vers edge of the Hat exhibited magnificent specimens of the sea 

 urchin (Heterocentrotus sp.), with spines as thick as cigars. 

 The limestone caverns sheltered smaller crabs (Eriphia sp. and 

 \i taea sp. ). The edge of the reef Hat dropped abruptly some 

 5-6 m, in places forming an overhanging ledge that overlooked 

 the most abundantly populated and significant portion of the 

 reef, namely the buttress zone with its rich variety of coral 

 species. 



Caroline's lagoon enclosed a unique natural structure, an 

 unusual Acropora-Tridacna reef ( Fig. 1 . Chapter Frontispiece). 

 This reef extended from the southernmost islet (Ana-Ana) in 

 the Southern Leeward Islands into the interior of a shallow 

 lagoon. In the middle of the lagoon, it divided into two 

 branches: a northern branch that ended in the center of the 

 lagoon, and an eastern branch that extended all the way to the 

 opposite shore. The reef, in effect, partitions the lagoon into 

 two parts; a wide channel had to be dug to permit a small flat- 

 bottomed rubber boat to pass. The width of the reef varied from 

 1 5 to 20 m, attaining 30 m at its fork. Most of the reef surface 

 stood above sea level, with many of the Tridacna maxima and 

 corals partially drying out during very low ebb tides. 



The distribution of organisms over a typical cross section 

 of the reef is shown in Fig. 3. Five distinct zones (two edges, 

 two lateral strips, and a central strip) were clearly evident. 



The edge strips, populated mostly by fragile colonies of 

 Acropora secale, A. palmerae, and Acropora sp., are growth 

 zones where vital activity keeps increasing the reefs width. 

 The dominant corals of the genus Acropora completely cover 

 the steep slopes of the reef as well as the neighboring rudimentary 

 reefs that do not reach the surface of the water in the lagoon. 

 The lush coral growth resembles a huge domed topiary. 



The thickness of the living coral layer ranges from 20 to 70 cm. 

 Parts of the reef slope exhibited scarps that probably formed as 

 a result of the collapse of fragile coral colonies unable to carry 

 their own weight. It is at these scarps that measurements were 

 made of the longest coral branches, some of which were found 

 to attain a length of 70 cm. The spaces between the fine coral 

 tentacles of the reef slope were filled with beaded ( moniliform ) 

 algae (Halimeda sp.). Also evident at the base of the reef slope 

 were a few, mostly large, specimens of Tridacna maxima. 

 Beginning at a depth of 5 m and extending to the reef base were 

 sparse, isolated growths of fungiform coral ( Fungia granulosa). 

 The colonies of Acropora were fine and fairly fragile, as is 

 typical in the still-water portions of many lagoons. It was risky 

 to approach the edge of the reef, since the loose and brittle coral 

 colonies tended to crumble underfoot. 



Beyond the edge zones (including the reef slopes, and 

 extending several tens of centimeters into the reef flat) lie 

 lateral zones where Tridacna maxima clams are especially 

 abundant. These lateral zones have a width of 3-5 m on either 

 side of the reef (Fig. 3). Particularly striking was the very high 

 density of Tridacna. firmly attached to the reef surface by the 

 byssus. which formed veritable bunches atypical for these 

 large mollusks in other regions. Not uncommon were bunches 

 of five or six clams attached to one another by the byssus. The 

 average shell length for Tridacna found atop the reef was 

 12-13 cm, with a maximum length of 1 9-20 cm for individual 

 specimens. No less striking was the variety of coloration 

 exhibited in their mantles, probably occasioned by the presence 

 of symbiotic algae. We were able to identify as many as 

 10 shades of blue, green, and light brown pigmentation. 

 The average Tridacna population density in the lateral zone 



^ - coral 



(Acropora spp.) 



| - mollusks 



(Tridacna maxima) 

 <X> - coral 



(Fungia granulata) 



«* - Holothurioidea 

 (Ludwigothuria sp.) 



mm - algae 



(Porolithon sp.) 



$ - algae 



(Halimeda sp.) 





r 





edge i later; 



Coral limestone 



central 



lateral | edge 





ZONES 



i ross section of the Kcropora rrirfacna reef in the lagoon of Caroline Atoll. 



168 



