Concentration of secondary metabolites 



The concentration of common secondary metabolites produced 

 by Halimeda also varied with segment age (Figs. 6 and 7, and 

 Table 1 ) . The concentrations of halimedatrial and 

 ephihalimedatrial as a function of time and segment type are 

 shown separately in Table 1. We know little about the 

 biosynthesis of these two compounds, but their concentrations per 

 ash-free dry mass of plant material were almost identical for all 

 times and segment types. For this reason and because the 

 compounds differ only stereochemically in structure (Paul, 1985), 

 we pooled these two compounds to produce the top panel in Figure 

 6. With increasing segment age, the halimedatrials 

 (halimedatrial and ephihalimedatrial combined) decrease as a 

 percentage of the ash-free dry mass of the segments (Fig. 6); 

 they constitute up to 4% of the ash-free dry mass of newly 

 forming segments but constitute only 0.3% of the ash-free dry 

 mass of old segments. Differences in concentrations between tips 

 (2000-0400 hr), young segments (0800-1800 hr) and old segments 

 (several days old) were large and significant (p<.05, Kruskal- 

 Wallis Test and a nonparametric parallel of the Student-Newman- 

 Keuls (see Zar, 1974) after randomly reducing all sample sizes to 

 N=9 ) . Halimedatetraacetate did not occur in newly developing 

 tips of H^ incrassata (Fig. 6 lower panel). The 0400 hr samples 

 showed no halimedatetraacetate; by 0800 hr, halimedatetraacetate 

 concentration had risen to approximately 1% of the ash-free dry 

 mass of the young segments. Its concentration then decreased in 

 older segments. The concentration of halimedatetraacetate 

 differed significantly between tips, young segments, and old 

 segments (p<.05, Kruskal-Wallis Test and a nonparametric parallel 

 of the Student-Newman-Keuls ) , with the maximum concentration 

 occurring in young segments that had been formed the previous 

 night. Since the rapid rise in halimedatetraacetate occurs at 

 about sunrise when pigments are being pumped into the newly 

 formed segments, it seems probable that halimedatetraacetate is 

 being moved from the older to the younger segments or that 

 biosynthesis of the tetraacetate is dependent upon 

 photosynthesis . 



For Halimeda simulans , we conducted only preliminary 

 analyses on the timing of changes in secondary compound 

 concentration. However, the available data suggest that patterns 

 for H_^ simulans (Fig. 7) are similar to those documented for H. 

 incrassata . 



Herbivory on Halimeda 



Diel patterns. The rapid nocturnal expansion of the newly 

 formed and uncalcified segments coincides with a predictable 

 decrease in grazing that starts in the afternoon and continues 

 throughout the night (Fig. 3). Grazing in the sandplain habitat 

 was much less intense (0-6%/4 hr ) than on the reef slope (as much 



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