CORAL REEFS 221 



Darwin estimated that the greatest depth at which the 

 reef-building corals can flourish is between 20 and 30 

 fathoms, and he inferred from that estimate that the reefs 

 could not have been formed by up-growth from a stationary 

 sea-bottom of any considerable depth. 



It is interesting to find that, as a result of the extensive 

 investigations of more recent times, Darwin's estimate is 

 confirmed and the conclusion is reached that reef-forming 

 corals do not flourish at greater depths than 25 fathoms.^ 

 It is true that some genera such as Madrepora, Porites, 

 Millepora, Heliopora, have been found alive at depths of 

 35-50 fathoms of water, but the conditions at these greater 

 depths do not appear to be favourable to the formation 

 of luxurious plantations. Some forms such as Heliopora, 

 Millepora, and Goniopora are more frequently found in 

 depths of over 20 fathoms than others, such as the Seriato- 

 poridae, which are usually confined to quite shallow water, 

 but there seems to be no doubt that they all flourish most 

 abundantly in water of less than 25 fathoms. 



The genus Dendrophyllia is one of the few reef-building 

 corals which appears to be rarely found in water of less than 

 20 fathoms and to flourish in depths of 20-50 fathoms, and 

 it is interesting that this genus is also one of the few corals 

 that occur not only in the tropical seas but extend into the 

 cooler waters of the Mediterranean Sea and Atlantic Ocean. 



The coral Algae, Lithothamnion and Lithophyllum, which 

 play such an important part in the constitution of some reefs, 

 are sometimes left exposed at low tide even in the Tropics, 

 but are more usually found in shallow water down to a 

 depth of 40 fathoms - ; but unlike the typical reef-forming 

 corals, these plants have a world-wide distribution, occurring, 

 sometimes in great abundance, not only in tropical seas but 

 also in temperate and arctic waters. 



The corals of the order Stylasterina have a much greater 

 range of distribution in depth than any of the true reef- 



1 J. Stanley Gardiner, Fauna and Geography of the Maldive and Lacca- 

 dive Archipelagoes, vol. i. pt. 3. 



^ Madame Weber van Bosse in Science of the Sea, edited by G. H. 

 Fowler, 1912, p. 152. 



