TRADEOFF BETWEEN FORAGING AND ANTIPREDATOR BEHAVIOR IN A MACROPHYTE DWELLING OSTRACOD

The ostracod Cypridopsis vidua is a highly mobile crustacean which fre quently occurs in beds of submerged macrophytes in lakes or ponds. A l aboratory experiment consisting of eight different treatments was perf ormed to test if this species responds to fluctuations in predation ri sk, food abundance, hunger, and plant structure by trading off conflic ting demands such as searching for food in the sheltered lower zone of a macrophyte or shifting to nutritionally more rewarding patches in t he exposed upper zone of this plant. Stems of the macrophyte Chara fra gilis covered with a variable amount of periphyton were used as plant structures. To signal predation risk, chemical stimuli derived from ta nks containing cyprinid fish were used. The ostracods responded to pre dator signals by spending significantly more time in the lower zone of the aquaria and by increasing swimming activity especially during 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 Cham was offered, time spent swimming in the lo wer zone of the aquarium increased markedly compared to the presence o f three stems. In response to different predation risk and food abunda nce, C. vidua ostracods performed shifts in their vertical microhabita t use as a result of a tradeoff between foraging and antipredator dema nds. In three experimental treatments the ostracods showed considerabl e temporal variations in microhabitat choice indicating additional beh avioural fine-tuning after a quick preliminary adjustment. In agreemen t with earlier experimental studies our data support the conclusion th at these phytophilous crustaceans are characterized by a well develope d capability for exploring their surroundings and by responding to ext ernal and internal factors in a flexible and behaviourally dynamic man ner.