(3) seaweeds that grow as upright blades during periods when 

 grazers are inactive, but grow as herbivore-resistant, encrusting 

 or creeping forms during periods when herbivores are active 

 ( Lubchenco and Cubit, 1980; and Lewis et al., 1987). These 

 previous studies have focused on seasonal or year-to-year 

 variability in consumption. In this study, we show that some 

 tropical seaweeds reduce losses to grazers by exploiting diel 

 patterns of herbivore activity. These seaweeds produce new, 

 nitrogen rich, and more vulnerable, growth only at night when 

 herbivorous reef fishes are inactive. Immediately after 

 production, the new, nutritionally valuable, tissue contains high 

 concentrations of terpenoid compounds that deter grazing by the 

 most common herbivorous fishes. Over the next 2-3 days, 

 morphological defenses increase, the tissue becomes less valuable 

 as a food source, and chemical defenses decrease. Although 

 nocturnal growth appears to occur in numerous genera of tropical, 

 chemically-defended seaweeds in the order Caulerpales, we focused 

 our investigations exclusively on species in the genus Halimeda . 

 Halimeda is an ecologically and geologically important component 

 of several tropical habitats and is one of the most abundant 

 seaweeds in the tropics (Hillis-Colinvaux, 1980; Drew, 1983; 

 Davies and marshall, 1985; and Drew and Abel, 1985). We ask the 

 following questions: 



1 ) Do seaweeds in the genus Halimeda initiate new growth 

 primarily at night? 



2) Does this nocturnal production of new growth coincide with a 

 diel decrease in the overall rate of herbivory on Halimeda ? 



3) Is young Halimeda tissue more susceptible to herbivory than 

 older tissue? 



4) How do nutritional value (nitrogen and organic content) and 

 secondary metabolite concentration of Halimeda vary with 

 tissue age? 



5) Do the terpenoid metabolites produced by Halimeda deter 

 grazing by common reef herbivores? and, 



6 ) Are the patterns documented for Halimeda unique or do they 

 occur in other organisms? 



Organisms and study sites 



Species in the genus Halimeda grow by the addition of new 

 segments at the apex (Fig. 1). This makes quantifying new 

 growth particularly easy since very simple field procedures can 

 be used to mark older segments and count the addition of new 

 segments with time (Drew, 1983). Halimeda and related genera 

 produce calcium carbonate at rates of 32-185 g m"^yr _1 (Bach, 

 1979; and Drew, 1983) and account for more than half of the 

 carbonate production in tropical reef and lagoon systems 

 (Stockman et al . , 1967; Milliman, 1974; Neumann and Land, 1975; 

 Bach, 1979; and Wefer, 1980). The unconsolidated carbonated 

 sediments produced by these seaweeds are often quantitatively 

 more important in contributing to the bulk of reefs than are the 



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