DEPARTMENT OF MARINE BIOLOGY. 



201 



Book of the Carnegie Institution of Washington for 1914, were obtained by 

 determining the average weight of colonies of the several species taken from 

 reefs in shallow water. When working upon the deeper reefs, where it was 

 impracticable to remove the colonies from any given area, the number of colo- 

 nies was ascertained and their average weight was assumed to be the same 

 as that determined for the colonies from the shallow reefs. 



The addition to the equipment of the laboratory this season of a "Dunn 

 diving hood," by the use of which the study of any bottom in less than 30 

 feet of water is made practicable, has made it possible to obtain extensive 

 collections of the alcyonarian fauna from the deeper reefs and to correct an 

 error which had affected all of my estimates of the bulk of the gorgonian col- 

 onies growing on the deeper reefs. When seen from above, through depths of 

 water greater than 15 feet, all except the very smallest gorgonian colonies 

 appear to be of about the same size, and this average size does not appear to 

 differ to any marked extent from that of colonies growing in shallow water. 

 When the colonies growing on one of the deeper reefs were viewed from their 

 own level, however, the aspect of that feature of the fauna was entirely 

 changed. In many instances the surface of these reefs was covered with a 

 dense shrub-like growth of gorgonians of an average height of at least 3 feet. 

 Since the surfaces of all of the reefs are very irregular and the gorgonian colonies 

 are commonly attached to the higher points on the reef, many of them would 

 reach above the level of one's shoulders as he was walking about over the 

 reefs. In general, the bulk of the colonies of the most common species of 

 gorgonians was about twice as great as that determined for the same species 

 from specimens collected on the shallow- water reefs. The average weight of 

 the colonies of a number of these forms taken from a reef in 18 feet of water is 

 given below: 



Weiglit of gorgonian colonies from deep reefs. 



The proportion of spicules in the tissues of those forms for which spicule 

 determinations were made of specimens from the deeper reefs did not differ 

 materially from those determined for specimens from the shallow reefs, so 

 that the estimate of 5.38 tons to the acre as the amount of spicules held in the 

 tissues of living gorgonian colonies on the reefs about Tortugas would, for 

 those reefs in more than 15 feet of water, be only about half of the amount 

 actually present. 



These observations tend to emphasize more strongly the importance of the 

 alcyonaria as a contributing factor in reef limestone formation and necessitate 

 a reexamination of the question of the life cycle of these forms, especially the 

 relation of the depth at which they are attached to their growth-rate and length 

 of life. As a preliminary attempt to secure this information, specimens of the 

 same age of several species of gorgonians were cemented on tiles, some of which 

 were fastened on the reef in 18 feet of water, while a corresponding set was 

 planted in 3 feet of water in order to determine the difference, if any, in the 

 rate of their growth under the two sets of conditions. 



A number of forms of which small fragments only had been previously 

 procured by dredging were found to be quite common constituents of the alcyo- 

 narian fauna on the deeper reefs. 



