Abstract

The comparative present-day rarity of megafaunal herbivores means that fish and sea urchins are likely the dominant grazers in much of the modern Caribbean. Grazing by these smaller organisms has been shown to regulate the canopy structure and species composition of seagrass beds, but the comparative impact of the two groups may not be equal. Fish are generally thought to be the more important of the two functional groups in all but the most overfished seagrass beds. Grazing assays and surveys of benthic cover and grazer abundance in seagrass beds across various hydrographic regimes were used to determine the relative importance of grazing by fish and urchins and whether high grazing by either group excludes Syringodium filiforme from certain seagrass beds. Grazing on deployed grazing assays was greater during the day than at night suggesting that generally diurnally active fish consume more seagrass than generally nocturnally active urchins. Grazing on deployed S. fililforme was also greater than on deployed Thalassia testudinum at most sites regardless of time of day suggesting the former species is preferred by both fish and urchins. Grazing on deployed S. filiforme, however, was generally higher at sites where this species is present than where it is absent and may suggest that grazing does not primarily limit the distribution of S. filiforme in Bocas del Toro. This conclusion, however, must be interpreted cautiously given that the deployment time of a grazing assay is necessarily shorter than the lifespan of a seagrass shoot and this may make assays more likely to be visited by small, abundant, and relatively evenly distributed herbivores than by larger rarer ones and further research may be desirable to determine which group typically has a greater influence on the structure of Caribbean seagrass beds.

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