Calcium Carbonate 273 



These conditions favor the secretion of hnie and tlie building of coral 

 formations in the tropical oceans. 



Floating in the open water of the warmer seas are countless trillions 

 of planktonic organisms that use the easily accessible Ca^+ and 00^= 

 ions to build their shells. Prominent among these are biflagellated 

 coccolithophorids, foraminiferans, and pteropods. When these or- 

 ganisms die, their calcareous skeletons rain down upon the bottom, 

 and, if the water is not too deep, their remains accumulate to take 

 part in the formation of globigerina ooze and pteropod ooze. Globi- 

 gerina ooze is particularly extensive in the Atlantic and Indian Oceans, 

 and this type of bottom deposit sometimes contains as much as 86 

 per cent CaCO^. In deeper regions of the ocean with lower tempera- 

 tures and pH values the bottom deposits contain much less calcareous 

 material. These areas of the sea bottom are covered with red clay, 

 or have deposits of diatom ooze or radiolarian ooze formed from the 

 remains of plankton with siliceous skeletons that strongly resist solu- 

 tion (Kuenen, 1950, Ch. 5). 



Calcareous formations around shores and on shoals in the marine 

 environment are the result chiefly of the activity of bottom-living or- 

 ganisms. Calcareous sediment resulting from wave action is de- 

 posited on the bottom and may possibly be supplemented in some 

 instances by direct precipitation from supersaturated water. The 

 plants and animals taking part in lime production range from micro- 

 scopic forms to large solitary species and those that form huge colonial 

 aggregations. Particularly notable are certain bacteria, specialized 

 representatives of the Protozoa, Coelenterata, Annelida, and Crus- 

 tacea, as well as the calcareous algae and a great many kinds of 

 mollusks (Sverdrup et al., 1942, Ch. 20). 



Lime-forming organisms are present in all seas, but they are much 

 more prominent in warm waters. Although the physiological proc- 

 esses involved in shell secretion are highly complex (Bevelander, 

 1952; Wilbur and Jodrey, 1952) and not thoroughly understood, we 

 do know that shell formation is greatly accelerated at high tempera- 

 tures. In really cold water shell development tends to be suppressed, 

 and exoskeletons of many invertebrates are thin or even absent, as in 

 the naked species of the pteropod Clione. Shells in tropical regions 

 are typically larger and thicker than those of related species in tem- 

 perate regions and often display extra spines, ridges, or other pro- 

 tuberances. The grandfather of them all is Tridacna, a bivalve mol- 

 lusk with a shell that may approach 2 m in length and weigh 250 kg 

 (Fig. 7.11). These animals, hidden among the coral formations, are 

 large enough to be a danger as a man trap since they may close on the 



