66 The Substratum 



animate objects. The surfaces of other organisms are also susceptible 

 to invasion by attaching organisms. Plants which grow on the ex- 

 ternal surfaces of other organisms but do not obtain nourishment 

 from them are called epiphytes. Similarly, animals which gain attach- 

 ment or shelter without using the tissues of the host organism as a 

 source of food are termed epizoans. If the attaching forms obtain 

 nourishment from the host, they are external parasites. The bro- 

 meliads and orchids perched on the branches of forest trees in the 

 tropics are typical epiphytes. The barnacles on the backs of whales 

 are examples of epizoans, although they are often erroneously referred 

 to as parasites. 



When the substratum of an organism is another animal or plant, 

 new relations appear. The organism may find that its substratum 

 moves around, grows, or is destroyed. In such instances the inter- 

 dependencies between the epiphyte or epizoan and its host become 

 extremely complex. For example, the distribution of sessile rotifers 

 attached to the water plant Utricularia depends upon the relative rates 

 of the migration of the rotifers and the elongation of the stems of the 

 plant (Edmondson, 1946). When the growth of the plant is rela- 

 tively slow, the rotifer population tends to become concentrated. 

 But, in the reverse situation, the rotifers become spread out with cor- 

 responding changes in the age distribution of the population. 

 Epizoans on the gulf weed Sargassum are similarly subject to the 

 vicissitudes of a living substratum. The floating gulf weed elongates 

 at its distal end and dies and breaks off at its basal end. A slow- 

 growing hydroid attached to the Sargassum might not be able to 

 maintain itself if its substratum grew out from under it. 



ROCK, SAND, AND MVt> IN AQUATIC ENVIRONMENT 



Influence of the Aquatic Substrata 



Although many different materials can serve as substrata in the 

 water environment, by far the most common are rock and its deriva- 

 tives. Whether the substratum consists of smooth rock, loose stones, 

 sand, or mud has a profound effect on the distribution of aquatic or- 

 ganisms and on the regulation of their growth. Different textures, 

 various degrees of stability of the material, and a great variation in 

 the nutrient content have an important selective action. 



In shallow water the difference in fauna and flora on a rocky bot- 

 tom, on a sandy beach, or in soft mud can easily be studied by anyone 

 visiting the shore. On the sea coast, for example, a rock substratum 



