UNEVENNESS OF OCEANIC PLANKTON 537 



migration, but the remarkable patchiness in distribution of P. gandichaudi has been 

 well demonstrated at South Georgia (Hardy and Gunther, 1936, p. 260). 



Young Euphausiacea occur in all records but 1, 2, 4, 23, 24 and 27. Whilst also 

 affected by vertical migration they show marked fluctuations with as a rule the peaks 

 close together. Here, too, there are indications of a certain rhythm in the fluctuations. 

 The apparent restricted zones of adult Euphausiacean distribution in records 5 and 6 

 are most likely explained by vertical migration, but not so the dense patch of Euphausia 

 longirostris in record 22 in daylight. 



The pteropod Limacina (records 7-1 1, 27, 28, 30-33) shows marked fluctuations and 

 particularly a tendency to occur in isolated patches. The young gastropods Ici7ithina 

 fluctuate in record 3. 



Salps show remarkable fluctuations in record 7 and a tendency to occur in isolated 

 patches in records 2 and 33, and also in the continuous net series (St. WS 133, Fig. 18). 

 The numbers of Doliolum (record 3) are too small for the fluctuations to be significant. 



On reviewing the records as a whole it is difficult not to be impressed by the sugges- 

 tion of an apparent rhythm in the fluctuations ; a rhythm which is not constant for dif- 

 ferent groups, but one which appears to have a definite pattern on different occasions. 

 A few examples will illustrate what is meant. In Fig. 17 the young Euphausiacea 

 fluctuate with fairly closely spaced peaks in record 34, whereas the peaks are far apart in 

 record 35. In records 8 and 9 (Fig. 8) the copepod fluctuations follow one another in a 

 pattern of alternating low and high peaks. Records 18-22 (Fig. 11) suggest a rhythm of 

 widely spaced major fluctuations in the copepods, and in record 31 (Fig. 15) a rhythm of 

 lesser fluctuations combined with vertical migration. Even the small numbers of cope- 

 pods in record 2 (Fig. 3) show wave-like fluctuations. See also the euphausian larvae in 

 Fig. 9. It is perhaps easy for one's imagination to exaggerate sporadic fluctuations into 

 such rhythmic patterns, and for the moment they must remain as tentative suggestions. 

 If such rhythmic fluctuations should be established by further observations as a 

 common phenomenon of plankton distribution, then it seems likely that they must be 

 caused by some dynamic principles involving vertical migration and water movements 

 such as have been recently outlined by the author (Hardy and Gunther, 1935, pp. 343- 

 56). If they were due to the action of the sea alone one would expect all organisms in the 

 same area to fluctuate to the same "rhythm"; we see, however, different organisms 

 fluctuating on the same records with their peaks at different intervals apart, suggesting 

 that these differences are due to differences in the behaviour (vertical movements ?) of 

 the animals in relation to the water. 



Whether there are in reality such rhythms or not it is evident from the foregoing 

 records that the plankton is seldom uniformly distributed even in the most open ocean 

 regions. That part of Haeckel's classical attack upon the theories of Hensen and the 

 Kiel planktologists based on his conception of the unequal distribution of the oceanic 

 plankton appears to have been fully justified. 



