ECOLOGY AND TAXONOMY OF Holimeda 



291 



rhizomes (Fig. 96), and some of the less dense may contain very sizeable 

 populations of Halimeda, or the dense Halimeda patches may be in 

 adjacent areas. These especially thick Thalassia stands are restricted 

 to shallow water between — 0-5m and —3 m. Even in apparently 

 optimum sites the distribution of the Thalassia stands is patchy, and 

 there is evidence that the patches themselves are unstable. Most 

 patches terminate in what could well be called a blow-out : there is an 



Fig. 96. Edge of a dense stand of Thalassia testudinum, partly fringed by other sea 

 grasses such as Diplanthera wrightii. Here the calcareous alga Penicillus capitatus, with 

 some Udotea, is prominent towards the edge of the bed, and extends into the sand 

 of the lower left. There is a suggestion of a rotational process of colonizing sand, 

 consolidation of the sand by sea-grass rhizomes, development of the dense Halimeda 

 and other populations, and physical destruction of the community. 



abrupt edge, with barren sand adjacent, and the discontinuity between 

 the Thalassia community and the bare sand reveals a vertical section 

 of the sand consolidated by the Thalassia rhizomes which in places is 

 about 10 cm high. These observations suggest very strongly that there 

 is a rotational process of colonizing sand, consolidation of the sand by 

 sea grass rhizomes, development of the dense Halimeda and other alga 

 or sea grass populations, and then physical destruction of the com- 

 munity. The dynamics of this process are of immense importance to 

 the total Halimeda productivity of the reef. 



