filaments; e.g., Pol vsiphonia t Space1aria » and Taenioma ) , 2) MACROPHYTES (larger, 

 more ridged forms; e.g.» Laurencia , Dictvota , Jania , and Asperogopsis ) , and 3) 

 ENCRUSTING CORALLINES (calcareous algal crusts; Porolithon , Neogoniol ithon and 

 Paragoniol ithon ) . Note that the "turfs" referred to here follow Neushul 1967, 

 Randall 1967, Dahl 1972, John and Pople 1973, Van den Hoek et al_. 1975, Adey et_ al_. 

 1977, Benayahu and Loya 1977, Pichon and Morrisey 1981, but not Hay 1981a). Among 

 the ecological properties correlated with algal functional groups (and this 

 simplified scheme) is toughness of the thallus. From an herbivore's perspective 

 turfs are easiest to consume, rnacrophytes intermediate and coralline crusts most 

 difficult (Littler and Littler 1980, Steneck and Watling 1982, Littler ejt_ aj_. 1983). 

 Herbivores fall into three categories with respect to grazing (Steneck 1983, see 

 Table 1 for species): 1) NON-DENUDING (those incapable of, or unlikely to, denude 

 the substratum of algae; e.g., some damselfishes [ Eupornacentrus ], anphipods and 

 polychaetes; Brawley and Adey 1977, Kaufman 1977), 2) DENUDING (those that denude 

 the substratum of turfs and smaller macroalgae but are incapable of excavating 

 corallines and large leathery rnacrophytes; e.g., yellowtail damselfish 

 [ Microspathodon ], tangs [ Acanthurus ] several non-limpet archaeogastropods and 

 mesogastropods; Randall 1967, Jones 1968), and 3) EXCAVATING (those capable of 

 consuming even the toughest forms of algae, such as encrusting corallines e.g., 

 limpets C Acmaea J, chitons, some regular echinoids [ Diadema ] and parrotfishes C Scarus 

 and Sparisoma ] : Randall 1967). For a more complete discussion of these categories 

 see Steneck 1983 . 



Study Sites 



This study was conducted at nine sites in three locations along the north shore 

 of St. Croix (Fig. 1). The locations were selected as representative of three 

 common reef types in the Caribbean 1) Alcal Ridge (Boiler Bay), 2) Bank barrier-reef 

 (Tague Bay), and 3) Deep wall-reef (Salt River Canyon). All three are 

 geographically close to one another, thus giving them about the same exposure to 

 light, destructive storms and recruitment events. 



The predominant algal growth form at the bank-barrier and deep wall-reef sites 

 are turfs with an average canopy height of 2mr.i. Turfs are highly diverse with about 

 30 to 50 species found in an average four square centimeter area (Adey and Steneck 

 in ms.). Species composition is temporally variable with up to an 80/3 change in 

 community dominance every 3-4 months (Steneck in prep.). Patches of encrusting 

 coralline algae are also scattered throughout. Macroalgae are only abundant in 

 places at the algal ridge site (see Connor and Adey 1977). 



At each site, circular slabs of coral (1 cm thick, and 10 cm diameter) were 

 placed on racks and affixed to the reef (locations and depths in Fig. 1). The 

 plates were placed in December of 1979, and most of the experiments were conductea more 

 than three years later from March to May 1982. At the time of the experiments, the 

 coral plates had the same algal community structure, canopy height, and biomass as 

 the surrounding substrata. All the plates at the nine stations were covered with 

 the same (algal turf) functional group. Details of specific experiments are 

 described below where appropriate. 



RESULTS AND DISCUSSION 



To reduce some of the variables related to herbivory, the study was conducted at 



one spot and at one time at each of the nine sites. Several of the experiments ran 



simultaneously, focusing on the same six (100 to 200 crn sq. ) planar coral plate 

 surfaces or their surrounding areas. 



104 



