diel pattern of segment production (M. Hay, personal 

 observation) that we document for H^ incrassata and H^ simulans 

 in the Virgin Islands. By using the microcosm and its connected 

 refuge tanks, we were able to reverse day-night photoperiods and 

 assess the importance of light cycles versus diel changes in 

 water chemistry in regulating segment production. 



METHODS 



Diel patterns of segment production in Halimeda incrassata 

 growing near the Hydrolab in St. Croix were documented by 

 collecting 30 haphazardly selected plants every 4 hrs for 56 hrs 

 and investigating each branch tip on each plant to see if it 

 contained a new bud (a small white protuberance of less than 1 mm 

 wide), an expanding but unpigmented tip (larger than the bud, 

 with the shape of a segment, but still white), or a relatively 

 mature, pigmented segment (see Fig. 1). At each sampling period, 

 plants were taken into the Hydrolab where newly forming buds, 

 immature tips, and mature segments (4-6 segments below the apex) 

 were blotted dry on paper towels and carefully cut from the 

 plant using a razor blade or small scissors. Each type of 

 segment material was stored in small vials that were taken to the 

 surface within 4 hrs of collection and either extracted for 

 chemical analyses in a mixture of 25% methanol and 75% 

 dichloromethane or dried and frozen for analyses of nitrogen and 

 organic content. These samples were later analyzed for nitrogen 

 content, ash content, and concentration of three secondary 

 metabolites commonly produced by the genus Halimeda . In order to 

 get enough material for an adequate determination of nitrogen, 

 ash, and secondary metabolite content, we had to pool the new 

 buds from several plants and the immature tips from several 

 plants (usually about 10). Independent replicates for each time 

 interval were obtained by making collections on 4 separate days. 

 While processing these samples, some replicates were lost due to 

 spillage or a breakdown of the high performance liquid 

 chromatography (HPLC) equipment used for quantifying 

 concentration of secondary metabolites. This resulted in less 

 than 4 replicates for some time intervals. 



During the 56 hrs when we quantified diel patterns of 

 segment production, we simultaneously quantified herbivory on 

 Halimeda incrassata . Two mature branches of H^ incrassata were 

 affixed into sections of 3-strand rope so that 5 segments on each 

 branch were exposed to grazers. These ropes were placed on the 

 sand plain at a depth of 21-24 m and on the reef slope at a depth 

 of 15 m. Grazing was quantified as the number of segments eaten 

 on each rope during each 4 hr interval . Grazed branches were 

 replaced at each 4 hr monitoring interval and all branches were 

 replaced every 12-24 hrs. Fifteen separate ropes were placed on 

 the sand plain at intervals of 3-4 m. Twenty-one ropes were 

 placed on the reef slope in a similar manner. 



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