ABSTRACT
A. P. Orr (1933), in the course of a study of the physical and chemical conditions prevailing in the sea near coral reefs, and in shallow pools on their surface, found that the oxygen content of the water in pools containing coral may rise to a very high figure (sometimes as high as a supersaturation of 278 per cent.) by the end of the period of low water during the daytime, falling to a low figure (e.g. 18 per cent, saturation) by the end of low water at night. Respiration by plants and animals uses up oxygen in both light and darkness; photosynthesis produces an excess of oxygen during daylight. The photosynthesis is due not only to free-living algae, but also (and in some places predominantly) to the symbiotic algae known as zooxanthellae which abound in the tissues of nearly all reef-corals. The effect can be detected in open water over coral, but is naturally much less marked there than in pools. C. M. Yonge, M. J. Yonge and A. G. Nicholls (1932) and S. M. Marshall (1932) studied the oxygen production of corals and their larvae experimentally. Oxygen determinations on coral reefs, and experiments on corals, have also been made by other authors—for references see Yonge, Yonge and Nicholls (1932) and Verwey (1931).
See Legendre, 1909, 1909 a and 1922; McClendon, 1918; Powers, 1920; Atkins, 1922; Allee, 1923; Marshall and Orr, 1928 and 1930; Thompson, Miller, Hitchings and Todd, 1929; Gran and Thompson, 1930; from these papers further references may be obtained.
Unfortunately we cannot obtain in South Africa a paper entitled “A study of the tide-pools on the West Coast of Vancouver Island” by I. Henkel, published in Postelsia, 1906.
The material of one species was sterile ; it cannot therefore be stated with certainty whether it belonged to Cheilorporum or Corallina.
This figure represents perhaps 90 per cent. Pycnophycus, the remainder Gigartina and Hildmbrandtia.
We wish to express our sincere thanks to a number of workers who made this experiment possible by taking temperatures simultaneously, throughout the day, at places far removed from one another.
It is known, for instance, that in certain fish and invertebrates oxygen consumption is lowered at low oxygen tensions ; that oxygen consumption may vary with temperature and pH ; that water very much supersaturated with oxygen has harmful effects on certain animals ; and that some marine animals can survive for long periods under anaerobic conditions. See Collip, 1920 and 1921; Haempel, 1928; Hyman, 1929; Galadziev and Malm, 1929; F. G. Hall, 1929 and 1931 ; Keys, 1930; Koller, 1930; V. E. Hall, 1931 ; Raffy, 1931 ; etc.
