was estimated at 35 living individuals/m 2 , with some 0.25-m 2 

 patches containing as many as 20 of these mollusks. In addition 

 to Tridacna clams, thu lateral strips included small colonies of 

 Acropora sp. coral and some less abundant lamellate Montipora 

 sp. Also present were algae {Halimeda sp. ) occupying spaces 

 between Tridacna shells and coral colonies, as well as calcareous 

 fouling algae encrusting the mollusk shells and dead portions 

 of coral colonies. 



A central strip 7-10 m in width accounts for most of the 

 reef flat. This site harbored most of the dead coral colonies and 

 empty Tridacna shells, firmly bonded to the reef surface by 

 calcareous fouling algae (Porolithon sp.). Nearly 809r of the 

 surface area of the central strip was covered by these algae, 

 which gave the middle portion of the reef added strength. The 

 live Tridacna population density in this strip was one order of 

 magnitude lower (an average of 4-5 individuals/m : ) than in the 

 lateral strips. The strip also contained widely scattered colonies 

 of Acropora sp. and, more commonly . bluish scales of Montipora 

 sp. coral colonies, including small clumps of Halimeda sp. 

 Another denizen of the central strip is the holothurian 

 Ludwigothuria sp. (approximately 1 specimen/nr ). 



The central and lateral strips exhibited a few small holes 

 4-8 cm in diameter surrounded by empty shells of smaller, 

 thoroughly consumed Tridacna. These were probably the 

 remains of meals taken by predators, namely small octopuses 

 that are able to open bivalve shells without damaging them. 

 The typical division of the reef into the aforementioned /ones 

 is disrupted in some places. In these instances, the central strip 

 of the reef contained a shallow depression of friable structure 

 populated by corals and Tridacna, with small amounts of 

 calcareous fouling algae. This is probably an intermediate 

 stage in the merging of individual smaller reefs with the larger 

 reef traversing the entire lagoon. One such smaller and still- 

 growing reef is shown on the left side of the reef cross section 

 in Fig. 3. 



It is interesting to note that the Acropora-Tridacna reef is 

 a natural farm producing large bivalves of commercial value. 

 The efficiency of the "farm" is difficult to assess without data 

 on its productivity. However, a count of the Tridacna present 

 is possible. According to the most conservative estimates, the 

 surface of the Acropora- Tridacna reef, extending 1 km into the 



lagoon of Caroline Atoll, contains approximately 300,000 

 Tridacna clams, the raw weight of their flesh equaling not less 

 than 30 tons. It should be noted that the Acropora-Tridacna 

 reef actually investigated was not the only one in the lagoon 

 (Chapter Frontispiece: Kepler cial.. Subchapter 1.1. this volume. 

 Figs. 47,48,57). There were, in fact, several such reels, and if 

 we assume that they are of similar structure, the above figures 

 can be multiplied by an appropriate factor to show the actual 

 reserves of valuable food protein available in Caroline"s 

 waters. 



The unusually high density of living Tridacna in Caroline's 

 lagoon was especially striking, exceeding any previously known 

 populations of both Tridacna maxima and T crocea (usually 

 more abundant in other parts of the World Ocean). For 

 example, the Palau Islands (western Pacific) were reported to 

 have just six T. maxima and 153 T. crocea within an area of 

 1 . 1 00 m- ( Hardy & Hardy, 1 969), whereas the same area on the 

 Acropora-Tridacna reel in Caroline Atoll lagoon contained 

 16,500 T. maxima. Richards ( 1 985 ) found that T. maxima in the 

 Tuamotus numbered 6-20/nr at Takapoto Atoll, and up to 

 60/rrr at Reao Atoll. Although these are the highest densities 

 previously reported, they do not equal the numbers found in the 

 densest patches on Caroline. 



The very considerable effect of such an enormous mass of 

 large mollusks on the entire atoll is also noteworthy. This is 

 because in symbiosis with zooxanthellae. which are the principal 

 food of the Tridacna clam, the latter experience intensive 

 growth and in turn enrich their habitat with proteins (Ricard& 

 Salvat. 1977). Dataconcerning the natural Acropora-Tridacna 

 reef could be put to use in creating artificial reefs in other parts 

 of the ocean to achieve considerably enhanced productivity. 



The sound condition of the coral reef around Caroline 

 Atoll, as well as the presence in its lagoon of a uniquely 

 interesting natural feature in the formof the Acropora-Tridacna 

 reef, may be deemed sufficient grounds for organizing a marine 

 reserve in the area. 



The authors are grateful to US colleagues Kay and Cameron 

 Kepler and D. Smith, participants in the 47th voyage of the research 

 vessel Akademik Korolev, lor their active assistance in field work on 

 Caroline Atoll. They also wish to thank Yu.N.Latypovforidentifying 

 the corals. 



169 



