ZOOPLANKTON I 



I. Protozoa 



Protozoa arc single-celled — or if you prefer it, acellular — animals and 

 almost all are microscopic creatures. They occur in marine and fresh water, 

 in the damp soil and as parasites of almost every living thing. In the plankton 

 they may live freely, or attached to larger planktonic creatures and there are 

 also many protozoan parasites of plankton. The free protozoa feed on bac- 

 teria and small particles of detritus which occur as particles either from land 

 drainage or from the disintegration of marine creatures and they thus serve 

 a useful role in the food cycle in the sea. Some, highly magnified, are illus- 

 trated in Fig. 12. Although many swim about as naked forms, others build 

 shells of minute sand grains or secrete their own shells of very delicate and 

 sometimes extremely intricate design. The tintinnids have fairlv simple 

 bell-shaped shells as shown in Plate X and Fig. 12; 13-17. Two most im- 

 portant groups of planktonic protozoa are the Foraminifera and the Radio- 

 laria. The Foraminifera secrete calcareous shells, often resembling minute 

 molluscan shells (Fig. 12; 20) punctured with many small holes, called fora- 

 mina, hence the name. On death these shells sink to the bottom, sometimes 

 in sufficiently vast numbers to form an ooze there. The most prolific of 

 these is called Globiocriiia (Fig. 12; 22); Globigerina ooze covers the greater 

 part of the Atlantic sea bed and most of the floor of the Pacific ocean south 

 of the equator. Although the planktonic Foraminifera are extremely numer- 

 ous individually, the number of species is small, in double figures only, but 

 there are some 1,200 known species living on the bottom and about 18,000 

 fossil forms. These are important as the different species are known to have 

 lived within certain temperature limits and examination of the Foraminifera 

 from 'core samples' of mud and ooze from the sea bed can reveal the climatic 

 conditions at the time the deposit was formed. A similar technique is used 

 in dating sedimentary rocks. 



The Radiolaria do not secrete external calcareous shells like the Fora- 

 minifera but an internal siliceous skeleton, sometimes fantastically delicate 

 and precise, and often in a radial form (Fig. 12; 21) from which the name is 

 derived. These, too, have been sufficiently numerous in the plankton for 

 their deposited skeletons to form an ooze — the Radiolarian ooze — which is 

 found most characteristically as a band across the Pacific from Panama to 

 about 160° W lying to the north of the Globigerina ooze. Radiolaria are 

 abundant in the north Atlantic today, especially in the warmer water and 

 are carried by it to the North Sea, Faroe and Icelandic areas. One species of 

 Radiolaria, ThalassicoUa (Fig 12; 27), is relatively huge for a protozoan, and 

 has a lar^e central zone of about a millimetre in diameter surrounded bv a 

 foam of protoplasm up to about 5 millimetres, making it look very like 



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