GROWTH-RATE OF THE MARINE PLANKTONIC CILIATE STROMBIDINOPSIS-CHESHIRISNYDER AND OHMAN AS A FUNCTION OF FOOD CONCENTRATION AND INTERCLONAL VARIABILITY

A clonal culture of the ciliate Strombidinopsis cheshiri Snyder and Oh man was established from British Columbian coastal waters. Using the s olitary 4 mu m diatom Thalassiosira pseudoanana (Hustedt) Hasle and He imdal as food, a numerical response was obtained by keeping the ciliat es in semi-continuous culture, measuring specific growth rates at diff erent prey concentrations, and fitting the data to a rectangular hyper bolic function; this provided growth and mortality rates. From the exp eriments four pieces of information were obtained for S. cheshiri: 1) the threshold concentration (i.e., the prey concentration where ciliat e growth = 0) was similar to 10(3) . ml(-1) (similar to 6 ng C . ml(-1 )); 2) the major increase in growth rate occurred between 10(3)-10(4) prey . ml(-1) (similar to 6-60 ng C . ml(-1)); 3) the maximum growth r ate was mu = 0.985 (similar to 1.4 divisions . day(-1)); and 4) the ci liate was capable of selfing conjugation. The growth rate data were, h owever, variable due to sub-clonal differences (i.e., clones secondari ly isolated from the original clone). Data obtained from examining cel l isolates suggested that the sub-clonal differences were caused by cl onal ageing and selfing conjugation. Consequently, abnormally low grow th rates were assumed to be due to ciliates with a high mutational loa d; outliers were removed and not included in the fnal numerical respon se. This is the first observation of selfing conjugation in oligotrich s and one of only a few observations of clonal ageing for these ciliat es. The results suggest that 1) this genus may be useful for future wo rk on oligotrich mating and 2) older cultures of these ciliates should be used with caution when estimating ecological parameters.