388 MARINE DINOFLAGELLATES AND RED WATER CONDITIONS 



cence in the sea. Allen (1939) stated categorically that they are the 

 most common cause of this phenomenon. It follows that any consid- 

 eration of the factors contributing to the occurrence of marine bio- 

 luminescence must include a consideration of the ecological factors 

 that influence and control the growth and distribution of its princi- 

 pal causitive agents, the dinoflagellates. The following discussion will 

 be concerned with the general ecology of these organisms. In addi- 

 tion, special attention will be devoted to a possible explanation for 

 the frequently reported occurrences of dense "blooms" or "swarms" of 

 dinoflagellates in many parts of the world. 



No attempt will be made in the following discussion to differen- 

 tiate between luminous and nonluminous dinoflagellates, but it will 

 be assumed that the presence or absence of this property will not 

 appreciably affect the interrelationships between the organisms and 

 their environment. A consideration of their ecology as a group is com- 

 plicated, however, by the fact that individual species may employ 

 almost any type of nutritional habit, including autotrophic, holozoic, 

 saprophytic, and parasitic. Although the number of species which 

 employ the second mode of nutrition, either obligatorily or faculta- 

 tively, is apparently quite large (Kofoid and Swezy, 1921), in total 

 abundance they are probably insignificant in comparison with the 

 photosynthetic forms. For this reason we will consider here primarily 

 the autotrophic dinoflagellates. 



General Ecology of Dinoflagellates 



Temperature 



Dinoflagellates are found in all parts of the ocean and are usually 

 present in some quantity at all times of the year. It is therefore im- 

 possible to make broad generalizations concerning tHe temperature 

 relationships of so large and widespread a group. Some species are 

 obviously arctic forms and may be observed living in subzero tem- 

 peratures associated with the polar seas (Gran, 1924; Braarud, 1935). 

 However, there does appear to be some justification for classifying the 

 great majority of the dinoflagellates as warm-water organisms. This 

 is particularly true if they are compared as a group with the diatoms, 

 which, in contrast, are often considered cold-water forms. 



