8 



COPEPODS OF LAST CRUISE OF CARNEGIE 



of those in the Pacific, each listing 50 species or more. 

 In the Atlantic, the Caribbean Sea stations yielded a 

 larger number of species than those in the Sargasso Sea. 

 But, as already stated, both these regions were found by 

 Jespersen (1926) to be much inferior to other parts of the 

 North Atlantic. 



Diversity of Distribution 



We have just seen that there is great disparity in the 

 quantity of macroplankton obtained from different 

 localities of the various oceans. In dealing with the 

 copepods we may go farther and say that there is not a 

 single species that does not show more or less diversity in 

 its distribution. In more than 90 per cent of the species 

 the diversity is so great that it becomes the predominant 

 feature of the distribution. This diversity is evident not 

 only in the horizontal or geographic distribution, but 

 even more in the vertical distribution, as will be shown 

 later. Any attempt, therefore, to calculate the number of 

 copepods per liter or per cubic meter of the ocean water 

 gives us no practical information. One might as well try 

 to calculate the number of gulls and terns per cubic meter 

 of the atmosphere above the ocean. 



The recurrence of specific names in successive station 

 lists might suggest at first thought a more or less uniform 

 distribution. But when we consider the relative abun- 

 dance, we are likely to find that the species was repre- 

 sented at one station by hundreds of individuals, whereas 

 perhaps at the very next station the entire tow had to be 

 carefully examined in order to secure a single specimen. 



The concept of diversity is greatly strengthened by a 

 study of the data contained in the station lists here 

 recorded, especially those on relative abundance. Simply 

 the name of the species has heretofore been recorded, and 

 usually we have been left in complete ignorance of its 

 comparative abundance and its vertical distribution. The 

 interesting and valuable information which may be 

 derived from such data furnishes an abundant warrant 

 for their publication. Of the 263 different species recorded 

 from 166 localities in the Pacific Ocean, 105 appear at 5 

 localities or less, 60 being each confined to a single local- 

 ity. Of the remainder, 120 species are restricted to a total 

 of 50 localities or less, which is only a two-sevenths dis- 

 tribution. Only 7 species, or less than 3 per cent of the 

 entire number, have any claim to universal distribution, 

 and each of them shows great diversity in its relative 

 abundance. 



A remarkable instance of inequality in distribution is 

 shown by two surface tows taken between stations 50 

 and 51. The tows were made consecutively at the same 

 locality, on the same day, and with the same net. To- 

 gether they yielded 42 species, and 23 of these, or more 



than half, were found in one tow only and did not 

 appear at all in the other. Two surface tows between 

 stations 62 and 63, taken on the same day, with the same 

 net, but a short distance apart, yielded 50 species, 32 of 

 which, or 64 per cent, were confined to one of the tows. 

 Between stations 65 and 64 three surface tows were made 

 on January 2 and three others on January 5, all with the 

 same net, but short distances apart. The first three tows 

 yielded 44 species, of which 26 appeared in one tow only. 

 The second three tows gave a total of 37 species, and 22 

 of the.se were found in one tow only. 



Such data not only confirm the irregularity of distribu- 

 tion in the plankton, but also suggest that, even in 

 restricted areas, what may be termed the "personnel" of 

 the plankton is constantly changing. Two tows over the 

 same area, no matter how quickly one may follow the 

 other, will never yield identical results. Having thus 

 established the lack of uniformity in the horizontal or 

 geographical distribution of the copepods, we turn to 

 their vertical distribution. 



Daytime Stratification 



The different species of copepods that are found near 

 the surface of the ocean in the daytime show a marked 

 tendency to arrange themselves in zones or layers parallel 

 \vith the surface. Some species are practically confined to 

 the surface tows, of which they often constitute a very 

 large percentage, rarely as high as 90 to 95 per cent. 

 Others are confined entirely or very largely to the 50- 

 meter tow, and still others apparently do not approach 

 nearer to the surface than the lOO-meter tow. 



If the station lists are compared, it will be found that 

 this stratification is apparent in every one of them. Per- 

 centages large enough to constitute definite evidence of 

 layering are always present. This does not necessarily 

 mean that any one species is always and everywhere 

 found at one of the three depths and never at either of 

 the other two, although even that is true in some 

 instances. But it does signify that at the time the tow 

 was taken and under the existing conditions, very re- 

 spectable percentages of the copepods showed a decided 

 preference for one of the three depths to the exclusion of 

 the other two. On an average, about half the copepods 

 present at each station show such an exclusive preference, 

 and most of the other half show a similar but less marked 

 liking for some one of the zones. They are found in 

 abundance in the preferred zone, but only rarely or in 

 small numbers in the other two zones, being often wholly 

 absent from one of them. In all probability a closing net 

 would eliminate many of these secondary appearances 

 and credit more species with exclusive preference for the 

 zones where they were most abundant. Two or three 



