292 L. HILLIS-COLINVAUX 



(b) Grazing and the Opuntioid forms. Straggling growth-forms of 

 opuntia on rocks form the densest Halimeda stands, reaching covers 

 of about 90%, which is probably more than the equivalent of 900 

 Rhipsalian Halimedae per square metre. There seems to be good 

 reason for thinking that the sites where densities like these can be 

 attained are limited by grazers for many opuntia patches show direct 

 signs of having been grazed, and apparently suitable sites are both 

 barren of opuntia and are the habitat or territory of some urchins or 

 fishes. It is possible that there are seasonal or periodic fluctuations in 

 grazer populations, in which event total Halimeda densities may 

 fluctuate very widely from time to time. 



(c) Factors restricting Halimeda on bare bottoms. Halimeda populations 

 are low on the sandy bottom of the moat and on the fore-reef terrace. 

 For the sandy, duned parts of the moat that lacked vegetation, the 

 explanation may be that the sands are kept too mobile by the wave 

 surge, and that only exceptionally well-anchored plants survive. On the 

 barrens of the moat floor urchin grazing is a possibility, because high 

 densities of urchins were found, and the vegetation which was there, a 

 short soft algal fuzz, is a life-form that might result from heavy grazing. 



B. Enewetak Atoll 



There are more data from Bikini and Enewetak (the spelling 

 preferred by the Marshallese) than from any other atoll, a consequence 

 of their use for bomb tests. Thirty-five atomic devices have been 

 exploded at Enewetak, and the bomb craters themselves are sometimes 

 interesting sites for marine botany. As always, research has been con- 

 centrated on processes leading to the construction, maintenance, 

 productivity and geology of the reef framework, with processes in the 

 lagoon and among the macrophyte populations examined to a lesser 

 extent. Geology and the structure of the reefs are described in Emery 

 et al. (1954) and Ladd (1973), with Ladd also providing a list of the 

 chapters of U.S. Geological Survey Professional Paper 260 which reports 

 on the Marshall Islands. Some of the results of core drilling of the reef 

 are reviewed in Section IX. The classic studies of production on reef 

 ridges by Odum and Odum (1955), as well as the more recent studies 

 of the SYMBIOS group (Johannes et al, 1972; Pomeroy et al, 1974; 

 Webb et al, 1975; Wiebe et al, 1975), also described Enewetak systems, 

 with the earlier work of Sargent and Austin (1954) carried out in 

 another atoll of the same group of islands. 



