270 L. HILLIS-COLINVAIJX 



populations of the Glory Be reef is estimated from these three 

 components of carbonate production. 



(a) Carbonate contents of Halimeda thalli. Figures for the amount of 

 calcium carbonate in Halimeda thalli are given in Table XXX, and 

 for comparison information is provided for other Caulerpales in 

 Table XXXI. The data from our laboratory were obtained using the 

 Hutchinson-MacLennan method (Barnes, 1959). Dried and cleaned 

 young field specimens which I collected in Jamaica provided most of the 

 material, but the fertile incrassata and the last two incrassata samples 

 listed of "about to fall" and "fallen" segments were collected in a labora- 

 tory environment though much of the calcification occurred in the reefs 

 of Jamaica. Although no uncalcified one- to two-day-old segments were 

 included in the study, or any old white sub-basal and basal segments, 

 the data show some of the range encountered within a species, as 

 provided by segments of different ages and with different rates of 

 metabolism. The variation among species, if the subtropical cuneata is 

 excluded, is not as great, and might be even less if younger and older 

 incrassata were better represented in the samples. 



These data show, with the possible exception of cuneata, that the 

 carbonate content of a segment is a function of segment age, which is a 

 reasonable conclusion. Older segments generally have more carbonate 

 than young segments, and this is particularly true of the final death and 

 loss of segment stages, as is shown clearly in the data for incrassata, 

 where the segments with most carbonate were those collected after 

 death (the very heavy basal segments are specialized structures not 

 typical of the segment flux). In interpreting the data for cuneata two 

 factors must be considered: that apical segments older than about 48 

 hours generally are as physiologically mature as most other segments on 

 the thallus (Section VI) and hence the separation into "apical" and 

 "midway" segments is not necessarily meaningful, and that cuneata 

 has, in addition to the regular segments, stalk and collar segments 

 (Section IV). The stalk region is uncalcified; the extent of the aragonite 

 deposits in collar segments has not been critically examined. 



These conclusions are in agreement with results from an earlier 

 calcification study by Wilbur et al. (1969). There must be further 

 variations in carbonate content with local circumstance and with time of 

 year. Thalli of some species, for example, appear to be more heavily 

 calcified in deeper water (Fig. 88; Bohm, 1973a). But in spite of these 

 caveats it seems reasonable to proceed to compute a carbonate flux 

 using the estimate for a typical dead incrassata segment as given in 

 Table XXX. 



