THE DRIFTING SWARMS 



indeed, have the characteristics of both plants and 

 animals. 



When we turn from planktonic plants to planktonic 

 animals we enter a vast world of drifting life-forms with 

 a teeming population of living creatures millions of 

 millions of times greater than our world of humans. 

 Enormous numbers of these plankton feed upon the 

 planktonic plants. The chief plant-eaters are the cope- 

 pods, which vary considerably in size, although the vast 

 majority are microscopic. Their numbers are so prodi- 

 gious that any small harbour or bay contains in its sea 

 water thousands of times more copepods than there are 

 human beings on earth. All kinds of fishes feed on plank- 

 tonic swarms which contain large numbers of copepods, 

 and (like the whale) use various devices to filter them 

 from the seawater. 



Fishes cannot discriminate between the various forms 

 of planktonic food, but in some cases the size of the 

 planktonic animals eaten by them varies with the age 

 of the fishes. In the earlier stages of a herring's existence, 

 for instance, its "sieve" (the gill-rakers) is finely meshed, 

 so that numbers of the smallest diatoms are caught in 

 it. As the herring grows, its gill-rakers coarsen, allow- 

 ing many of the smaller varieties to pass through them. 

 When adult, the herring's diet consists mainly of the 

 larger copepods and plant-feeding plankton. 



As a general principle in the "feeding chain", fishes 

 feed on copepods (planktonic animals), and copepods 

 feed on microscopic floating plants — diatoms or other 

 floating forms of planktonic life. A herring may have as 

 many as ten thousand copepods in its stomach, and each 

 of these copepods may have hundreds of planktonic 

 plants in its own. 



Planktonic animals — free-swimming sea animals of all 

 kinds whose powers of locomotion are not strong enough 

 to overcome the transporting forces of tides and currents 

 — ^vary considerably in size. Many are microscopic, and 



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