MODERN CORAL REEFS 391 



Depth of Water. The greatest deptli at which reef-huilding 

 corals flourish is 20 to 25 fathoms, though occasionally reef-build- 

 ing species may extend down to 40 or 45 fathoms. At this depth 

 ]\Iadrepora has been found growing on the Macclesfield and Tizard 

 banks of the China Sea. These corals were, however, as a rule, of 

 much slighter build than those found in shallow water. The depth 

 here given is of course not the downward limit of corals as a whole, 

 for some species are known to extend to great depths (see Chap- 

 ter XXVIII). Reef corals, however, flourish best in shallow water, 

 their most luxuriant growth being above 15 or 10 fathoms and to 

 just below low-tide level. Indeed many coral reefs luxuriate in 

 spite of the fact that they are exposed at exceptionally low tides, 

 as shown in Great Barrier Reef of Australia (Saville-Kent-77). 

 The alcyonarian Heliopora and the hydrocoralline Millepora, how- 

 ever, are active reef builders at 35 and 40 fathoms, and nullipores 

 extend to a depth of 35 fathoms. 



Intensity of Light. That the range in depth is in part to be cor- 

 related with the intensity of the sunlight is shown by the fact that 

 species of reef corals placed in a light-proof life car die after the 

 light has been excluded for several weeks. At Fort Jefferson in the 

 Tortugas, reef corals grow on the peripheral piles of the govern- 

 ment wharf, but are absent from the permanently shaded portions. 

 (Vaughan-87 -.240.) 



Now, as has been shown by the investigations of the optics of 

 sea water (see ante, p. 204), the visible depth in the open ocean 

 in 33° north latitude is roughly 50 meters (27J/3 fathoms), and in 

 50° north latitude it is 40 meters (22 fathoms). In the tropics 

 the depth is even greater, and it thus appears that the reef-building 

 species flourish within the zone of visibility and therefore of great- 

 est intensity of the sunlight. 



Temperature. As long ago shown by Dana, the minimum an- 

 nual temperature of the water must be 68° F. (20° C), or the mean 

 annual temperature must be above 70° F. (21.2° C). (Murray-68: 

 2^21.) All the principal coral-reef regions lie within the equatorial 

 belt where the range of temperature is not over 10° Falirenheit 

 (5.5° C), and recent observations and experiments have shown 

 that the precipitation of carbonate of lime both by organisms and by 

 inorganic agencies is most active at high temperatures. Saville- 

 Kent {77:98), indeed, insists that reef building is influenced by 

 temperature much more than by the species of corals, for he finds 

 that the same species which build the reefs in tropical waters are not 

 reef-building in extratropical regions. He further holds that it is 

 only in the shallower water where precipitation of lime is caused 



