Copepoda 223 



frequent. Further information on effect of different temperatures on 

 other species is wanting, nor is it known to what degree constancy of 

 temperature must be maintained. 



Illumination. The effect of light on the well-being of copepods has not 

 been adequately investigated. Since the constant temperature tanks 

 which were used in these experiments were open at the top only, the cul- 

 ture dishes placed in them received indirect illumination reflected from 

 the ceiling. Direct sunlight is certainly harmful and there is no doubt 

 that the illumination should be weak. The copepods placed in the re- 

 frigerator in complete darkness survived well for several weeks, and it 

 is possible that light is not necessary for these animals at any time. 



Food. The question of what forms the chief food of copepods is a 

 controversial one and is being intensively investigated at various labora- 

 tories and from various angles. The matter has been reviewed by Clarke 

 ( 1934) and the specific experiments carried out at Woods Hole have been 

 described in detail by Clarke and Gellis (1935). Since a satisfactory 

 food for culturing has not yet been found, a brief statement of methods 

 only will be attempted here. The amount of food and frequency of feed- 

 ing required in different cases is highly variable for in early experiments 

 with Centropages it was found that death supervened within a few days 

 if food material was not added, whereas in the case of Calanus a few 

 specimens lived for 14 days in continuously flowing water from which all 

 particulate matter had been removed by a membrane filter. 



The survival of Centropages, Acartia, and Labidocera was improved 

 by adding to the culture dishes planktonic material obtained by centri- 

 fuging seawater, or in larger quantities, by making short hauls with a 

 diatom net. "Persistent" cultures of diatoms and green flagellates grown 

 in the laboratory* were also used. Experience showed that organisms 

 which grow encrusted on the bottom and walls of the vessel were not 

 suitable for food, probably because copepods, being filter feeders, can 

 take in material in suspension only. However, the addition of none of 

 these foods prolonged the life of the majority of the animals for more 

 than about two weeks. When the flagellates were added to the water, 

 green material could be seen in the intestines of the copepods and many 

 excretory casts were found in the bottom of the container, but molted 

 shells were observed only rarely. 



In the case of Calanus more elaborate experiments have been carried 

 out since this article was originally prepared. Reference had best be 

 made to the published reports (Fuller and Clarke, 1936, and Fuller, 

 1937). Briefly, these experiments show that Calanus will live for several 

 weeks and molt readily when provided with fine planktonic material. 

 Bacteria are not, however, an important article of diet. Precisely which 



♦For method see Clarke and Gellis (1935)- 



