The living coral is a carnivorous animal and feeds 

 upon small floating or swimming creatures which it 

 captures by means of stinging tentacles, and by means 

 of a slimy secretion of its skin. Since the coral is unable 

 to move in search of food it follows that in order to 

 flourish it must be exposed to water currents or wave 

 action which will bring food to it. 



The degree of salinity of the seawater is a further 

 factor of importance to the coral. Salts in ocean water 

 are in a concentration of about 35 parts per thousand. 

 When this is reduced to less than 25 parts per thousand 

 the reef coral begins to suffer. Similarly an increase of 

 salinity to 40 is harmful to many corals. 



The tropical zones with seawater temperatures rang- 

 ing from 25° Centigrade to 30° Centigrade (75° Fahren- 

 heit to 85° Fahrenheit) contain the only areas where 

 reef corals are able to flourish at their maximum. Since 

 the ocean currents are forced into a clockwise spiral 

 movement by the earth's rotation and associated wind 

 distribution, warm water travels towards the poles 

 along the eastern shores of the continents. On these 

 shores, therefore, there is a much wider extension of 

 warm water suitable for vigorous coral growth. On 

 the western shores of continents the reverse is true. 

 Cold currents of water running towards the equator, 

 combined with upwellings of cold water from the depths, 

 greatly restrict the extent of shoreline favorable to 

 coral growth. Thus it is that reefs are for the most 

 part formed only upon the eastern shores of continents, 

 and in the open tropical seas. 



Sunlight is rapidly absorbed as it passes into sea- 

 water. The requirement of strong sunlight therefore 

 restricts reef building to depths of less than 150 feet 



i8 



