Arch. Hydrobiol. 137 119-133 Stuttgart, July 1996 Tradeoff between foraging and antipredator behaviour in a macrophyte dwelling ostracod Franz Uibleinl, Josep R. Roca2'3, Angel Baltanäs2'a and Dan L. Danielopol2 With 4 figures and 2 tables in the text Abstract: The ostracod Cypridopsis vidua is a highly mobile crustacean which fre- quently occurs in beds of submerged macrophytes in lakes or ponds. A laboratory experiment consisting of eight different treatments was performed to test if this spe- cies responds to fluctuations in predation risk, food abundance, hunger, and plant structure by trading off conflicting demands such as searching for food in the shel- tered lower zone of a macrophyte or shifting to nutritionally more rewarding patches in the exposed upper zone of this plant. Stems of the macrophyte Chara fragilis cov- ered with a variable amount of periphyton were used as plant structures. To signal predation risk, chemical stimuli derived from tanks containing cyprinid fish were used. The ostracods responded to predator signals by spending significantly more time in the lower zone of the aquaria and by increasing swimming activity especially dur- ing the initial phase of encountering the signals. Both with increased hunger level and high food abundance the time spent on Chara increased. When only a single stem of Chara was offered, time spent swimming in the lower zone of the aquarium increased markedly compared to the presence of three stems. In response to different predation risk and food abundance, C. yidua ostracods performed shifts in their vertical micro- habitat use as a result of a tradeoff between foraging and antipredator demands. In three experimental treatments the ostracods showed considerable temporal variations in microhabitat choice indicating additional behavioural fine-tuning after a quick pre- liminary adjustment. In agreement with earlier experimental studies our data support the conclusion that these phytophilous crustaceans are characterized by a well devel- oped capability for exploring their surroundings and by responding to external and in- ternal factors in a flexible and behaviourally dynamic manner. 1 Authors' addresses: Konrad Lorenz-Institut für Vergleichende Verhaltensforschung, ÖAW, Savoyenstr. I a, A-1 160 Vienna, Austria. 2 Institut für Limnologie, ÖaW A-5310 Mondsee, Austria. 3 Unitd PAGE, Pal6ontologie et Paldog6ographie, B-1348 Louvain-la-Neuve, Bel- gium. a Dept. of Ecology, UniversidadAut6noma de Madrid, E-28049 Madrid, Spain. 0003-91 36/96/0137-01 1 I $ 3.75 o 1996 E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart