CORALS AND CORAL REEFS VAUGHAN. 215 



where the temperature is between 4.5° and 10° C; these are mostly 

 cup-corals or delicately branching forms. It seems that this fauna 

 lives in shallower water in higher latitudes than it does in the 

 Tropics. 



2. Between depths of 46 and 74 meters in the Tropics, there is a 

 moderately distinctive fauna that is more closely related to the 

 shallow-water than to the deep-water fauna. 



3. In the shallow waters of the warm parts of the tropical oceans 

 there is another fauna, the one that forms coral reefs, and its local 

 adaptations to the character of motion of the water, sediment, and 

 other factors have been described. The conditions necessary for the 

 vigorous growth of reef-forming corals are as follows: (a) Depth 

 of water, maximum, about 46 meters (25 fathoms) ; (b) bottom 

 firm or rocky, without silty deposits; (c) water circulating, at times 

 strongly agitated; (d) an abundant supply of small animal plankton; 

 (e) strong light; (f) temperature, annual minimum not below 18° 

 C; minimum average temperature for the coldest month in the 

 year not lower than about 22° C; (g) salinity between about 27 

 and about 38 parts per thousand. 



4. According to conservative estimates, reef corals can build a 

 reef 46 meters (150 feet) thick within a period ranging from 1,800 

 years to 7,500 years ; but, in places, a reef of such a thickness might 

 be formed within 1,000 years, according to Gardiner. 



THE FORMATION OF CORAL REEFS. 



DEFINITION OF THE TERM "CORAL REEF." 



The preceding pages are devoted to a general account of corals and 

 the conditions under which they live, and no definition of "coral 

 reef " has as yet been given, although the term has been used. In 

 order to give some idea of a coral reef several illustrations are intro- 

 duced. Plate 26, figures A, B, represent the reef off the west face 

 of Loggerhead Key, Tortugas, Florida, as exposed at very low tide 

 on June 6, 1910. The heads projecting above the water are Orbicella 

 annularis, the principal reef -building coral of the Floridian and 

 West Indian region; the fanshaped objects arc the alcyonarian coral, 

 Gorgonia flabeUum; while the rod or whip like objects are other 

 Alcyonaria that belong mostly to the genus Plexaura. Plate 26, 

 figure C, is from an undersea photograph taken at Carysfort Eeef, 

 south of Miami, Florida. This illustration shows the beautiful, 

 waving gorgonians, especially the fan coral, and large heads of 

 Orbicella annularis, as well as some other stony corals ; but it does not 

 show the highly colored fishes that dart in and out among the coral 

 heads and constitute one of the enchanting sights to be seen on coral 



