210 MARINE ANIMALS 



Laminaria in other situations. This food relationship also explains 

 how corals can form the basis for a great animal community. Inci- 

 dentally the presence of the Zooxanthellae produces the bright colors 

 of the polyps, which may be yellow, yellowish-red, or red. Corals from 

 deep water have colorless polyps. 



Reef corals are sensitive to any decrease in salinity from the 

 normal, though increase in this respect does not affect them, as is 

 shown by their abundance in the Red Sea. Broad gaps in fringing 

 reefs occur opposite river mouths, and the great gaps in the Australian 

 barrier reef seem to be opposite the river mouths on the mainland. 

 In spite of the aridity of the African coast of the Red Sea, its harbors, 

 which represent gaps in the coral, are opposite the valleys which 

 descend from the upper Egyptian mountains. 57 One may conclude 

 from this fact that the rainfall in this region was greater in times not 

 far past. 



The sensitiveness of corals to river water is increased by the fact 

 that they are adversely influenced by the mud and sand brought 

 down by floods. Hence coral reefs are absent in the neighborhood of 

 active volcanos, since ash deposits kill them. They are absent in the 

 region of the mouth of the Amazon, and much reduced and absent in 

 some areas in the Java Sea, on account of the sediment brought down 

 by the rivers of the surrounding large islands. 58 



A certain amount of movement of the water is also necessary to 

 the corals, and this must be neither too active nor too slow. In deep 

 bays and inlets where the tides are reduced, the coral growth is weak. 

 The best collecting and observing stations for the scientist are always 

 on the sides of the reef, especially at capes which extend somewhat 

 seaward, 59 and on barrier reefs and atolls, at the channels which lead 

 into the lagoon. Moderate motion of the water prevents deposition of 

 mud, which is constantly formed by chemical and wave action and by 

 the activity of the reef-boring animals. Such motion also distributes 

 food and oxygen. Too much motion prevents the ciliate larvae from 

 gaining a foothold and carries them away. 



The rate of growth of corals is probably greatest near the surface. 

 The diameter of a large madrepore on the wreck of a ship sunk 64 

 years before was 5 m., an average annual increase in diameter of 

 39 mm. on each side. Other estimates are lower, about 20 mm. for the 

 more compact forms. The rate of growth of the Maldive reef is ap- 

 parently 27-29 m. in 1000 years. 60 The charts of a reef may be wholly 

 valueless after 20 years on account of the varied growth. The great 

 blocks of hard coral, such as the Astraeae, may have an age of several 

 hundred years. 



