274 Oxygen and Carbon Dioxide 



foot of a person walking on the reef or on the hand of a pearl diver 

 in deeper water. 



Since higher temperatures facilitate the secretion of CaCOg and 

 accelerate the growth of organisms producing calcareous structures, 

 we are not surprised to find that coral reefs are most abundant in the 

 western portions of the tropical oceans where extensive areas of warm 

 water are found (Fig. 7.12). Coral animals themselves require a 

 temperature above 20°C for good growth, and many other organisms 

 taking part in reef formation flourish only in tropical seas. Foramini- 

 ferans, millepores, alcyonarians, barnacles, serpulid worms, and mol- 



fln.l,, J. I. T>at,v. 1 1 , / .S^ (.,,nlvgi,al Survey 



Fig. 7.11. Underwater photograpli oi the giant elam Tridaciia on the reef flat at 

 Bikini. The specimen is about 45 cm long. 



lusks growing in unbelievable profusion among the corals add their 

 calcareous parts to the formation. The reef would not grow as an 

 enduring structure, however, if it were not for the cementing action 

 of lime-secreting Bryozoa and particularly of coralline algae, such as 

 Lithothamneon and Halimeda. These calcareous plants, known as 

 nullipores, often produce a sort of pavement protecting the new reef 

 growth from destruction by the surf and thus contribute enormously 

 to reef formation although they are not as conspicuous as the coral 

 animals (Gardiner, 1931; Ladd and Tracey, 1949; Kuenen, 1950, 

 Ch. 6). The lavish development of calcareous structures (Fig. 



