carbonates bound in framework organisms such as corals and 

 encrusting coralline algae (Milliman, 1974). 



The ability of Halimeda to maintain high standing stocks on 

 reefs where other seaweeds are rapidly eaten to extinction has 

 generally been attributed to its calcified thallus and its 

 ability to synthesize numerous novel terpenoid metabolites (see 

 Paul, 1985 for the structures of these compounds) that appear to 

 function as herbivore feeding deterrents (Paul and Fenical, 1983; 

 Hay, 1984b; Paul, 1985; Paul and Fenical, 1986; and Targett et 

 al . , 1986). The consequences of nocturnal segment production for 

 Halimeda-herbivore interactions have not been investigated. 



The majority of this study was conducted from the National 

 Oceanic and Atmospheric Administration's Hydrolab facility 

 located in Salt River Canyon, St. Croix, U.S. Virgin Islands 

 (17°47'N, 64°45'W). The habitats near this facility have been 

 described previously (Adey et al . , 1977). Most of our 

 observations and experiments were made on populations of Halimeda 

 incrassata that occurred abundantly on the sandy canyon floor at 

 a depth of 18-21 m. Some herbivory assays were conducted on the 

 nearby reef slope at a depth of 15 m or in areas of the back reef 

 at a depth of 2-3 m. Herbivorous parrotfishes ( Scarus 

 taeniopterus , S. iserti , Sparisoma aurofrenatum , and S_^ viride) 

 and surgeonfishes ( Acanthurus coeruleus and A^ bahianus ) were 

 common in the back-reef habitat; on the deeper reef slope, 

 parrotfishes were the most common herbivores. Sea urchins are 

 rare in these habitats, and spatial patterns of herbivory on the 

 reef at Salt River Canyon, St. Croix are typical of those 

 occurring on most Caribbean reefs that are not subject to heavy 

 fishing (Hay, 1984a). 



Observations and experiments on a second Halimeda species, 

 H. simulans, were conducted in a shallow (1-3 m deep) mangrove- 

 lined bay next to the Hydrolab shore base in St. Croix. In this 

 habitat, H_^ simulans and documentation of its diel pattern of 

 segment production were conducted on a 20-24 m deep sand plain at 

 Coki Point, St. Thomas, U.S. Virgin Islands (18°21'N, 64°52'W). 



We investigated the proximate causes of nocturnal segment 

 initiation in Halimeda using a large (7000 1) coral-reef 

 microcosm at the Smithsonian Institution's National Museum of 

 Natural History in Washington, D.C. A simplified schematic of 

 the microcosm is shown in Figure 2; the system has been described 

 in detail by Adey (1983). A wave generator produces turbulence 

 within the microcosm and powerful metal-halide bulbs provide 

 intense light that facilitates seaweed growth. Diel variations 

 in water chemistry are similar to those that occur on natural 

 coral reefs (Adey, 1983). The microcosm supports several hundred 

 species of organisms found on Caribbean reefs. Halimeda opuntia 

 grows particularly well in the microcosm and undergoes the same 



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