BETWEEN-HABITAT DIFFERENCES IN HERBIVORE 

 IMPACT ON CARIBBEAN CORAL REEFS 



Mark E. Hay 

 University of North Carolina at Chapel Hill 

 Institute of Marine Sciences, Morehead City, North Carolina 28557 



Tim Goertemiller 



Marine Systems Laboratory, NHB W-310 



Smithsonian Institution, Washington, DC 20560 



ABSTRACT 



Transplanted sections of the seagrass Thalassia testudinum were used as a 

 bioassay to assess between-habitat differences in herbivory on three Caribbean 

 reefs. Consumption of Thalassia by herbivorous fishes on shallow (1-10 m) reef 

 slopes was significantly higher than on deep (30-40 m) reef slopes or on shallow 

 reef flats. Seaweeds typical of reef flat habitats were rapidly consumed when 

 placed on shallow reef slopes. Seaweeds typical of either deep or shallow reef 

 slopes were relatively resistant to herbivory and a high proportion of these 

 species are known to contain secondary chemical compounds that appear to deter 

 herbivorous fishes. Shallow reef flats provide seaweeds with a predictable 

 spatial escape from major reef herbivores; algae characteristic of these habitats 

 have evolved few, if any, characteristics that significantly reduce losses to 

 herbivory. 



INTRODUCTION 



Although herbivory plays a major role in determining the distribution and 

 abundance of seaweeds on coral reefs (Stephenson and Searles 1960; Randall 

 1961, 1965; Ogden et al_. 1973; Hay 1981a, b; Luchenco and Gaines 1981; Hay et 

 al . 1983), few studies have addressed spatial variations in herbivory or the 

 relative susceptibility to herbivory of seaweeds from different habitats. 

 Recent investigations have focused on between- and wi thin-habitat variations in 

 herbivory on individual reefs (Hay 1981a, b; Hay et a]_. 1983) and on changes in 

 herbivory that occur over depth gradients on several reefs scattered throughout 

 the Caribbean (Hay 1984). 



In this study we use transplanted sections of the seagrass Thalassia 

 testudinum as a bioassay for herbivore activity in different habitats [reef 

 flats, shallow (1-10 m) reef slopes, and deep (30-40 m) reef slopes] on 3 

 Caribbean reefs. We also transplant seaweeds from each of these habitats 

 into areas with high herbivore activity in order to assess their relative 

 susceptibility to herbivory. 



METHODS 



Thalassia was chosen as the bioassay organism because it is eaten in the 

 field by both herbivorous fishes and urchins (Randall 1965, Ogden et _aJL 1973, 

 Ogden 1976) and it is readily available on most reefs. Freshly collected 

 sections of Thalassia were fastened in wooden clothespins; the latter were 

 attached to small coral fragments and distributed haphazardly within the habitats 

 where herbivory was to be measured. All Thalassia sections were 5 cm long and 

 separated by a distance of 1-3 m when placed in the field. At the end of each 



97 



