ABSTRACT 



This investigation was conducted to determine the effects of 

 Gymnodinium brevis Davis, a naked dinoflagellate definitely associated 

 with the sporadic mass mortality of marine animals in the Gulf of 

 Mexico, on fish under controlled conditions. A series of experiments, 

 seven with unialgal cultures and two with bacteria-free cultures, 

 demonstrated the toxicity of this organism to six species of fish. 

 Bacteria-free cultures were just as toxic as unialgal cultures. The 

 cultures employed contained 0.6 to 4-. 8 million G. brevis per liter. 

 Apparently the test fish were differentially sensitive to G. brevis 

 cultures. In order of decreasing sensitivity, the test fish were: 

 Membras vagrans , Mugil cephalus , Fundulus grandis , Mollienisia 

 latipinna . Fundulus similis . and Cyprinodon variegatus . The lethality 

 of bacteria-free G. brevis cultures to fish clearly indicates that 

 this dinoflagellate is the direct cause of the mass fish mortalities 

 with which its "blooms" are associated. 



Toxicity of G. brevis cultures does not depend on the presence 

 of living organisms. The more toxic portion of the cultures passes 

 through a millipore membrane whereas it is retained by filter paper. 

 In some experiments such possible lethal factors as oxygen deficiency, 

 svLffocation due to clogging of gills with masses of organisms, and 

 bacterial growth were eliminated thereby establishing that G. brevis 

 produces a toxic substance(s) . 



Two chromogenic marine bacteria, Flavobacterium piscicida Bein 

 and an unidentified red-pigment-produclng form, from the west coast 

 of Florida were tested for toxicity to fish. The results of these 

 tests are discussed. 



