Build-Up of the Substratum 69 



(Fig. 3.5). These holothurians, and other types of bottom animals, 

 pass the mud through their intestines and extract the organic matter 

 from it. For such forms the nutritive vakie of the substratum may 

 have an even greater ecological importance than its physical nature. 



Breakdown of the Substratum 



We have seen the various ways in which the nature of the sub- 

 stratum in the aquatic environment limits the growth and distribution 

 of organisms living on it or in it. Turning the ecological picture the 

 other way around, we find that the presence of organisms often has a 

 profound influence on the substratum. Even solid rock can be bored 

 into and broken down by animals living on it. In the intertidal zone 

 of the Oregon coast sea urchins have carved craters for themselves in 

 the sandstone. So abundant are the sea urchins and their craters that 

 in many places the shelving rocks present a honeycombed effect. 

 Even more remarkable is the ability of bivalve molluscs of the family 

 Pholadidae to drill into gneissic rock and into concrete. Some of the 

 pillars of the causeway to Key West have been pockmarked by these 

 animals. Empty shells which form the major portion of the sub- 

 stratum in some regions are broken down by the sponge Cliona. The 

 basal part of this sponge produces an acid secretion that hastens the 

 disintegration of the shells. In some situations the complete destruc- 

 tion of an organism's substratum by its own activity has led to its 

 undoing, for without a suitable substratum it is eliminated from the 

 habitat. 



Build-Up of the Substratum 



In addition to the breaking down of solid materials many aquatic 

 organisms play an important part in building up their substratum. 

 Calcareous algae and many types of coral animals cause calcium 

 carbonate to be deposited in and around their tissues. As a result 

 limestone formations of various sorts are brought into being. Anyone 

 who has visited the tropical ocean is well aware of the vast extent of 

 reef-building activity (Fig. 7.12). In fresh water certain organisms 

 similarly cause the precipitation of calcium carbonate. The marl 

 deposits that are formed in this way often come to be the chief com- 

 ponent of the substratum in ponds and lakes. 



In some situations the new substratum produced by the activity of 

 living agents consists essentially of the surfaces of the organisms 

 themselves. Occasionally a few mussels become attached to individ- 



