ECOLOGY AND TAXONOMY OF Halimeda 261 



classes, liut the data in their present state are sufficient to suggest 

 strongly that Halimeda contributes to the mass of a coral atoll to a 

 significant extent compared with the coral contribution. And this 

 observation comes from cores taken on islets or the reef ridge, not in the 

 lagoon where the contribution of Halimeda would be largest. 



The evidence of drill cores, therefore, is that a significant part of the 

 mass of coral atolls is made of the segments of this one green alga, 

 Halimeda. Much of the work of maintaining the atoll habitat is done by 

 these plant populations, and the solar energy flux they transduce not 

 only fuels food chains but also contributes to the structure of the eco- 

 system in a way that may be necessary to the system's homeostasis. 



2. The shedding of segments and the segment fates 



The aged, white, yellowish or greyish segments commonly are shed 

 from the apical portions of a Halimeda thallus by a natural separation 

 from the node, rather like a tree leaf being shed in the autumn. In 

 sprawlers such as macrophysa and opuntia they also may be lost basally 

 (Hillis-Colinvaux, 1977; Section VI). 



Segments are also lost basally and deposited in situ when lower 

 portions of a thallus are buried. 



Initially much of the material contributed to the reef environment 

 may be entire segments, but the segments of a few species such as 

 macrophysa and favulosa are delicate, and sometimes may not be shed 

 intact. It is noteworthy that these two species have the largest utricles 

 (Hillis, 1959; Section III, IV), that in macrophysa the utricles do not 

 adhere laterally, and in favulosa they are attached only lightly. 



Segments of some of the rock growers-sprawlers, when shed, may 

 shift deeper into the rock crevices in which the living thallus is clois- 

 tered. The species macrophysa is a good example, opuntia somewhat 

 less so. Most commonly, however, the segments floor some surface in 

 the immediate vicinity, from whence they may be transported by the 

 currents to other sites, or they may be transported directly. The 

 pattern of movement will depend both on their weight (branches may 

 be involved initially as well as segments) and on the strength of the 

 currents. Their transport out of the region may be slowed by the growth 

 of blue-green algal mats over them (Figs 102, 103), or by various 

 gelatinous materials within the sediments (Moore et al., 1976). 



At Enewetak (Fig. 83) some of the lagoonal sediments may have 

 originated on the fore -reef, and some lagoonally derived Halimeda 

 material may be transported through the passes in the reef to outer 

 areas. Such movements may be somewhat seasonally controlled, since 



