ALBERT

Notes: Abstract In contrast to terrestrial systems, few positive plant-plant associations have been recorded in tropical reef environments. This study, conducted at Carrie Bow Cay, Belize during 28 March–10 April 1984, provides the first documentation of herbivore escapes for natural combinations of palatable and unpalatable marine plants. For example, there was a highly significant association of several macrophyte taxa (Laurencia poitei, Dictyota spp., Amphiroa fragilissima, Cladophoropsis macromeres, Galaxaura cylindrica, rhodophycean turf) within a 2.0-cm radius of the herbivore-resistant brown alga Stypopodium zonale. Almost twice as many taxa occurred within 10 cm of S. zonale as within 10 cm of an equal number of random Stypopodium-free points, and there were no algal species negatively associated with S. zonale. The association of A. tribulus, L. poitei, Digenia simplex, rhodophycean turf, and Jania adherens with S. zonale provided them a fourfold greater survivorship per 48 h in the presence of grazing activity by fishes (mainly Acanthuridae and Scaridae). Reduced herbivory by fishes on macroalgae associated with S. zonale was not solely a consequence of its structural aspect. Losses of the palatable alga Acanthophora spicifera were significantly greater for thalli spatially remote (30 and 60 cm) from either a real or simulated Stypopodium; however, losses of A. spicifera adjacent to actual Stypopodium plants were significantly less than the losses next to models. The inter-relationships studied here, where an abundant and well-defended plant provides a significant refuge habitat for at least five relatively edible macroalgae, clearly facilitates the survival of certain taxa in the reef system and concomitantly enhances the within habitat diversity. Our findings also suggest an interaction counter to the process of competitive exclusion, since the single predominant plant has a positive rather than negative net effect on the abundances of other species that utilize the same general resources (e.g., light, space, nutrients).

Notes: Abstract Field and laboratory research at Carrie Bow Cay, Belize showed that macroalgae, grouped in functional-form units resisted fish and urchin herbivory in the following order (from high to low resistance): Crustose-Group, Jointed Calcareous-Group, Thick Leathery-Group, Coarsely Branched-Group, Filamentous-Group and Sheet-Group; thereby supporting the hypothesis that crustose, calcareous and thick algae have evolved antipredator defenses and should show the greatest resistance to herbivory with a gradation of increasing palatability towards filaments and sheets. Of the 21 species examined, several (e.g.,Dictyota cervicornis on grids,Laurencia obtusa andStypopodium zonale) had exceptionally low losses to fish grazing, probably due to chemical defences. The sea urchin,Diadema antillarum, was more inclined to feed on algae with known toxic secondary metabolites than were herbivorous fishes; hypothetically related to the differences in mobility and concomitant modes of feeding. Tough leathery forms such asSargassum polyceratium andTurbinaria turbinata resisted grazing by bottom feeding parrotfishes (Scaridae) and surgeonfishes (Acanthuridae) but were susceptible when suspended midway in the water column, possibly due to the presence of rudderfishes (Kyphosidae) which readily consume drift Sargassaceae. The overall tendencies support our predicted relationship between grazer-resistance and algal morphology. In conjunction with our previously reported findings concerning primary productivity, toughness and calorimetry for many of the same species, these results lend credence to generalizations relating form with function in marine macroalgae.

Notes: Abstract Field and laboratory research at Carrie Bow Cay, Belize showed that macroalgae, grouped in functional-form units resisted fish and urchin herbivory in the following order (from high to low resistance): Crustose-Group, Jointed Calcareous-Group, Thick Leathery-Group, Coarsely Branched-Group, Filamentous-Group and Sheet-Group; thereby supporting the hypothesis that crustose, calcareous and thick algae have evolved antipredator defenses and should show the greatest resistance to herbivory with a gradation of increasing palatability towards filaments and sheets. Of the 21 species examined, several (e.g., Dictyota cervicornis on grids, Laurencia obtusa and Stypopodium zonale) had exceptionally low losses to fish grazing, probably due to chemical defences. The sea urchin, Diadema antillarum, was more inclined to feed on algae with known toxic secondary metabolites than were herbivorous fishes; hypothetically related to the differences in mobility and concomitant modes of feeding. Tough leathery forms such as Sargassum polyceratium and Turbinaria turbinata resisted grazing by bottom feeding parrotfishes (Scaridae) and surgeonfishes (Acanthuridae) but were susceptible when suspended midway in the water column, possibly due to the presence of rudderfishes (Kyphosidae) which readily consume drift Sargassaceae. The overall tendencies support our predicted relationship between grazer-resistance and algal morphology. In conjunction with our previously reported findings concerning primary productivity, toughness and calorimetry for many of the same species, these results lend credence to generalizations relating form with function in marine macroalgae.

Notes: Summary When the common sea urchin Diadema antillarum was removed from a 50 m strip of reef in St. Thomas, US Virgin Islands, cover of upright algae and the grazing rates and densities of herbivorous parrotfish and surgeonfish increased significantly within 11–16 weeks when compared to immediately adjacent control areas. Sixteen months after removal, Diadema had recovered to 70% of original density, abundance of upright algae no longer differed between removal and control areas, and the abundance and grazing activity of herbivorous fish in the removal was approaching equivalence with control areas. On a patch reef in St. Croix that had been cleared of Diadema 10–11 years earlier (Ogden et al. 1973b), urchins had recovered to only 50–60% of original density. This reef still showed significantly higher rates of grazing by fish and a significantly greater density of parrotfish and surgeonfish than a nearby control reef where Diadema densities had not been altered. These results indicate that high Diadema densities (7–12/m2 for this study) may suppress the densities of herbivorous fish on Caribbean reefs.

Notes: Summary Some shallow habitats that surround mangrove islands exhibit abruptly discontinuous macrophyte boundaries; in other regions, plant distributional patterns are less defined. Where distinct boundaries do occur, fleshy algae predominate on the roots of the red mangrove, Rhizophora mangle, which do not contact the bottom sediments (“hanging roots”), while calcifying algae dominate on the substratum-penetrating roots and banks (=embedded-root habitat) surrounding the mangrove thickets. Considerable natural-history and floristic information reveals that the fleshy hanging-root species are not specialists, for that type of habitat. Experimental transplants showed that on banks and embedded roots where there typically are abundant macroherbivores (particularly sea urchins), most fleshy algae are eliminated. The dominants of the hanging-root habitat (e.g, Acanthophora spicifera, Spyridia filamentosa, Caulerpa racemosa var. Occidentalis) are 6–20 times more susceptible to herbivores than the dominants of the embedded-root habitat (e.g., Halimeda opuntia f. triloba, H. monile). Consequently, we suggest the former are relegated to the spatial refugia from herbivores (=non-coexistence escapes) provided by the hanging roots. Factors associated with these palatability differences include higher average calorific values (6.5 times) of the fleshy hanging-root dominants, greater proportions of organic content (2.6 times) and the general absence of calcification. The dominants of the embedded-root habitat show reduced edibility as a probable consequence of low calorific values, heavy calcification and potential herbivore-detering secondary metabolites. Correlative evidence and preliminary experimental results tentatively indicate that, in the absence of macroherbivores, the hanging-root dominants, which exhibit production rates 4.7 times greater than the dominants of the embedded-root habitat, are better competitors for space. We suggest that variations in herbivory are responsible, in part, for maintaining greater algal diversity in mangrove systems. At a study site with abundant sea urchins, five algal species were found only in the embedded-root habitat three species were confined to the hanging roots, while three others occurred in both. At an urchin-free site, no macrophytes were found only on embedded-root substrata, while one (in trace amounts) was found only on hanging-root habitat and eight occurred in both. We predict that in the absence of herbivores, the species assemblage characteristic of the hanging-roots would exclude many of the dominants from the embedded-root habitat.