272 THE BIOLOGY OF HYDRA : 1961 



duced by approximately fifty per cent on a cloudy day under other- 

 wise similar conditions. By contrast, the calcification rates of some 

 shallow water ahermatypic corals lacking zooxanthellae do not re- 

 spond significantly to changes in light intensity. The stimulant 

 effect of light on reef coral calcification disappears when the zoo- 

 xanthellae are removed by culturing corals in darkness for about 

 three months. 



Inherent species specific factors, independent of the zooxanthel- 

 lae, also exert an important influence on calcium deposition. 

 One example of this is the growth gradient of ramose corals such as 

 Acropora cerviconiis where the large pale apical polyps that con- 

 tain relatively few zooxanthellae calcify several times faster than 

 the much smaller adjacent lateral corallities the tissues of which 

 are packed with large masses of zooxanthellae. The enzyme car- 

 bonic anhydrase also appears to play an important role in coral 

 calcification. We have found carbonic anhydrase activity in repre- 

 sentative species of all major groups of Coelenterata. The occur- 

 rence of the enzyme has no relationship to the calcareous habit, 

 or to the presence of zooxanthellae, which themselves do not 

 contain significant amounts of carbonic anhydrase. The treatment 

 of reef corals with a specific carbonic anhydrase inhibitor ( Diamox, 

 Lederle) results in an average fifty per cent reduction of the calci- 

 fication rate in the light, and a seventy five per cent reduction in 

 darkness. The effect of carbonic anhydrase inhibition on the calci- 

 fication rate is partially reversed in the light when the zooxanthel- 

 lae are photosynthesizing. It therefore appears that carbonic anhy- 

 drase and the zooxanthellae act in synergy to potentiate calcium 

 deposition in corals ( 3 ) . 



The mechanisms responsible for the stimulation of skeleto- 

 genesis in corals by photosynthesis of zooxanthellae are not clear- 

 ly understood. If the two reactions are linked through some common 

 pathway, the coupling must be of a facultative type since cal- 

 cification can proceed in the absence of photosynthesis, although 

 at a much reduced rate. We have observed that calcification is 

 speeded up very quickly following the exposure of the corals 

 to adequate light intensities. The short time constant of the 

 potentiation makes it unlikely that the stimulation is due to produc- 

 tion of nutrients by the zooxanthellae, but rather to prompt changes 



