ECOLOGY AND TAXONOMY OF Holimeda 241 



this column eventually may be shown to be pantropical. If the remain- 

 ing taxa evolved later, then the calcareous genera Tydemania, Penicillus 

 and Ehipocephalus, which are the ones of interest herein, would be 

 more recent than Halimeda and Udotea. More detailed lineages have 

 not been shown but Tydemania and Penicillus appear more closely 

 related to Halimeda than to Udotea. Caulerpa, as the only genus of the 

 Caulerpaceae, has not been included in the scheme which consequently 

 is restricted to the Udoteaceae. 



The uniaxial construction of Tydemania has an interesting parallel 

 with that of H. cryptica, a species known only from —25 m and deeper 

 in the eastern Atlantic. It seems likely, however, that this morpho- 

 logical feature evolved independently in the two taxa, and is an adapta- 

 tion of these two algae to a particular niche, rather than a primitive 

 characteristic, as suggested by Gilmartin (1966). This niche may well 

 involve successful growth under conditions of low light intensity and 

 low energy reserves, leading to economy of form. In addition to the 

 reduced numbers of medullary filaments that both taxa possess, the 

 economy would extend, in H. cryptica, to the secondary utricles which 

 are less fully developed on the shaded surface. So far a similar Halimeda 

 species has not been found outside the Caribbean and the niche may be 

 occupied by Tydemania or other taxa elsewhere, or perhaps, as with 

 other Halimeda species, appropriate regions have not yet been explored 

 adequately. 



These phylogenetic schemes, both past and present, remain largely 

 speculative. Some advances in understanding the relationships of the 

 genera have been made, however, and there is an increased appreciation 

 of avenues in which subsequent research would be particularly profit- 

 able. The long geological record of Halimeda and the widespread 

 recent distribution of this genus indicate some of the impact the genus 

 may have had, and is continuing to have, on tropical reefs. This contri- 

 bution will be examined in more detail in the final sections. 



IX. Productivity 



All Halimedae are plants of shallow coasts or of coral atolls and as 

 such they are members of some of the very few highly productive 

 communities of the oceans (Ryther, 1969; Koblentz-Mishke et ah, 1970; 

 Colinvaux, P. A., 1973, 1974, 1978; Bunt, 1975). The record of then- 

 own high productivity is preserved in many a coral reef, as in those at 

 Funafuti where a significant part of the reef structure was found to be 

 made of Halimeda segments (Section II). Understanding the productive 



