IN THE FORMATION OF CORAL REEFS. 



357 



cent (Gorgonia acerosd) to 35.86 per cent {Plexaurella dichotoma) of the fresh 

 weight of the colonies is made up of spicules. The average spicule content 

 for the 12 species was 27.4 per cent. 



3. The fresh weight of the Gorgonian colonies growing upon a square 

 yard from different reefs varied from 1.5 pounds to 25.0 pounds. The 

 amount of spiculae held in the tissues of these colonies varied from 0.45 

 pound to 6.94 pounds. The average weight of the spicules for the 20 deter- 

 minations was 2.1225 pounds. 



4. The number of colonies per square yard along a series of lines extend- 

 ing over a number of the reefs (see map) was as follows: 



Table 9. 



5. An estimate of the weight of the Gorgonian colonies from each square 

 yard in the series of counts just mentioned, based upon the average weight 

 of the colony for common forms (see table 4), gives an average of 2 pounds 

 for line 1. The number of colonies to the square yard along this line was the 

 least of any line on which counts were made. Even on the basis of this 

 estimate, the weight of the spicules held in the tissues of living colonies would 

 average 5.38 tons per acre. 1 



6. The coenenchyma of all the species studied will be disintegrated, 

 setting free the spicules, within 120 hours of the time they are broken from 

 their attachment on the reefs (table 6). 



7. There is no evidence of death from old age among Gorgonians. 

 The most important destructive agent is wave-action, with its many effects. 

 Incrusting organisms {Millepora, Bryozoa, etc.) are also a constant factor of 

 considerable importance. 



8. A series of records extending over 5 years for one reef shows that on 

 an average about one-fifth of the total Gorgonian fauna is destroyed each 

 year. (Table 7.) While the fades of the Gorgonian fauna remain prac- 

 tically constant approximately 1 ton of limestone, as spicules, is therefore 

 added to the surface of each acre of reef area annually. Records of the rate 

 of growth of the most common species of Gorgonians found in the Tortugas 

 region have shown that nearly all of them reach medium size in from 2 to 5 

 years, and that growth takes place very slowly after that time. (Table 8.) 



■See table 5. 



