find optimal conditions here for' their growth: d high degree of water 

 turbulence, sufficient illumination, intensive development of plankton, 

 consuming the organic matter which flows down from the reef. The large 

 fan-shaped and branched forms of coral predominate: Agaricia , Mussa , 

 Fa V i a , Acropora , Ser i a to po ra , Madracis , Pterogorgia , Gorgonaria bloom 

 ( Ell isella , Nicella , Viminel la , Pseudopterogorgia , Plexaura) along with 

 soft coral s " (Sclerophytum ) , which compete wittt the corals for the solid 

 substrate and frequently form continuous growths. The Bu zone is rich 

 in sponges (Ircina , Kycale , Agelas , Verongia ) , some of which reach 

 almost 1 meter in diameter. The fauna includes crinoids ( Nemataster ) , 

 boring sponges ( CI ionidae ) and mollusks (Li thophaga , Fungiclava ) , which 

 make paths through the living coral. An unusually rich fauna inhabits 

 the dead colonies of coral and the rocky base. In one such colony, 

 brought up from a depth of 15 m, some 8000 individual organisms of 

 macrobenthos of 50 species were found (f'tCloskey, 1970). 



The PR (fore-reef) zone, depth 20-40 m. This zone occupies a steep 

 slope, on which a kind of cliff (so-called "sill-reef") is sometimes 

 forming, which consists primarily of the massive corals Montastrea , 

 Agaricia and Madracis . This zone is characterized by a rapid growth of 

 corals, which form a large colony ( Astrangia , Agaricia , f-tonti pora , 

 Millepora , Mussa ). Calcareous algae ( Halimeda , Peyssonellia ) develop 

 intensively here as well as a rich zoobenthos, which includes various 

 attached forms (polychaetes , sea lilies, Bryozoa, hydroids, 

 gorgonarians, ascidians, colonial sea anemones, soft corals). A 

 significant role in the deposition of lime in this zone is played by the 

 calcareous sponges (Sclerospongia) , which reach a significant size and 

 d.re encountered in large quantity, particularly on the walls of the 

 canyons and caverns (Goreau, Goreau, 1973). 



The discovery of the abundant development of coral communities at 

 significant depths at the foot of a reef (Goreau, 1959) was one of the 

 most important achievements of Goreau in the study of coral 

 ecosystems. His observations on the structure of the deep zones of the 

 various reefs have shown that the structural form of the reef is 

 determined not by processes of erosion, but by the localization of the 

 growth of corals. The localization of the coral growth, in turn, is 

 regulated to a significant extent by the complex movement of the bottom 

 sediment and clastic material over the profile of the reef, since in 

 sediment covered areas of the reef, over which it flows, colonies of 

 massive corals cannot settle and develop, creating the buttresses of the 

 reef. Their surface can be inhabited only by solitary corals ( Fungia ) , 

 which lie freely on the ground, or by highly branched forms (certain 

 Acropora). As the corals grow, the troughs filled with reef sediment 

 Are converted into deep canyons with steep walls. These canyons are 

 gradually covered over by massive colonies of coral growing on their 

 side walls, and converted into caverns. In the Br, Mix and Bu zones, 

 caverns are normal elements of the external side of a reef. 



Individual species of organisms are rather clearly restricted to 

 specific zones of the reef. However, if we analyze the species 

 composition of the madrepore corals and algae in various zones of the 

 Jamaica and Great Barrier Reefs (Goreau, Goreau, 1973; Grassle, 1973), 

 we can see that certain species of coral inhabit all zones of the 



170 



