bioassay, removal of Thalassia was quantified by measuring the remaining length 

 of each blade to the nearest .5 cm. During all tests, the clothespin and coral 

 apparatus was positioned so that it would be equally approachable by both fishes 

 and urchins. However, on most sections of these reefs, urchins are relatively 

 uncommon and almost all Thalassia removal was due to grazing by fishes (Hay 

 1984). 



Removal of Thalassia on reef slopes varies with depth; portions of this 

 pattern have been extensively analyzed elsewhere (Hay et a]_. 1983, Hay 1984). 

 In this paper we compare herbivory on (1) reef flats that are exposed at lowest 

 tides, (2) shallow (1-10 m) portions of reef slopes, and (3) deeper (30-40 m) 

 portions of reef slopes. 



Seaweeds from each of these habitats were exposed to herbivorous reef 

 fishes by placing small (3-4 cm long) pieces of each seaweed in a 3-stranded 

 rope that was fastened to the reef slope at a depth of 1-5 m. Ten to 37 

 individuals of each test species were used at each location (for a description 

 of each site, see Hay 1984). Seaweeds within a length of rope were separated 

 from one another by a distance of about 7 cm. Thus, when an herbivore encountered 

 a rope, all species of seaweed should have been equally apparent and available. 

 At the end of an experiment, each species on each rope was recorded as either 

 still present or totally eaten. Ropes were only placed in the field under 

 completely calm conditions and were shaken to be sure that all individuals were 

 securely attached. On the reef at Becerro, Honduras, where the feeding trial 

 was of short duration (1.75 h), we were able to directly observe the ropes for 

 most of the test period; no individuals were lost to any source other than 

 herbivory. For the feeding trials of longer duration (19-24 h), we cannot 

 absolutely rule out the possibility that some individuals were lost to breakage. 

 However, the magnitude of such loss would have to be very small given the calm 

 conditions and our inability to observe breakage during any of the observation 

 periods. 



Assignment of seaweeds as characteristic of reef flat, shallow reef slope, 

 or deep reef slope habitats was based on qualitative observations at each study 

 site. For example, species that were common between 30 and 40 m deep and 

 present but rare at 10 m deep were listed as characteristic of the deep reef 

 slope. 



RESULTS 



Consumption rates for sections of Thalassia placed on shallow reef slopes 

 were significantly higher than consumption rates on either reef flats or deeper 

 sections of reef slopes (p<.05, AN0VA and Student Newman-Keuls Test) (fig. 1). 

 A deep reef slope comparison could not be done at Becerro, Honduras, since the 

 reef slope extended to a depth of only 9m. On all 3 reefs, all daytime removal 

 of Thalassia was attributable to fishes, as evidenced by their crescent shaped 

 feeding scars (see Hay et a]_. 1983, Hay 1984). On the two reefs where comparisons 

 were made, removal of Thalassia on the deep reef slope was significantly higher 

 than removal on the reef flat (fig. 1). However, the magnitude of this difference 

 was small when compared to differences between the shallow reef slope and 

 either of the other habitats. 



When seaweeds were transplanted onto shallow reef slopes, those from reef 

 flats were consumed rapidly while those from either deep or shallow reef slopes 

 were consumed slowly if at all (fig. 2). The one exception to this pattern was 

 Padina sp. at Becerro, Honduras (fig. 2C). This reef flat species was not 

 eaten when placed on the shallow reef slope, although Padina sanctae-crucis was 

 rapidly consumed at Carrie Bow and Lighthouse reefs (figs. 2A, 2B). 



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