DISCUSSION 



Relationships among plants and fierblvores on a tropical reef are 

 obviously complex, ye.t certain patterns are clear. Evidence 

 gatRered d-axing tlie present study supports the^ findings of Randall 

 C1961, 196"?, 1967), BCLatt and Strasburg C1960) and Stephenson and 

 Searles (1960) suggest that tropical marine vegetation is markedly 

 influenced by herbivores, especially fishes, and fishes are affected 

 by the availability of food plants. 



Distribution of Plants and Plant-eating Fishes 



Distribution of benthic plants in Lameshur Bay is basically limited 

 by availability of suitable substrate, light conditions, and other 

 physical factors in the environment. But the diversity and abundance 

 of vegetation on and in the immediate vicinity of the reef is directly 

 related to the occurrence of herbivorous fishes. This is illustrated 

 by the gradual increase in number and kind of attached plants along 

 two transect lines extending seaward from the reef, with concurrent 

 diminuition of herbivorous fishes. Further evidence is provided by 

 the increase in plant growth within cages designed to exclude grazing 

 fishes. After seven weeks, within cages placed in areas where grazing 

 fishes were common, there was appreciable increase in plant growth. 

 Within cages placed where grazing fishes were rare (more than 100 m 

 from the reef), plant growth was comparable with the adjacent 

 unprotected area, and in some, there was apparent reduction in plants 

 within the cages in this area, perhaps in part because of the activity 

 of small fishes that entered the cages , in part because of light 

 reduction due to heavy growth of filamentous algae on the outside of 

 the cages (much greater than on cages close to the reef) . At the 

 outer margin of plant cover, approximately 200 m from the reef, 

 physical factors, notably light and unsuitable substrate, may limit 

 growth. 



Results compare favorably with a study of Randall (1961) in Hawaii, 

 where growth of plants within wire enclosures on a reef attained 

 as much as 15 to 30 mm while algal outside averaged 1 mm in height, 

 the difference attributed by Randall to heavy grazing outside the 

 enclosure. In Lameshur Bay, Randall (1961) also observed appreciable 

 growth of attached algae within a wire enclosure over one square yard 

 of reef during ten weeks of observation, although the enclosure 

 contained at least 16 herbivorous urchins , Diadema antillarum , that 

 were being studied. The urchins tripled in size during the study, 

 but algae inside the cage nonetheless grew muck more than algae outside. 



VI-167 



