ACTIVE DISPERSAL OF STENASELLUS VIREI BOUI MAGNIEZ (ISOPODA) THROUGH THE ALLUVIAL SEDIMENTS OF A PYRENEAN STREAM (SOUTHERN FRANCE)

During a 13 month project Stenasellus virei boui Magniez, 1968, living in the alluvial sediments of the Nert stream (near Saint Girons, Ariege), were sampled monthly in an area of 30 m² at depths of 0.6 and 1.0 m. Isopods were found over the whole area at low frequencies: 91% of the samples contained 1-5 specimens and a total of 140 individuals was caught. After each sampling series, the sites were baited and re-sampled the next day. The isopods aggregated rapidly (up to 148 specimens/sample) around the bait, especially in interstitial areas with high permeability, and more than 2,000 additional individuals were caught with this technique. The experiment documents that S. virei boui is represented in the field by an unexpectedly large perennial population, even in superficial sediments. The animals actively react to chemical cues, moving rapidly through the interstitial space. The data also point out that the interstitial habitat and the isopods that are adapted to live within it, together build a complex ecological system.

Lors d'une etude poursuivie pendant treize mois, Stenasellus virei boui Magniez, 1968, qui vit dans les sediments alluviaux du Nert (pres de Saint-Girons, Ariege), a fait l'objet de prelevements mensuels sur une aire de 30 m², a 0,6 m et 1,0 m de profondeur. Cet Isopode a ete recolte sur la totalite de l'aire prospectee mais avec une frequence peu elevee: 91% des prelevements contenaient 1 a 5 specimens et un total de 140 individus ont ete recoltes. Apres chaque serie de prelevements, les sites etudies ont ete appates puis preleves a nouveau le jour suivant. Les Isopodes se rassemblaient rapidement autour des appats (jusqu'a 148 individus par site), essentiellement dans les zones a permeabilite elevee. Plus de 2000 individus additionnels ont ete captures par cette technique. Les donnees obtenues montrent que S. virei boui peut former des populations abondantes dans des sediments relativements superficiels; les animaux repondent activement aux informations chimiques en se deplacant rapidement dans les espaces interstitiels. Ainsi est suggere le fait que l'habitat interstitiel et les Isopodes qui lui sont adaptes forment un systeme ecologique plus complexe qu'on ne le pensait.