298 L. HILLIS-COLINVATJX 



somewhat different from fringing reefs. An especially fine-grained 

 unconsolidated substrate for Halimedae at Enewetak is provided by 

 atom bomb craters. 



(a) Halimedae on the fore-reef and the spur and groove zones. We have 

 no data on Halimeda distributions on the fore-reef or at the greater 

 depths below, but Jamaican experience suggests that there may well be 

 dense populations or new species waiting to be found there. If these 

 populations exist, they could be important to the carbonate budget of 

 the lagoon because the strong onshore currents might be expected to 

 carry shed segments over the reef. Species to be expected there include 

 copiosa, opuntia, gracilis, macrophysa and gigas, with Tydemania 

 expeditionis. This region is yet part of the "Mare Incognitum" (Ladd, 

 1961 ; Smith and Harrison, 1977) for plants as well as for corals. 



There is no published account of Halimeda on the spur and groove 

 region, but I was able to make one collection in a transitional spur and 

 groove region in 1975. This was on the seaward side of the islet of Mut 

 at the south-western corner of the atoll (Fig. 97) in about —10 m to 

 — 15 m of water. Four species of Halimeda were found on the spurs: 

 opuntia, distorta, gracilis and macrophysa, with opuntia being much 

 less common than the others. The species distorta and gracilis sprawled 

 over, through and around the uneven reef surface, and together with 

 macrophysa, which has a discrete rather than spreading habit (Fig. 99), 

 filled many of the crevices between living coral heads. Halimeda 

 macrophysa, in addition, hung somewhat more openly on the sides of 

 the spur. 



At the time of the visit, which was mid-December, some of the 

 branches of distorta bore rows of several flabby, relatively uncalcified 

 segments at their growing tips, which I interpreted as indicating very 

 rapid growth. There were also many Halimeda segments in the sand in 

 the immediate vicinity of the spur, which could indicate an equally 

 rapid death and separation of older segments. Blue -green algae with 

 unbranched trichomes were associated with the Halimedae. Cover of the 

 spurs by Halimeda was estimated as approximately 15%, and the 

 relative conspicuousness of Halimeda on them presents a marked 

 contrast to the buttresses of Jamaica where Halimedae were rare. No 

 Rhipsalian species were noted in the vicinity of the spurs, but this 

 observation is not necessarily significant because a search could not be 

 made in the time available. 



(b) Algal ridge and hack-reef. At the seaward edge of this region, 

 where much of the force of the breaking waves is spent, especially on 



