T. F. GOREAU 271 



stood. The so-called "zooxanthella problem has been the sub- 

 ject of much controversy because some investigators ha\e 

 failed to recognise the multiplicity of host-symbiont relationships 

 in the different groups of coelenterates: ranging from total nutri- 

 tional dependence on zooxanthellae in some xeniid Alcyonacea 

 (2) to nutritional independence in the Scleractinia which are spe- 

 cialised carni^'ores (21). There can be little doubt that zooxanthella- 

 coelenterate symbioses have exolved independenth' in many unre- 

 lated groups at different times, thus accounting for the haphazard 

 variety of the association in the \ arious classes and orders of the 

 phylum. For further details and references regarding the zooxan- 

 thella problem, the papers of Yonge (19, 20), Vaughan and Wells 

 (17), Odum and Odum (10), and Goreau (4) should he consulted. 



CALCIUM DEPOSITION AND PHOTOSYNTHESIS 

 IN REEF CORALS 



Growth in corals is achie\ ed b\' an increase in mass of the cal- 

 careous skeleton and a concommittant proliferation of the overly- 

 ing tissues. Our recent underwater studies on reef corals suggest 

 that e\'en within any given species there may be no constant re- 

 lationship between these two kinds of growth and that colony shape 

 is to a certain extent controlled by ^'ariations in the ratio of new 

 skeleton to new tissue. To study the factors which regulate calci- 

 fication in corals and other calcareous organisms, we ha\e dexeloped 

 new methods for the fast quantitative assay of growth by the use of 

 radioactive tracers. Calcification is determined from the rate with 

 which Ca^"* ions added to the sea water medium is deposited into 

 the skeleton as Ca^' CO.,. under various conditions, e.g. light and 

 dark. The procedure, which has been described elsewhere (3, 6), 

 requires only a few hours; the experimental runs can be carried out 

 in the field, and growth gradients are determined by sampling 

 different parts of experimental colonies. 



Our observations demonstrate that calcification in reef-build- 

 ing corals is dependent on the ambient light intensity to the ex- 

 tent that growth in foiuteen species tested is on the a\'erage ten 

 times faster in sunlight than in darkness (6). Calcification is re- 



