COMPARISONS OF PLANKTON 



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rella, Pleuromamma, and other genera. The temperature 

 at 2000 meters was below 3° C, and yet these copepods 

 seemed as healthy and active as other specimens of the 

 same species captured within the upper 100 meters, where 

 the temperature ranged from 15° to 21° C. It is evident 

 that even considerable variations in temperature would 

 affect such copepods very little, if at all. 



Salinity (Map ]) and Density 



In contrast with the temperature, both the salinity and 

 the density differ so little within the upper 100 meters 

 that they probably exert no influence on the vertical dis- 

 tribution of the copepods. But they do appear to affect 

 the regional distribution to a certain extent. The surface 

 salinity is a little higher in the Atlantic than in the Pacific 

 Ocean, which may be a contributing factor in the superior 

 quantity and variety of the Pacific plankton. None of the 

 Carnegie stations came within the area of maximum sur- 

 face salinity in the North Atlantic, but those numbered 

 from 22 to 34 were in the zone of minimum salinity, and 

 it is worthy of note that they yielded a larger average 

 number of species than any of the others. Again, the area 

 of maximum salinity in the Pacific Ocean lies around the 

 Society Islands. The Carnegie passed through this area, 

 and the station lists from within it are below the average. 

 All the stations where the largest numbers of species 

 were obtained show salinities at or a little below the 

 average. This would seem to indicate that such average 

 salinities are most acceptable to copepods and that there 

 is a tendency to avoid both extremes. The salinities and 

 densities at the three tow depths are exactly the same in 

 many of the station lists. When they do differ, it is within 

 such narrow limits as to preclude any appreciable in- 

 fluence on vertical distribution. 



Hydrogen-Ion Concentration 



The same statement may be made about the hydrogen- 

 ion content of the sea water as about the salinity and 

 density. The hydrogen-ion concentration is too often 

 exactly the same for all three tow depths, and when it 

 does vary the extent of the variation is too slight to war- 

 rant an assumption that it has any material influence on 

 the vertical stratification of the copepods. 



Pliototropism 



The preceding discussion has shown that neither tem- 

 perature nor salinity, hydrogen-ion concentration, or 

 density could produce the daytime stratification of which 

 we find definite evidence in every station list of species. 

 Temperature and salinity are concerned in the regional 

 distribution of the species and may answer the cjuestion 

 as to why a given copepod is found at one station or in 

 one locality and not in another; neither density nor 



hydrogen-ion concentration appears to enter appreciably 

 into even this kind of distribution. We are forced to look 

 elsewhere for the cause of the daytime stratification. We 

 must search for something that is everywhere present in 

 the daytime, that is at work all the time from dawn to 

 dusk, and that is strong enough to produce definite and 

 uniform results even in the face of antagonistic influences. 

 The very fact that this is a daytime stratification suggests 

 sunlight as its cause, and after the elimination of every- 

 thing else, this is the only cause left that answers all the 

 requirements that have just been enumerated. 



It has been found by many observers that light is the 

 most important factor controlling the nocturnal migra- 

 tion of copepods, and it is perfectly Icjgical that it should 

 also control their diurnal movements. Many species of 

 copepods in both salt and fresh water migrate regularly 

 from greater or lesser depths toward the surface as soon 

 as it begins to be dark, and begin to return between mid- 

 night and sunrise. It is evident that the depth from which 

 a copepod will be able to reach the surface is necessarily 

 limited by its swimming ability. The jerky movements 

 characteristic of copepod locomotion are not conducive 

 to even moderate rapidity, and they are never continued 

 steadily for any length of time. The progress is always 

 leisurely, interspersed with frequent rests and periods of 

 floating. The only occasion when a copepod tries to 

 move rapidly is in an attempt to escape impending 

 danger, and even then its efforts are spasmodic and 

 nearly always futile. Moreover, its course is never in a 

 straight line for any considerable distance, but is full of 

 turns and twists and often spiral movements. 



Another fact must also be considered, namely, that the 

 interval between sunset and sunrise varies greatly with 

 the season of the year and with the latitude of the loca- 

 tion. All the Carnegie stations were located in the tropical 

 and temperate zones. The observations in the North 

 Atlantic were made from the middle of May to the first 

 of October, those in the South Pacific from the first of 

 November to the last of April (southern summer), and 

 those in the North Pacific from the first of May to the 

 middle of October. .At practically every one of the stations, 

 therefore, the time of year and the latitude were such as 

 to give considerably reduced periods of darkness, with a 

 corresponding increase in the periods of daylight. 



Such considerations lead inevitably to the conclusion 

 that the diurnal migrations are confined to the upper 

 layers of the ocean water and do not reach any con- 

 siderable depth. It would, seem as if from 100 to 150 

 meters were about the lowest depth from which the 

 average copepod would be able to reach the surface. But 

 of course the migration is not limited by the copepod's 

 ability actually to reach the surface within a given 

 period. It undoubtedly takes place down to the limit of 



