Cypridopsis vidua (O.F.M.) lives within dense stands of Chara fragilis Desv. in Mondsee. The species develops fast and is very voracious, feeding on the "Aufwuchs" of the Chara stems. Under experimental conditions, C. vidua actively explores the interstitial of a fine gravel sediment. The ostracodes are also able to find their way out of the interstitial labyrinth.The limited colonisation of deep groundwater habitats by C. vidua (e.g. in the Danube and the Rhône valleys the species lives in superficial alluvial sediments located near the bottom of surface waters) is presumably due to its high energetic requirements.C. vidua recognises the chemical alarm signals of predators such as the fish Abramis brama (L.) ; the ostracods react by drecreasing their motility in both the surface and the interstitial habitat.