THE INFLUENCE OF HERBIVORES ON THE 

 MARINE PLANTS OF GREAT LAMESHUR BAY, ST. JOHN, VIRGIN ISLANDS 



Sylvia A. Earle 

 Los Angeles County Museum of Natural History 



ABSTRACT 



Thirty-five species of plant-eating fishes in 14 families 

 were observed in Lameshur Bay, St. John, Virgin Islands 

 during TEKTITE II Mission 6-50. One hundred fifty-four 

 species of marine plants were found, including 26 species 

 newly reported for the Virgin Islands. Activities of the 

 fishes were noted, particularly feeding behavior and choice 

 of diet, and their influence on vegetation assessed. Fish 

 distribution was described and compared with plant distri- 

 bution. In general, in areas where physical requirements 

 for plant growth were met, attached vegetation was abundant 

 where herbivores were sparse, and where herbivores were 

 abundant, vegetation was sparse. Cages designed to exclude 

 fishes were established along two transect lines and growth 

 of plants within compared with adjacent plants outside. 

 After seven weeks, appreciable gro\vth was noted inside 

 cages where plant-eating fishes were abundant, but there 

 was little difference inside and outside of cages in areas 

 where these fishes were rare. Most adult plant-eating 

 fishes remained, day and night, close to the reef, but 

 young of many lived on the algal plain as much as 200 m from 

 a reef. All plant-eating fishes observed were active by day, 

 at rest during the night. Herbivorous fishes were concluded 

 to be a major influence on the abundance, diversity and 

 distribution of benthic marine plants in tropical seas, 

 with invertebrate herbivores of relatively small significance. 

 This is in contrast to cold and temperate situations where 

 herbivorous fishes are rare, but invertebrates of great 

 importance as grazers. The role of predators and their 

 indirect influence on vegetation is discussed. Annotated 

 lists of plants and plant-eating fishes in Lameshur Bay 

 are included. 



INTRODUCTION 



Perhaps the most striking aspect of plant life on a coal reef is the 

 general lack of it. It seems anomalous to even the casual observer 

 that tropical reefs, notable for their dazzling profusion of animal 

 life, are almost devoid of conspicuous plants. Frequently, not only 

 the reef proper, but also an area 8 to 30 or more feet surrounding a 

 reef may be almost bare, although beyond this area attached plants 

 may be quite abundant. Odum and Odum (1955) found that a substantial 

 part of the plant food available on a reef may be growing on and 

 within corals . 



VI-132 



