290 



L. HILLIS-COLINVAUX 



Fig. 95. The cobble-urchin-fuzz system of the moat. The density of Diadema for the 

 region averaged 6 in"^, in places reaching 20 m"^. 



Thalassia, together with white urchins Tripneustes esculentus Leske 

 and heart urchins Clypeaster rosaceus Linnaeus. The density of each of 

 the two urchin taxa was estimated as 1 m~2. Among the sea grasses 

 were occasional small thalli of incrassata, of density about 4 m~^. The 

 Thalassia stands, alternating with the grassless, ostensibly barren 

 areas, gave the impression of advancing and retreating, while the heart 

 urchins and fuzz remained. 



Where there are appropriate substrates, Rhipsalian species also 

 grow in the deeper portions of the reef (Section IV ; Fig. 90) such as the 

 upper fore-reef slope. However, these deep-water Rhipsalian populations 

 are never dense, as they are in the shallows. This is, to some extent, 

 remarkable, for rock-growing pendant species like copiosa a,nd cryptica 

 do achieve very dense populations in deep water. The disturbance of 

 suitable substrates by silting, or by slumping of reef, perhaps accounts 

 for these smaller Rhipsalian populations at depth. 



4. Summary: factors affecting Halimeda distributions at Glory Be 



(a) Association with sea grasses. One of the best places to find a 

 dense (c. 450 m~^) population of Rhipsalian Halimedae is in the region 

 of dense patches of Thalassia. The Thalassia stands constitute well- 

 defined communities, the sand consolidated by a network of Thalassia 



