pressure for the evolution of grazer deterrents would be much greater in the 

 deeper habitats since losses would be a larger proportion of net growth and 

 take longer to replace. Future studies on spatial patterns in herbivory should 

 attempt to quantify herbivore impact as a proportion of plant production within 

 each habitat studied. 



Most of the reef slope seaweeds are known to have naturally occurring 

 chemical substances that appear to serve as defenses against herbivory (Fenical 

 1975, Norris and Fenical 1982). Many species in the genus Halimeda produce 

 diterpenoid trialdehydes (Paul and Fenical 1983), Rhipocephalus contains similar 

 compounds (Norris and Fenical 1982), Stypopodium zonale contains several related 

 C27 compounds derived from a mixed biosynthesis of diterpenoid and acetate 

 precursors (Gerwick and Fenical 1981), and Liagora produces an unusual acetylene 

 containing lipid (Norris and Fenical 1982). These compounds are toxic to or 

 deter feeding in reef fishes, and some even stop cell division in fertilized 

 sea urchin eggs or motility in sea urchin sperm (Norris and Fenical 1982, Paul 

 and Fenical 1983). The polyphenol ic compounds produced by Turginaria and 

 Sargassum (Norris and Fenical 1982) do not appear to be especially effective, 

 as evidenced by the feeding data in figure 2. The difference in susceptibility 

 of Sargassum polyceratium from the deep reef slope at Carrie Bow and the reef 

 flat at Lighthouse could result from population differences in defensive 

 compounds or from between-reef differences in herbivorous fishes. 



In general, it appears that herbivores consume a significant proportion of 

 reef slope production and that this has resulted in strong selection for 

 herbivore deterrents in seaweeds from this habitat. Reef flats provide predictable 

 escapes from herbivory, and seaweeds from these habitats are characterized by 

 very little resistance to herbivory. 



ACKNOWLEDGMENTS 



This study was supported by a post-doctoral fellowship from the Smithsonian 

 Institution and by the Marine Systems Lab of the Smithsonian Institution. 



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