CORAL REEFS OF TORRES STRAITS 221 



pora but now called Acropora, are killed at 97.5°, and cease to take food 

 at about 90.5% while the more resistant forms such as various species of 

 Siderastrea, and Porites and some of the brain corals, survive to 100°. 

 or even 102° F., but cease to feed at from 95° to 97.5° F. Now, in 

 general, those corals which are most sensitive to high temperature are 

 correspondingly so to the smothering effects of silt. The more deli- 

 cate forms such as Acropora, Pocillopora and Seriatopora are killed by 

 being buried for only 10 to 1-i hours beneath the mud, whereas, those 

 corals which die at 100° can withstand as much as 40 to 50 hours of 

 burial and Siderastrea radians of the Atlantic, which dies at 102°, can 

 survive being buried 72 hours without apparent injury. It seems, 

 therefore, that high temperature may produce death by causing asphyxi- 

 ation, and thus those corals which can withstand the highest temperature 

 are usually those which are best able to resist being covered by mud and 

 silt, and these are the very corals which live in the hot, muddy shallows 

 near the shore, while corals which require pure water live far out upon 

 the reef, where the temperature is lower. 



It will be recalled that Winterstein who studied frogs decided that 

 the nervous paralysis that results from high temperature was caused 

 by asphyxiation; but later Babak, and Amerling showed that some of 

 the frogs and toads are very resistant to lack of oxygen but easily para- 

 lyzed by heat, while the reverse is the case with others. Becht also casts 

 doubt upon the asphyxiation theory by showing that recovery from heat 

 paralysis can take place in the absence of oxygen in the water surround- 

 ing the nerves of the frog or of the horse-shoe crab (Limulus). Oxygen 

 may however have been derived from the tissues themselves. 



Among corals we find that Favia fragum and Ma?andra areolata are 

 more resistant to the effects of C0 2 or of silt than one would expect from 

 their death temperatures, which are fairly low. If however they be 

 buried in the sand and then heated they are still nearly as resistant as 

 if in the open water, whereas sensitive corals such as Acropora or 

 Oroicella are killed at lower temperatures 'if buried than if heated in 

 the open water. 



The explanation may be that Favia and M. areolata can survive at 

 a low as well as at a high rate of metabolism, in other words " hibernate " 

 under the mud; and thus be almost as well able to resist heat in this 

 condition as when living at a higher rate of oxygen-consumption in the 

 free water of the ocean. 



We still incline to the belief, therefore, that high temperature may 

 produce death in corals by asphyxiation, although other factors may 

 complicate the matter, as is so often the case in physiological reactions. 



Vaughan, indeed, has shown that those corals which live in muddy 

 regions are quite able to free themselves from silt by means of the cilia 

 which cover their surfaces, but those forms which live in the pure water 

 of the outer parts of the reef are not so efficient in this respect. 



