202 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1917. 



indicate 18.15° C. as the minimum temperature which a reef will 

 survive — this is 1.85° C. lower than the figure given by Dana. It is 

 not probable that a reef could withstand a continuous temperature 

 so low as this. Wherever the depth of water is great enough to 

 lower the bottom temperature below 18.15° C., more probably about 

 22° C, reef corals will not live. This temperature appears to be 

 attained around the Hawaiian Islands within a depth of 183 meters. 

 According to Agassiz's " Three Cruises of the Blake " the bottom 

 temperature in the Gulf of Mexico and the Caribbean Sea is usually 

 too low for the growth of reef corals at a depth of 183 meters, and 

 in places it is too low at a depth of 87 meters. Recent records of 

 temperature near Bermuda, the Bahamas, and Florida, show that 

 in those areas the temperature at 300 meters is uniformly too low 

 for the life of reef corals; it is usually too low at 200 meters; and 

 occasionally too low at 100 meters, in an area where the surface tem- 

 perature is high enough for the life of reef-forming corals. 



RELATION OF CORALS TO SEDIMENT. 



. One of the important factors affecting the life of corals is their 

 relation to sediment. Of course any coral permanently buried in 

 sediment would be killed, but nearly all corals can remove some sedi- 

 ment from their surfaces, and some can rid themselves of considerable 

 quantities. The outer-reef corals proper have their surfaces kept 

 clean b}^ the movement of the water, that is, by waves, surf, and cur- 

 rents ; but as the species living on the inner flats and in the lagoons 

 have not sufficient assistance of that kind, they require special adapta- 

 tions for keeping their surfaces clean. One of these adaptations is 

 for the colony to be divided into upward-pointing branches, which 

 present very small or no flat areas on which sediment can lodge. 

 Other corals, Maeandra areolata for example, has greatly developed 

 cilia, which move the sediment toward the periphery of the colony 

 and cause it to drop off. Some species, Siderastrea radians for in- 

 stance, can stand temporary burial. A. G. Mayer discovered that 

 those corals that can witlistand the highest temperatures can endure 

 the longest burial. The capacity to resist the effects of high tempera- 

 ture and that to resist the effects of burial are, therefore, brought 

 into relation, and one seems to be the correlative of the other. Ac- 

 cording to Mayer, high temperature produces death by asphyxiation, 

 as also does burial. 



RELATION OF CORALS TO LIGHT. 



Light is another factor that affects corals. Plate 16, figure A, 

 represents the wharf at old Fort Jefferson, Tortugas, Florida. Coral 

 larvae have attached themselves to the peripheral piers and many 



