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 t he alluvial sediments of the Nert stream (near Saint Girons, Ariege), were sampled monthly in an area of 30 m(2) at depths of 0.6 and 1.0 m. Isopods w ere found over the whole area;It low frequencies: 91% of the samples contai ned 1-5 specimens and a total of 140 individuals was caught. After each sam pling series, the sites were baited and re-sampled the next day. The isopod s aggregated rapidly (up to 148 specimens/sample) around the bait, especial ly in interstitial areas with high permeability, and more than 2,000 additi onal individuals were caught with this technique. The experiment documents that S. virei boui is represented in the field by an unexpectedly large per ennial population, even in superficial sediments. The animals actively reac t to chemical cues, moving rapidly through the interstitial space. The data also point out that the interstitial habitat and the isopods that are adap ted to live within it, together build a complex ecological system.