70 



The Substratum 



ual stones scattered over a clam flat of fine sand or mud. During the 

 next season young mussels may attach to the shells of the old mussels. 

 Subsequent "sets" gain foothold on the surfaces of the second genera- 

 tion until gradually a mat of mussels spreads across the area that had 

 previously been covered by soft material quite unsuitable for the 

 growth of a mussel population. Mussel mats of this sort have some- 

 times been a serious economic concern because they have smothered 

 valuable populations of clams beneath. 



In other situations the action of the living part of the ecological 

 system is that of binding together loose particles of the bottom material 

 so that a firm substratum is provided in place of the shifting sand or 

 mud previously existing in the area. The eel grass that was formerly 

 prevalent along our Atlantic coast played this important role in many 



Aluiiay and Hjort, 1911, copyright Macmillan & Co. 



Fig. 3.6. A washed sample of pteropod ooze from the Indian Ocean showing the 



conical and angular shells of pteropods, the rounded tests of Foraminifera, and the 



remains of other types of plankton, enlarged about 10 X- 



