The circular reefs of atolls develop on the peaks of submerged 

 mountains or guyots, rising nearly to the surface of the water. The 

 growth of the reef in this case is not limited in space, and its 

 circular form is a result of the more favorable conditions for the 

 growth of coral on the outside of the reef. Inside the reef is a sandy 

 lagoon with the residue of a broken rocky base, on which sandy islands 

 usually develop, or underwater pinnacle banks. Inside the lagoon, 

 around the islands and on the remainder of the reef flat, the coral 

 organisms develop in groups, forming so-called "spot reefs." 



The structure of a circular reef is shown in Fig. 3, where we see 

 the plan and cross section of the reef of Conflict Atoll (New Guinea). 

 The structure of this atoll reef is similar to that of the barrier reef, 

 except that there are no large islands within the lagoons of the 

 atoll. The largest atolls are Kwajalein Atoll (Marshall Islands) and 

 Suvadiva (Mai dive Islands). Their areas approach 2500 km . 



The zonal ity of the structure of the reefs has been studied in 

 detail by T. Goreau. The plan which he has suggested rather completely 

 reflects the zonal ity of the distribution of the primary biotopes as 

 well. The zones distinguished by T. Goreau and his colleagues (Goreau 

 et al., 1972; Goreau, Goreau, 1973) are shown in the cross sections of 

 the primary types of reefs (see Figs. 1-3). They are most completely 

 represented in the barrier reef, so we shall use this plan to present 

 our detailed description of the individual zones and their corresponding 

 biotopes. 



The I (inshore) zone is the shore zone, depth less than 1 m. This 

 is the littoral beach. Its population is exposed to the influence of 

 drying out during low tides and to the terrigenous runoffs from the 

 land, leading to silting of the bottom. If silting is severe, mangroves 

 develop in the inshore zone (see III. 1). 



Frequently, the inshore zone consists of rocky beds--crust rock or 

 the remainders of an old reef, broken up with cracks. Here we find a 

 rich fauna of mollusks (the gastropods Tectarius , Littorin a, Nerit a, 

 Thais , Vasum , Conus , Cymatium , Purperita , Cypraea , Col umbel la, chitons), 

 hermit crabs ( Coenobita , Paguridae), crabs, sea urchins ( Diadema , 

 Echinometra , Eucidari s) and the Ophiuroidea ( Macrophiothrix ) . 



The main source of nutrition of the fauna is the plant fouling of 

 the rocks (periphytons) and plant material thrown out on the beach by 

 the waves. The biomass of the fauna sometimes is quite significant. 

 For example, off the coast of Florida, the biomass of chitons alone in 

 the inshore zone of the reef at the water line is as great as 40 g/m^ 

 (Glynn, 1973). 



The L (lagoon) zone, depth 2-20 m is present in all barrier reefs 

 and atolls. The lagoons of barrier reefs usually are subjected to the 

 influence of terrigenous runoff and differ from the lagoons of atolls in 

 their more eutrophic conditions the predominance in them of hetero- 

 trophic processes. The bottom of the lagoon of a barrier reef is 

 frequently silted. Silting of the lagoon greatly influences the 

 composition of its fauna (Goreau, Yonge, 1968). Upon silting, and under 



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