14 



COPEPODS OF LAST CRUISE OF CARNEGIE 



that this varies with each species and possibly with each 

 individual, and is affected by the physicochemical factors 

 of its environment. The optimum zone of distribution 

 changes with the age of the animal and with its physico- 

 chemical state at any moment. This explains the fact that 

 nearly all the copepods show a strong positive photo- 

 tropism during their development stages, although many 

 of them become negatively phototropic on reaching 

 maturity. Russell (igiyb) confirmed this and added: 

 "The gradual changes in vertical distribution throughout 

 the life of an individual have been termed ontogenetic 

 migrations." 



Russell (1925, 1926^, 1926^, i926r, 1927(7, 1928) has pub- 

 lished several papers on "The vertical distribution of 

 marine macroplankton." He deals with the entire plank- 

 ton, especially the young of fishes, and devotes one paper 

 to light intensity as a controlling factor in plankton dis- 

 tribution (1926c, p. 415). He gives the vertical distribution 

 of Calanits, Centropages, and Temora in water over 50 

 meters deep, at different hours of the day, under different 

 intensities of light, and during different months of the 

 year. We gather from these that the vertical distribution 

 sometimes begins below the surface, attains its maximum 

 at depths of 5 to 20 meters, and then diminishes and dis- 

 appears before reaching a depth of 50 meters. This distri- 

 bution was in water which was shallow as compared 



with the open ocean, but there is no reason why it should 

 not occur also in the open ocean at least to some extent. 

 If it does, then we should find communities in the 25- 

 meter zone differing more or less from those at the sur- 

 face and at the 50-meter level. In all probability, there- 

 fore, a tow taken at the 25-meter level would contain 

 species not found at the surface or at the 50-meter level. 

 Similarly, the 75-meter level would probably be found to 

 contain species different from those of the 5u-meter and 

 loo-meter levels. We are thus forced to the inference that 

 the Carnegie towings probably did not obtain all the 

 species that occur within the upper 100 meters of the 

 ocean water. 



Incidentally, also, these considerations indicate the in- 

 adequacy of results obtained by daytime towings at the 

 surface alone in an effort to determine the plankton of 

 any given locality. The number of species thus obtained 

 would constitute so small a fraction of the entire plank- 

 ton at the locality that it would have but little value. A 

 surface towing in the night, on the contrary, might give 

 a very respectable indication of the total plankton, since 

 many of the species that remain at deeper levels during 

 the daytime come to the surface at night. To obtain satis- 

 factory results, therefore, there must be tows at different 

 levels during the daytime and these must be supple- 

 mented by tows at or near the surface at night. 



