288 L. HILLIS-COLINVAUX 



thickness, composed of red and brown algae. Amidst this carpet grew 

 compressed thalli of H. monile and H. incrassata, their buried basal 

 portions white. In some places, about 300 of these Halimeda thalli 

 occurred in a square metre of turf, compressed amidst the fleshy algae. 



(i) Regions of low density of Bhipsalian Halimedae. Much of the 

 lagoon (5) is sandy, and one might expect extensive stands oi Halimeda, 

 the other calcareous Caulerpales, which are predominantly sand growers, 

 and sea grasses. Such stands did occur (Fig. 94). In these shallow sandy 

 areas, I found Lilliputian forests of Thalassia testudinum, and the 

 densest population of H. incrassata, 441 thallim^^, encountered on 

 this particular reef. But the vegetation was patchy, and there were 

 extensive barren areas along the edges of the reef rock and in the main 

 part of the channel. Where vegetation occurred the sea grasses were 

 usually the commonest, although the Caulerpalean genera Halimeda, 

 Penicillus, Udotea and to a lesser extent Bhipocephalus were prominent 

 too. These algae usually grew in parts of the channel which were some 

 distance from the bases of reef walls, although a conspicuous exception 

 was Udotea ad spinulosa which pushed up in tight clumps of about 

 40 clonal thalli under reef overhangs. The commonest Rhipsalian 

 species in the inner channel was incrassata, but simulans mingled with 

 it, particularly near the periphery of dense stands. Halimeda cover of 

 the lagoon sands was about 5%. 



Extensive patches of Rhipsalian Halimedae were anticipated in the 

 wide moat (7) where the next unconsolidated substrate occurred 

 proceeding seaward from the shore. There were, however, few obvious 

 calcareous greens when the study was made. Seaward of the reef flat 

 (6), with its few Acr opera palmata and 31illepora colonies, was first a 

 channel of ripple-marked sand, some of it halimedoid, with dune-like 

 waves being continually moved by the surge. Neither plants nor urchins 

 were obvious, but worm cones occurred at intervals of a metre or so. 

 The rippled area ended in a pebbled or cobbled region, also part of the 

 moat (7). The rounded cobbles, typically 5 cm or so across but many 

 larger, seemed to lie on Halimeda sand and occupied half the area. 

 Patches of coral rock occurred throughout the cobble region. This part 

 of the moat contained a Diadema population averaging about 6 m^^, 

 in places reaching a density of 20 m~2. So large a population indicates 

 heavy production, but sizeable algae such as Halimeda occupied only a 

 tiny fraction of the grazed surface. Instead, over much of the pebble- 

 cobble floor a fuzz of filamentous green algae grew, and this population 

 seemed to be maintained by urchin grazing (Fig. 95). About 20% of the 

 moat also contained reasonably dense stands, about 300 m.~^, of 



