CHAPTER 10 

 MARINE BIOTIC COMMUNITIES 



INTRODUCTION 



The marine communities possessing the qualities of land climaxes 

 appear to occupy the greater part of the surface of the globe. Ob- 

 viously, the corals and certain coralline algae produce reactions upon 

 the habitat equal to if not greater than those of forest trees, and 

 doubtless of greater duration in any particular place and set of con- 

 ditions (cf. Herdman, 1906; Bigelow, 1930). 



Several types of communities occur in the sea. Certain communi- 

 ties are commonly considered as dependent upon the character of the 

 bottom, but it has been pointed out (Shelford et al, 1935) that these 

 are frequently more closely related to the physiographic forces than 

 to the bottom materials. The amount of circulation and the force 

 with which it acts are of great importance in determining the entire 

 marine climatic regime and may sometimes overshadow bottom con- 

 ditions. 



The tidal community on hard bottom is distinctly marine and has 

 no counterpart in fresh water. It is dominated by acorn barnacles 

 and mussels in the Northern Hemisphere, and by mussels, barnacles, 

 tunicates, and oysters in the Southern Hemisphere (Oliver, 1923). 

 This type of community occurs between the average of the lower half 

 of the low tides and of the higher half of the high tides; it has no 

 counterpart on muddy and sandy shores (cf. Davenport, 1903; South- 

 ern, 1915). 



Subtidal communities of muddy and sandy shores or clam com- 

 munities on gently sloping beaches do not reach as high above low 

 tide as do the tidal barnacle groups. Clams usually extend about 

 two-thirds of the way between the low tides and the high tides. 

 This distinction prevents confusion and serves to emphasize the fact 

 that the subtidal community reaches up into the tidal area. The areas 

 farther landward from clam beaches on low depositing shores com- 

 monly represent serai stages to land occupied by halophytes mixed 

 with other land plants, or bare sand beaches. On sandy shores pro- 

 truding rocks are occupied by tidal communities. 



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