204 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1917. 



most luxuriant, it is reached by the blue and ultra-violet rays; 

 but many deep-sea species live in utter darkness. In higher latitudes 

 the deep-sea fauna of the Tropics, it seems, may live in shallower 

 water, where the light is stronger than in the deep water nearer the 

 Equator. 



At the Tortugas I made experiments on 17 species of shoal-water 

 corals to ascertain their relation to light. The specimens were 

 placed in a live car, specially constructed so as to be entirely dark 

 after shutting a trapdoor. At the end of 14 days one species, Acro- 

 pora rmiricafa, apparently had died, and the specimens of all the 

 other species were pale, the green plant cells in the coral tissues hav- 

 ing died or lost their color; at the end of 28 days specimens of 

 Favia fragum and Agcuricla purpurea had died; at the end of 43 

 days one specimen of Eusmilia aspera had died, and most of the 

 polyps of Omlma diffusa were dead. The notes at the end of 43 

 days were kindly made for me by Dr. A. G. Mayer, who put on the 

 rocks, imder the landing for the laboratory pump wharf, those 

 specimens that w^ithstood the exclusion of light. About a year later 

 I found seven of them and made notes on them on July 21, 1915. 

 These specimens not only survived being in the dark for 43 days, 

 but before the end of a year were again very nearly or quite normal. 

 The fact that shoal-water corals are not nonnal in the dark, although 

 they will endure the exclusion of light for a considerable period, and 

 the fact that they are absent on the central piers under Fort Jeffer- 

 son wharf where the light is weak, while they are abundant on the 

 peripheral piers, is strong evidence in favor of light being one of the 

 ecologic factors determining the locus of species of corals. The 

 commensal green algae, known as Zoanthoxellae, that as a rule are 

 embedded in the tissues of shoal-water corals, set free oxygen which 

 is intimately available for use by the corals, as it is in inunediate 

 contact with the animal tissues. Since these plants while in the 

 dark cease to set free oxygen, and the corals under such circum- 

 stances are deprived of oxygen from that source, it may be that the 

 poverty of coral growth in dark places is due to the suppression of 

 the activities of these plants. 



Notwithstanding the high degree of probability that this infer- 

 ence is correct, additional accurate photometric records at depths 

 from about 37 to 183 meters are necessary before completely con- 

 vincing results may be obtained. 



CAPACITY OF CORALS TO WITHSTAND EXPOSURE IN THE AIR. 



As the corals that live in very shoal water may be above water 

 level during low^-tide periods, it is of interest to know how long they 

 can endure beina: out of their natural medium. I made a number 



