ECOLOGY OF THE MURRAY ISLAND CORAL REEF. 45 



and this suggests that, physiologically speaking, the effect of high tempera- 

 ture may be to asphyxiate the corals, probably by accelerating their metabolic 

 processes to such a degree that the oxygen of the sea-water is unable to 

 sustain them. 



The ability of the various species of corals to resist heat or cold is a 

 physiological matter and the corals of Australia behave essentially as do 

 corresponding genera of Florida. Yet the Australian corals from Murray 

 Island are not called upon to withstand cold, nor do those of Florida commonly 

 have to withstand so high a degree of heat as must the corals of Murray 

 Island. In other words, natural selection has not improved the corals of 

 either region in respect to their ability to resist heat or cold. 



In comparison with the effects of heat, of silt, and of wave-action, the 

 influence of dilution of the sea-water due to rains is negligible. 



Of the 40 or more species of corals here recorded from the rich southeast 

 reef-flat of Maer Island, only 2 {Pontes midrewsi Vaughan and Cceloseris 

 niayeri Vaughan) are practically confined to within 1,200 feet of the shore; 

 while 14 species are practically restricted to the outer part of the reef-flat 

 and do not approach within 1,200 feet of the shore. Generally speaking, 

 corals thrive best in clear, cool water wherein the range of temperature is 

 but slight; and only a very few, such as Porites andrewsi and Montipora 

 ramosa, are confined to a habitat wherein the water is subject to wide range 

 in temperature and to being charged at times with silt. Seriatopora hystrix, 

 which is very sensitive to high temperature, silt, or dilution, and easily 

 broken by the waves, is confined to the middle zone of the reef-flat. 



Large stocks of Symphyllia or massive nodular Porites may grow so 

 as to enlarge in diameter at the rate of nearly 2 inches (48 mm.) per annum. 



The ocean's surface is a good absorber and radiator of heat, and thus 

 the temperature of wide, shallow areas of water over reef-flats is colder than 

 the air at sunrise and warmer than the air at about 3 o'clock in the afternoon. 



However they may have been formed, the lagoons of atolls and barrier 

 reefs have not been dissolved out by sea-water as such, the solubility of 

 limestone in sea-water being so slight that this factor is negligible as a cause 

 producing a deepening of lagoons. The effects produced by currents over 

 the reef-flats, and by "sand feeders," such as holothurians and echini, are 

 certainly important as agencies of disintegration, but no quantitative deter- 

 mination of their efficacy has yet been made. 



