288 
ZOOLOGY: L. R. CARY 
of Gorgonian colonies represent only a potential contribution to reef 
formation. The actual number of gorgonians destroyed on any reef 
area in a given year can not be determined save by actual observation 
extending over the entire period, a method that had been impossible to 
carry out. A considerable number of observations bearing directly 
upon this phase of the subject has, however, been accumulated in the 
course of ecological studies on the gorgonians of the Tortugas regions, 
extending over a period of five years. 
These observations have shown that when a gorgonian colony is re- 
moved from its place of attachment on the reef and allowed to lie on the 
bottom where it will be moved by the action of tidal currents death fol- 
lows within a short time. The amount of time necessary for the com- 
plete disintegration of the coenenchyma of a colony of each of the sev- 
eral species which make up the most important elements in the gorgon- 
ian fauna about Tortugas was determined. This series of observations 
showed that when torn from the bottom, the most resistant form — 
Gorgonia fiabellum — would be disintegrated within 120 hours, while a 
colony of Pseudoplexaura crassa would be broken down in 18 hours. 
Under normal conditions of the reefs the greatest number of the axial 
skeletons of dead colonies are found in positions which indicate that 
the tearing of the colony from its normal attachment by wave action is 
the cause of the greatest mortality among these animals. When de- 
stroyed in this manner the spicules of any gorgonians would be added to 
the limestone forming materials on the reef within a few days, at most, 
from the time when the colony was torn from its place of attachment 
and consequently before the spicules had undergone any noticeable 
erosion. 
Next to the destruction by wave action (storms) the greatest mor- 
tahty of the gorgonians is brought about by the overgrowth of their 
tissues by other organisms. The most important of these agents are 
the hydrocoralline Millipora alcicornis and some encrusting bryozoa. 
Under the influence of both these agents the tissues of the gorgonian 
are disintegrated very slowly and the spicules set free a few at a time. 
While the Millipora will eventually cause the complete destruction of a 
colony the bryozoa were never found to extend for more than a few 
inches above the base of the colony. The actions of the two sorts of 
organisms is markedly different in that the MilHpora causes the death 
of the gorgonian tissues by some chemical action and itself encrusts 
only the axial skeleton of the colony; while the bryozoa form a cylindri- 
cal tube surrounding the base of the gorgonian and cause the death of 
its tissues by the exclusion of food and oxygen. 
