370 Marine Microbiology 



type occurred regularly two to three days after transfer to a 

 fresh feeder culture. Upon re-isolation of mating pairs it was 

 found that the conjugants do not separate but die. Thus selfing 

 causes a drastic drop in cell numbers in the population. The 

 survivors vary greatly in size, from 15 /< to 40 /' in length. When 

 subclones were established from the smallest and largest in- 

 dividuals, they exhibited the same pattern of life cycle: the 

 feeding animals were 45 /t long and after selfing had occurred 

 the population consisted of diverse sizes and was remarkably 

 tliinned out. Among the sui-vivors the small individuals predomi- 

 nated. This morphological variability was not found in five out 

 of the original twenty isolates. During twenty-two passages no 

 sexual reaction was observed. The cells were large, 45 /x to 50 /a, 

 throughout the cultural life cycle. When a certain density was 

 reached in a population the cells became cannibalistic. Phenol 

 red incoi-porated in the media indicated that the cannibahstic 

 clones shifted the pH from 7.4 to 6.8 within a seven day culture 

 period. This pH change did not take place in the sexual strains. 

 Clone T5 was chosen as representative of the selfer stiains, clone 

 T16 for the cannibalistic form. 



Taxonomic Studies 



Taxonomic studies were carried out with material stained 

 with the Lwoff-Chatton silver line method. The number of cilia 

 meridians was found to be 10-13. The buccal apparatus is of 

 typical tetrahymenal character. A roundish macronucleus and 

 one spherical micronucleus were observed. At the posterior end 

 one caudal cilia can be seen. There is generally one contractile 

 vacuole and vacuole pore. These moiphological features place the 

 form in the genus Tetrahymena. Our strains represent probably 

 the macrostome and the microstome form of one and the same 

 species. Facultative parasitism is known for these holoti'ichous 

 ciliates (2). 



Growth Experiments 



A typical growth experiment of tlie carnivorously feeding 

 T16 strain is represented in Figure 1. While in maintenance 

 medium alone no increase is observed, the cell numbers rise 

 seven-fold above the input within 45 hours in the KB fed ciliate 



