286 L. HILLIS- COLIN VAUX 



surveyed. Beyond the very shallowest regions goreauii sometimes 

 accompanied opuntia, but seemed to be restricted to certain exposures 

 on the coral rocks in shallow water. Past the reef flat (6), goreauii was 

 more conspicuous and tended to replace opuntia in the deeper regions 

 of the reef (Fig. 90). 



(i) Regions of low Halimeda density on hard substrate. The seaward 

 edge of the inshore reef flat (region 4) was conspicuously barren of 

 Halimeda, appearing as a whitish border which can be seen on the 

 panorama (Figs 89, 91) running across Fig. 89 from the dock at the 

 right. Instead of supporting the heavy opuntia cover of the adjacent 

 regions, this band of about 0-66 m width had a partial cover of grey-green 

 algal fuzz, tiny opuntia and isolated opuntia cushions which showed 

 signs of being grazed, with curved bites out of the segments, while the 

 region was floored with similar segments. In addition, the few opuntia 

 plants growing in the fringe had a conspicuously rounded cushion or 

 cropped habit, and lacked the straggly projecting branches which 

 result from active growth (Fig. 51). The coral rock itself was pitted, 

 with numerous scattered hollows containing urchins at a daytime 

 density of 12 m~^. The densest opuntia cover on this urchin border 

 reached 5%, with overall cover not greater than 1%. The upper portion 

 of the adjoining vertical wall also was relatively barren of macroalgae; 

 lower on the wall Halimedae were present, although their distribution 

 was patchy and sparse. A 1% cover was estimated for the coral rock 

 sides and rock outcrops of the lagoon. Halimeda tuna appeared on some 

 of the latter, sometimes pendant and in shaded sites. 



About two-thirds of the surface of the reef flat : Zoanthus zone 

 (Fig. 91 ; region 6) also lacked Halimeda cover, and was urchin-barren 

 like the one described for the seaward edge of the shore reef. The remain- 

 ing third had an almost-closed cover similar to that of the inshore reef. 

 Halimeda opuntia was the most prominent siphonaceous alga, and the 

 thalli appeared to be grazed. Here also the seaward vertical face of the 

 reef lacked conspicuous vegetation, including calcareous green algae. 

 The Halimeda population was sparse throughout. 



The buttress zone (Fig. 93; region 9), a region of hard substrate, might 

 also seem a likely site for non-sand-growing Halimedae, including 

 opuntia. Our census did not extend to this region at Glory Be, but at 

 Runaway Bay (between Glory Be and Discovery Bay) Halimeda won 

 little space on this wall of living coral, although scattered thalli of 

 opuntia and goreauii occurred here and there. The absence of this genus 

 from the north-shore buttresses was noted by Goreau and Goreau 

 (1973). 



