270 THE BIOLOGY OF HYDRA : 1961 



ies becoming cemented into a single nnit by lithothamnioid algae. 

 The finer, more \'oIuminous, lagoon and forward slope sediments 

 are produced chiefly by calcareous green algae, with Scleractinia, 

 Gorgonia, Foraminifera, sponges, mollusks, arthropods and ech- 

 inoderms contributing in xarious proportions depending on local 

 factors. Owing to its stability and exposure to the seas, the frame- 

 work is probably the site where most of the calcium carbonate pro- 

 duction of the reef occurs. Only a fraction of this is ultimately de- 

 posited in situ since the greater part of the calcareous material pro- 

 duced here is washed out by waves and redeposited in the calmer 

 water of the lagoon or the seaward slope. 



A large proportion of the total biomass of coral reefs is due to 

 algae which grow in great abundance in all zones, ranging from the 

 shallowest parts of the rampart to depths exceeding two hundred 

 feet on the forward slope. The algal population of reefs can be divid- 

 ed into two categories: the free-li\'ing fleshy, filamentous, and cal- 

 careous algae; and the symbiotic unicellular zooxanthellae living 

 in coelenterates. 



All reef-building Scleractinian corals without exception contain 

 zooxanthellae. So do most Hydrocorals, Actinaria, Zoanthidea, 

 Alcyonaria and Gorgonia living in reefs. According to the existing 

 nomenclature, those calcareous coelenterates which have zooxan- 

 thellae are said to be hermatypic, or reef-building; whereas those 

 species lacking zooxanthellae are said to be ahermatypic or non- 

 reef building. The former are limited in their vertical distribution 

 to the upper parts of the euphotic zone and never grow in 

 dark places. The ahermatypes are usually found in deep water be- 

 low the euphotic zone although some species occur in shallow wa- 

 ter where they tend to favour dark crevices. The basic difference be- 

 tween hermatypic and ahermatypic coelenterates is that the 

 former grow much faster to much larger sizes than the latter. Never- 

 theless, some ahermatypic corals are known under certain condi- 

 tions to form deep-sea banks which bear a superficial resemblance 

 to shallow^ water reefs ( 14) . 



Although there is an absolute correlation between the pres- 

 ence of zooxanthellae in calcareous coelenterates and their ability 

 to build reefs, the relationship of the algae to their hosts and to 

 the bio-economy of the reef as a whole is not yet clearly under- 



