minutes for F, CTandis . However, all five species did not show 

 such extremes of sensitivity to Go brevis cultures. The minimum 

 "death times" for C. variegatus were 2-1/3 to 2-2/3 hours. 



The concentration of G. brevis in the cultures used for the 

 seven experiments varied from 0„6 to 2.1 million per liter. Since 

 these cultures were not free of bacteria, such organisms possibly 

 contributed to their toxicity. 



The results of a prellminar'y study suggest that the survival 

 period after exposure to G. bre\/-is depends on the exposure time 

 and that fish subjected for just a few minutes may not recover 

 when transferred to sea water. M. cephalus , subjected to unialgal 

 G. brevis for 3 and 15 minutes and then transferred to sea water, 

 lived for 165 and 12 minutes, respectively (Experiment 5). 



Moreover, there is some suggestion that G. brevis cultures 

 may become more toxic with each subsequent addition of test fish. 

 For example, in experiment 5, the "death times" of each of four 

 M, cephalus added in succession to the same culture decreased 

 progressively from 29 to 7 minutes. We do not know the reason(s) 

 for this apparent increase in toxicity. Since the test materials 

 were not aerated, possibly the oxygen content was progressively 

 lowered by the test fish and thereby decreased the "death times". 



The toxicity of unialgal G. brevis cultures does not depend 

 on the presence of living organisms. Cultures heated to 35 and 

 45 C. were no less toxic than the imtreated ones. The removal 

 of Go brevis from, a culture by milllpore filtration did not reduce 

 the toxicity. The relative toxicity of a filtrate appears to be 

 dependent upon the type of filter membrane employed. A paper 

 membrane (Whatman No. 4-2) apparently retains the more toxic portion 

 of the culture. 



Attempts to determine whether G. brevis or its associated 

 bacterial flora produces the toxic substance were not entirely 

 successful. G. brevis- free cultiires presiuned to contain the 

 bacterial flora associated with unialgal cultures of this organism 

 were not toxic to the test fish. However, when these experiments 

 were performed, the bacterial coimts of these cultures were con- 

 siderably lower than those of the G. brevis cultures. Nevertheless, 

 a culture of presumed associated bacteria with a count of slightly 

 more than a million per m^l proved non-toxic to M. cephalus . The 

 bacterial count in this culture of experiment 4 (Table 2) was 

 comparable to those of the unialgal cultures which were toxic 

 to M„ cephalus and C, variegatus in experiment 7 (Table 5). 

 Furthermore, in the latter experiment ^anlnoculated culture medium 

 containing about 2 million bacteria per ml, which was slightly 

 higher than the counts of two different Q. brevis cultures, was 

 not toxic to the test fish. 



19 



