ECOLOGY AND TAXONOMY OP Halimedtt 263 



from the island slope approximately 50% of the Halimeda material was 

 identified, as cryptica, 34% as goreauii, 7% as copiosa and 5% as gracilis. 

 The deep fore-reef is a likely source for most of the cryptica (Moore et ah, 

 1976) which is the most prominent species of this part of the reef (Fig. 

 86), but some may have been derived from the fore -reef, as goreauii and 

 gracilis would appear to be, since cryptica grows in this region also 

 (Colinvaux and Graham, 1964). 



The fate of segments in the sediments varies. That many of them 

 remain essentially entire or in sizeable pieces for some time is demon- 

 strated in numerous samples of loose sediments, or in the whole seg- 

 ments that frequently floor the immediate vicinity of a grove of Hali- 

 medae. Eventually, by processes of disintegration, cementation and 

 recrystallization, they may be bound, together with other reef organ- 

 isms, into carbonate rock such as that described from the Marshall 

 Islands (Emery et al., 1954) or the Halimeda-rich packstones and grain- 

 stones surrounding the Discovery Bay Canyon of Jamaica (Moore et al., 

 1976). They also may be weathered into carbonate muds early in their 

 history, as on some of the Bahaman Banks where Neumann and Land 

 (1975) report that calcareous green algae have produced 1-5-3 times 

 the mass of aragonitic mud and Halimeda sand now in the 7 m deep 

 Bight of Abaco. 



A significant portion of the disintegration oi Halimeda segments may 

 result from the activities of sediment-feeders such as holothurians and 

 echinoids, while living segments may be processed in situ by grazers. 

 Grazing activity will decrease the quantity of recognizable sediments 

 and affect the qualitative results of sediment determinations. 



3. Comparative calcium uptake by Halimeda and other reef organisms 



Goreau (1963) estimated rates of calcium uptake attained by many 

 reef organisms over short periods. His methods were to place fresh and 

 healthy terminal growing portions in light and dark bottles, to inocu- 

 late the bottles with ^^Ca, and to leave them on the reef in 1-2 m of 

 water for 1|— 2^ hours, ending in the full sunlight of noon. To be able to 

 compare rates of calcium uptake between animals and plants of very 

 different form he expressed all results in terms of nitrogen, determined 

 on the same samples. When considering Goreau's results it must be 

 remembered that the disturbance to the organism represented by the 

 incubation procedm^e was very different from taxon to taxon, the light 

 intensity being appropriate to the usual ambient conditions for some 

 species but not for others, and so on. The data are of great interest, 



