Freshwater ciliates as ecophysiological model organisms - lessons from Daphnia, major achievements, and future perspectives

Similar to Daphnia, many planktonic ciliates are algivores that occur in vir- tually every natural lake and reproduce primarily asexually. Due to their larger popula- tion size and shorter generation time, their significance as algal consumers and second- ary producers may exceed that of Daphnia during algal blooms and when averaged over the season. The high reproduction rate, the ease of culturing, the accessibility to experimental manipulation, and the potential to apply sophisticated measuring techni- ques such as flow cytometry render some ciliate species ideal candidates for ecophy- siological laboratory experiments. This paper summarizes recent research in which ciliates have been used as model organisms for investigating the effect of environ- mental key parameters on planktonic organisms. Special attention is given to the (com- bined) effect of temperature, food, pH and predators. Niche partitioning has been stud- ied at the level of genus, species and clone. Open questions and emerging perspectives of ciliate research for issues of general ecological relevance will be discussed at the end of each section.