104 Water 



the tide is out. In the situation shown in Fig. 4.4 the sharp upper 

 limit for the growth of the rock weeds is easily apparent. At a higher 

 level the longer period of air exposure and of lack of sea water im- 

 posed a clearly defined upper boundary upon the barnacle population. 



Photo by R. W. Miner 



Fig. 4.4. Rocky shore at low tide showing stratum of sea weeds (Laminarian 

 Zone) and of barnacles (Balanoid Zone). 



Stephenson and Stephenson (1949) have found that on rocky shores 

 in many parts of the world three fundamental zones may be recog- 

 nized: the Littorina Zone, characterized by snails; the Balanoid Zone, 

 characterized by barnacles; and the Laminarian Zone, characterized 

 by laminarian seaweeds. The relation of these zones to the tide marks 

 is shown in the diagram. 



Littorina Zone 



Balanoid Zone 



Laminarian Zone 



High Spring Tide 



TIDAL ZONE 



Lojv Spring Tide 



