ADAPTIVE RESPONSES IN CYPRIDOPSIS-VIDUA (CRUSTACEA, OSTRACODA) TO FOOD AND SHELTER OFFERED BY A MACROPHYTE (CHARA-FRAGILIS)

The ostracod Cypridopsis vidua prefers periphyton growing on Chara fra gilis to other feeding substrates. This species actively searches for periphyton, scrapes it off from Chara stems and handles it with a well organized feeding technique. When exposed to different quantities of periphyton on Chara stems, C. vidua selects foraging sites according t o their nutritional value. When exposed to low quantities of water fro m tanks containing fish, C. vidua responds to these weak chemical cues moving from uncovered areas into Chara plants. Experiments with juven ile cyprinids (Abramis brama) as predators and C. vidua as prey clearl y showed that the presence of Chara reduces fish foraging success and that C. vidua survival rite increases with increasing density of Chara .