240 



MARINE ANIMALS 



even globigerinas are found in great masses, especially in places where 

 currents meet which are rich in plankton. 27 



So-called "animal streams" may appear in both open sea and near 

 coasts, sometimes with a good deal of regularity, in which animals of 

 varied kinds are so abundant that one can dip a soup of plankton. 

 These streams may be distinguished by a smooth oily appearance of 

 the surface, and may extend for more than a kilometer, with a breadth 

 of 5 to 10 m. Their appearance may be dependent on wind and current; 

 they appear in the harbor of Messina, for example, twice daily. 28 



The composition of the plankton varies with time as well as locality. 

 It is usually composed of a great number of different animals, but at 



Fig. 61. — Curves of volume of various groups of organisms in the total plankton 

 at Laboe, in the Bay of Kiel, during the year. After Lohmann. 



a given place and special time a single species may flourish to such 

 an extent that it predominates and one speaks of monotonous plankton. 

 This is especially a phenomenon of shallow seas; in the North Sea 

 a pteropod or copepod, and in warm seas a salpa, may predominate. 

 Such plankton may be dominated either by mature or by larval ani- 

 mals. Seasonal variation in composition of the plankton is important 

 in temperate and cold seas. The maxima and minima of different 

 organisms depend in different ways on the temperature, and may 

 alternate in the single elements so that a varied plankton may have 

 very different compositions at different seasons (Fig. 61). Changes 

 may take place too rapidly to permit comparative studies by one 

 ocean-going vessel. Perhaps, in time, such plankton comparisons in 

 distant regions may be made by cooperative effort or by the use of 

 adequately equipped airplane expeditions. 



There is even a difference in the composition of the surface plankton 



