Growth responses of planktonic ciliates in the genera Strobilidium and Strombidium

Intrinsic growth and mortality rates need to be estimated to assess the potential for oligotrich ciliate populations to exploit prey populations, and since species specific differences exist, data for a variety of ciliates grazing on a variety of prey are also needed. In this paper, data are presented on the growth and mortality rates of 4 planktonic oligotrichs (2 Strobilidium species and 2 Strombidium species), at a number of prey concentrations. Growth and mortality data for these and other species in these 2 genera are reviewed. In general, these oligotrichs have maximum growth rates (p, d-l) ranging from -0.3 to 2.2 (at temperatures ranging from 6 to 31C); they have similar threshold concentrations (the food concentration where growth rate equals zero) of 10 to 100 ng C ml-'; they have rapid mortality rates at sub-threshold concentrations (cells only survive 1 to 3 d), and the range of food concentrations where the growth rate increases from zero to near its maximum is 20 to 300 ng C ml-l Although generalities can be made about ciliates in these 2 genera, the data suggest that in terms of growth and mortality rates, planktonic oligotrichs cannot be considered a single functional group. There is, however, a need to predict 'typical' oligotrich growth rates. To this end, several formulae have been established to predict maximum growth rates from ciliate cell volume and ambient temperature. In this paper, the growth data for Strobilidii~m and Strombidium are reviewed and compared to the predicted values from formulae In the literature; the formula presented by Muller & Geller (1993, Arch. Hydrobiol. 126: 315-327) appears to be the best pred~ctor of maxlmum growth rate from cell volume and ambient temperature. Finally, based on the review of growth data for these 2 genera, some speculations are made as to how these ciliates may exploit prey populations in a planktonic environment composed of small-scale patches.