Animal-plant defense association: the soft coral Sinularia sp. (Cnidaria, Alcyonacea) protects Halimeda spp. from herbivory

Abstract

An associationsl refuge occurs when an organism escapes predation by being closely associated with another organism. We describe a refuge provided by the soft coral Sinularia sp. for algae of the genus Halimeda growing at its bases. Total algal biomass was significantly greater in the 0–10 cm zone than in zones 10–20 and 20–30 cm around individual soft coral colonies. Total algal biomass was also significantly greater within clusters of colonies than in zones 0–10, 10–20, and 20–30 cm around the perimeter of the clusters. Halimeda incrassata (Ellis) and Halimeda opuntia (Linnaeus) Lamouroux comprised the bulk of the total algal biomass and were the only species that showed a pattern of growth similar to the total biomass. Fishes consistently grazed less algae ( Halimeda incrassata and Acanthophora spicifera (Vahl) Boergesen and seagrass ( Halodule uninervis Ascherson) strands placed next to soft corals than farther away. Fishes grazed indiscriminately around objects (rocks or outcrops of dead hard coral), whereas differential grazing occurred around soft corals. This suggested that the presence of an object alone is not responsible for reduced grazing next to the soft coral. When soft corals were removed, the pattern of grazing was disrupted, whereas around intact colonies the pattern persisted. Subsequently, funnel-shaped carrageenan models, that resembled the arborescent shape of Sinularia sp., produced a similar grazing pattern. Removing soft corals did not appear to encourage grazing of exposed Halimeda spp. by fishes. This may be explained by the chemically deterrent or heavily calcified nature of Halimeda spp. (the dominant algae around soft corals) which would prohibit vigorous consumption by herbivorous fishes. Algal species richness did not appear to be enhanced by the presence of Sinularia sp. We found no evidence for chemical defense in experiments with soft coral extracts, carrageenan models, and rehydrated Sinularia sp. colonies. Just as much seagrass was grazed around models and colonies with added extracts as around controls, suggesting that terpenes and other metabolites present in the extracts were not responsible for the protective refuge provided by the soft coral. However, a significant effect of distance was consistently obtained in experiments with funnel-shaped models and rehydrated soft corals, whereas, no effect of distance occurred with beaker-shaped models. These results suggest that the arborescent shape of Sinularia sp. provides a refuge for young recruits of Halimeda spp., and allows the algae to mature until they are not preferred by herbivorous fishes. This study is the first to directly test whether morphological features or chemical properties of a benthic marine organism are responsible for protecting closely associated algae.