296 L. HILLIS-COLINVAUX 



part by the tediousness, in the early stages of the taxonomy of the group, 

 of identifying the calcareous algae involved, for Lithothamnion is a 

 minor component, if present at all. The commonest members of the 

 ridge are species of Neogoniolithon, Porolithon and Lithophyllum, and 

 up to ten genera commonly may be present (Adey and Maclntyre, 

 1973). This ridge, showing various degrees of development, also occurs 

 in the Caribbean (Adey, 1975 ; Adey and Burke, 1976 ; Adey et al., 1976), 

 and it therefore does not represent a difference between the reefs of 

 Atlantic and Pacific as formerly believed (Stoddart, 1969; Milliman, 

 1974). To the region of shallow waters shoreward of the ridge I have 

 applied the general term back-reef (or reef flat). I have not subdivided 

 it according to predominant coral type as is commonly done, since these 

 divisions are not meaningful to our present understanding of Halimeda 

 distribution at Enewetak. 



Water flowing across this back-reef may continue directly into the 

 lagoon, or its passage may be interrupted by low, carbonate islets 

 built on the reef structure. Various vertical divisions of the lagoon reef 

 have been described, again based on types of coral, but for Halimeda 

 there is at present little reason for stressing them. Diff"erent environ- 

 ments among lagoon reefs are encountered in regions adjacent to inter- 

 island channels, on the lagoon side of the passage between islets and 

 bordering the islets themselves. The many pinnacles of the lagoon are 

 active, tall coral heads of immense significance to the productivity and 

 carbonate budget of the atoll. 



2. The distribution and diversity of Halimeda at Enewetak 



Fourteen species of Halimeda are known for Enewetak (Table 

 XXXIV), and although the number may be higher when some of the 

 problems of nomenclature are resolved, it is, even so, the greatest 

 number yet reported from any specific area in the world, and is about 

 the same as the total number of species of Halimeda known for the 

 Atlantic Ocean. The within-habitat diversity, sensu MacArthur (1965), 

 at any place in the atoll, however, is low (Hillis-Colinvaux, 1977). 

 Twenty-one sites were examined during a 3 -week visit to Enewetak, 

 and at no station were as many as half the species found (Fig. 97). At 

 only 10% of the stations were there six species, and at 40% of the 

 stations there were three or fewer. At two stations, including the site of 

 the Odum transect, no Halimedae were found. A station represented 

 about 1-2 h of diving time. A striking observation first made by 

 Gilmartin (1960) is that both sand- and rock-adapted species are 

 found at all depths of the lagoon, from the shallows to the bottom, 



