274 THE BIOLOGY OF HYDRA : 1961 



Given an adequate supply of oxygen in the medium, far reach- 

 ing effects on the rate and efficiency of metaboHc reactions can be 

 brought about by increasing the rate with which soluble waste 

 products are removed from the coral cells (20). This is a far more 

 potent metabolic stimulant than increasing the oxygen concentra- 

 tion. It has long been known that velocities of metabolic reac- 

 tions are strictly limited by the rates with which the end products 

 are removed from the immediate environment. In higher animals, 

 this is accomplished by specialised circulatory and excre- 

 tory systems which are lacking in the coelenterates. In the ab- 

 sence of zooxanthellae, or in darkness, corals are forced to rely 

 on diffusion alone to get rid of the soluble inorganic waste products 

 of cell metabolism. This is a slow process, especially when the sur- 

 face area for exchange is reduced by retraction of the polyps into 

 the calyces. This situation is radically altered in the presence 

 of zooxanthellae which require for photosynthesis and j)riniary pro- 

 duction those very substances that the coral host must get rid of, 

 e.g. COo, phosphates, nitrates, sulphates, ammonia, etc. Yonge and 

 Nicholls (21) showed for some corals that zooxanthellae are capable 

 of sufficiently high rates of photosynthesis to utilise not only all the 

 soluble inorganic phosphate produced by coral colonies, but that 

 additional phosphate is absorbed from the surrounding sea water. 



Under conditions of adequate illumination, the zooxanthellae 

 are to be regarded as combined intracellular lungs and kidneys. 

 The observed speeding up of calcification in reef corals exposed to 

 bright light may in part be due to an increase of the rate and effici- 

 ency with which metabolism can supply free energy to the car- 

 rier mechanism concerned with active calcium transport. The ques- 

 tion whether the calcification rate is indeed related to the metabolic 

 rate, and whether this is in turn influenced by the level of algal 

 photosynthesis in the manner indicated above is now under 

 investigation in our laboratory. 



CARBONATE DEPOSITION, GROWTH 

 AND PRODUCTIVITY 



Elsewhere, we advocated the view that Ca~^ and HCO^g ions 

 dissolved in the ambient medium are the source of the mineral de- 



