UNIALGAL AND BACTERIA-FREE CULTURES OF G. BREVIS 



495 



beon encountered in sufficient abuiulaiice to 

 implicate it as a fish-killing agent. Thus far, 

 neither the association of chromogenic bacteria 

 witli extensive fish kills nor the natural existence 



of toxic concentrations of such bacteria has 

 been established. On the otlier hand, Gymno- 

 dinium breiis has been clearlj' implicated on both 

 these counts. 



SUMMARY AND CONCLUSIONS 



1. Since 1947, blooms of the dinoflagellate, 

 Gymnodinium brevis, have been associated with 

 sporadic mass mortalities of marine animals 

 and discolored water in the Gulf of Mexico. 

 Extensive laboratory studies conducted with 

 unialgal and bacteria-free cultures, as well as 

 related bacterial studies, offer overwhelming 

 evidence that blooms of this organism are the 

 direct cause of the associated mortalities. 



2. Bacteria-free cidtures of G. brevifi with con- 

 centrations varying from 2.3 to 4.8 million or- 

 ganisms per liter were toxic to two species of test 

 fish. Five species of fish were killed when sub- 

 jected to unialgal G. brevis cultures containing 

 0.6 to 2.1 million organisms per liter. The 

 numbers of G. breds in areas of natural fish 

 kills often greatly exceed these toxic laboratory 

 concentrations. 



3. Bacteria apparently do not produce or di- 

 rectly contribute to the production of the toxic 

 substance present in G. brevis cultures. Bacteria- 

 free cultures were just as toxic to fish as the 

 unialgal ones. 



4. The toxicity of G. brevis does not depend on 

 the presence of the living organisms. Removing 

 the organisms from culture by millipore filtration 

 or killing them with gentle heat did not appear 

 to alter the toxicity. The high dissolved-oxygen 



content of aerated G. brevis cultures eliminates 

 oxygen deficiency as a factor. 



5. The toxic substance produced by G. brevis 

 readily passes through a millipore membrane, but 

 for the most part it is retained by filter paper. 



6. Studies with bacteria-free and unialgal 

 cultures indicate that the sLx species of test fish 

 are difTerentially sensitive to G. brevis cultures. 

 The test fish, listed in order of decreasing sensi- 

 tivity, were Membras vagrans, Mugil cephalus, 

 Fundulus grandis, Mollienisia latipinna, Fundulus 

 similis, and Cyprinodon variegatus . 



7. Some chromogenic bacteria isolated from 

 the Gulf of Mexico or adjoining bays are toxic to 

 fish in laboratory tests. However, association 

 of such bacteria with mass fish kills in the Gulf 

 of Mexico has not been established. Flavobac- 

 terium piscicida, previously found to be toxic to 

 several species of fish in undetermined concen- 

 trations, was not toxic to Mugil cejjhalus at an 

 initial concentration of about 2 million organisms 

 per ml. A red bacterium isolated from G. brevis- 

 infested waters appears to be toxic to fish at con- 

 centrations in the order of 0.5 to 2 million per 

 ml. This bacterium was uncommon during our 

 survey since only two colonies were obtained from 

 15 samples of 1.0 ml. each. 



LITERATURE CITED 



Bein, Selwyn Jack. 



1954. A study of certain chromogenic bacteria iso- 

 lated from "red tide" water with a description of 

 a new species. Bull. Marine Science Gulf and 

 Caribbean 4 (2): 110-119. [Contribution No. 

 121, Marine Lab., University of Miami.] 



CoNNELL, Cecil H., and Joy Barnes Cro.ss. 



1950. Mass mortality of fish associated with the 

 protozoan Gonyaulax in the Gulf of Mexico. 

 Science 112 (2909): 359-363. 



Davis, Charles C. 



1948 Gymnodinium brevis sp. nov., a cause of dis- 

 colored water and animal mortality in the Gulf 

 of Mexico. Botanical Gazette 109 (3): 358-360. 

 [Contribution No. 17, Marine Lab., University 

 of Miami.] 



Droop, M. R. 



1954. A note on the isolation of small marine algae 

 and flagellates for pure cultures. Jour. Marine 

 Biological Assoc. United Kingdom 33 (2): 511- 

 514. 



Galtsofp, Paul S. 



1948. Red tide. Progress report on the investiga- 

 tions of the cause of the mortality of fish along 

 the west coast of Florida conducted by the U. S. 

 Fish and Wildlife Service and cooperating organi- 

 zations. U. S. Department of the Interior, 

 Fish and Wildlife Service, Special Scientific 

 Rept. No. 46: 1-44. 



1949. The mystery of the red tide. Scientific 

 Monthly 68 (2): 108-117. 



