meridianus (Racovitza, 1919) colonizing the Charente River). - 3. A very recent stock (Holocene and present), from Asiatic origin (Gen. Asellus, very expansive and eurytope) presently invading the Western part of France (for example in the Garonne River basin). There, its presence is so recent that it is represented by surface and some stygophilic populations only.Presently, the Charente River basin can be considered, as well as NW Iberian Peninsula, a sanctuary for the more ancient Asellid stock of Western Europe. The permanence of underground freshwater aquifers in a continental island represents a factor of conservation of stygofauna biodiversity. Nevertheless, as the expansive Asellus stock is gradually, but ineluctably invading Western Europe, the future stygophilic and stygobitic Asellus forms likely will compete and eliminate the relict Gallasellus and still prosperous Proasellus.
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| dct:bibliographicCitation
| - Magniez, G. & Henry, J. 2002. Présence d'un asellide stygobie dans une ïle: causes et consequences. <em>Memoires de Biospéologie</em>, 28(55): 143-147.
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