214 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1917. 



Some species of Acroj)ora under favorable conditions on an average 

 grow in height from 40 to 45 millimeters per year. 



There is no average growth rate for corals generally gpeaking, 

 because growth-rate varies from species to species and varies for the 

 same species according to local environmental conditions. A colony 

 of a species of reef coral in a lagoon, if protected from sediment, 

 may grow more rapidly than a colony of the same species does on 

 the reef. The limitation of reef corals so largely to the outer edges 

 of platforms, is determined primarily by the freedom of the water 

 from silt and by the more uniform temperature. 



In order to estimate the rate at which a reef will grow, the upward 

 growth-rate of the true reef -forming species must be taken. The up- 

 ward growth-rate of Orhicella annularis^ the principal builder of the 

 Pleistocene and living reefs in Florida and the West Indies, is from 

 5 to 7 millimeters per year, according to station. At 6 millimeters 

 per year, it would form a reef 150 feet (=46 meters) thick in 7,G20 

 years; at 7 millimeters per year it would build the same thickness 

 of rock in 6,531 years. Acro'pora palmata, which grows more rapidly 

 might build a similar thickness in 1,800 years. The growth of corals 

 in the Pacific appears to be more rapid and according to Stanley 

 Gardiner they might build a reef 150 feet thick in 1,000 years. 



Growth-rate is one of the important factors in the battle between 

 corals and some of their natural enemies. For instance, if corals 

 grow less rapidly than sediment is being deposited on the bottom, 

 although other conditions may be favorable for their life, the}' will 

 surely be killed by smothering. In the competition between attached 

 and incrusting organisms, growth-rate is one of the most important 

 factors in determining which shall survive. Corals, as my experi- 

 ments showed, may grow with great rapidity in locations where they 

 cannot survive, or are only poorly represented, because the habitat is 

 suited to other organisms of a more rapid rate of groAvth. Among 

 these inimical organisms are various marine algae, including the 

 calcareous Halimcda and incrusting nulliperes ; other such organisms 

 are sponges, tunica tes, Br^^ozoa, and pelecypods. 



A study of the growth-rate of corals has an interest not only in 

 understanding the rate at which they may form rock, but also in 

 understanding their struggle for life against enemies, both organic- 

 and inorganic. 



SUMMARY OP STATEMENTS ON CORALS. 



The preceding pages show that in the ocean there are : 

 1. The deep-sea corals at depths of 180 meters or more, where the 

 light is weak or where there is perpetual darkness, and where the 

 temperature ranges from 1° to 15,6° C., although they thrive, best 



