354 



THE GORGONACEi^ AS A FACTOR 



of the tissues of the ccenenchyma takes place slowly in colonies overgrown 

 by the incrusting Bryozoa. Usually only the proximal portion of the colony 

 is affected, while the tissues of the distal portion retain their normal activity. 

 So far as can be determined from observations extending over a period 

 of five years on the reefs about Tortugas, there is no evidence of death from 

 old age in any of the Gorgonians. Every colony lives until it meets a violent 

 death through the agency of storms (wave action), by being overgrown by 

 some other organism, or by its being set free on the reef through the disinte- 

 gration of the material to which it was attached. Besides the tearing loose 

 from their place of attachment of the Gorgonian colonies, another result of 

 wave action is that frequently an amount of loose limestone debris sufficient to 

 cover up the colonies is brought upon a reef, causing their death by excluding 



food and oxygen. 



Table 7. — Percentage of dead colonies on a reef. 



'Computed upon the basis of the count of June 1910. 



The facies of the Gorgonian fauna on any reef remains practically con- 

 stant except for the introduction of the unusual factor of extensive destruc- 

 tion of colonies as an effect of severe storms. The determination of the 

 number of axial skeletons of dead colonies on any reef is, therefore, the only 

 practical method of determining the death-rate of these organisms on any reef 

 area. This method, unfortunately, has one very important source of error, 

 in that when a colony has been torn from its place of attachment it will most 

 often be carried for a considerable distance from its previous position and 

 will not be included in a count of dead skeletons on a restricted area of reef. 

 A record of the entire number of certain species of Gorgonians found on a 

 small protected reef east of Loggerhead Key has been kept continuously for 

 five years, and affords the most comprehensive data available on this point. 

 The record for this reef area from 1910 to 1914 inclusive is shown in the 

 above table. 



In this record the dead skeletons for the first and last two years alone 

 represent a normal condition. The percentage for the other years is abnor- 

 mally high on account of the fact that the unusual destruction on account of 

 the hurricane of October 1910 fell within the period of the record. On other 

 reefs within the group, the destruction of the Gorgonian fauna at that time 

 was in some instances complete, e. g., on the southern end of White Shoal. 

 In another locality, on the northern end of Bush Key, the amount of spicules 



