Corals Chemically Cue Mutualistic Fishes to Remove Competing Seaweeds

Coral reefs, some of Earth’s most important and fascinating ecosystems, are in global decline—coral cover in has declined nearly 80 percent in the Caribbean and nearly 50 percent in Australia’s Great Barrier Reef. Branching corals such as Acroporids are essential to the growth of coral reefs because they provide topographic complexity, among which many reef species depend. Herbivorous fishes also play a key role in coral reef ecosystems by gnawing on competing algae and facilitating the colonization and growth of new corals after disturbances such as cyclones or disease. Dixson and Hay (2012) show that symbiotic gobies defend the common coral Acropora nasuta by removing allelopathic alga. The process is mediated by chemical signals and cues and may be disrupted or reversed by changes in ocean environment such as pH. Results of this study are not only interesting, but also present the first example of a species chemically cuing consumers to remove competitors.—Kelsey Waite
Dixson, D.L., Hay, M.E., 2012. Corals Chemically Cue Mutualistic Fishes to Remove Competing Seaweeds. Science 338, 804–807.

            Dixson and Hay studied how coral-dwelling fishes affected seaweed-coral interactions. They placed Chlorodesmis fastigiata (an allelopathic seaweed) on the common coral Acropora nasuta, near four different commensal fishes. Coral health was analyzed using pulse-amplitude modulated fluorometry. In field experiments, transects were run across the reef to evaluate goby occupancy. Gobies were found in 81 ± 16% of the 207 colonies assessed.
              After three days of contact with C. fastigiata, the effective quantum yield of the corals had been suppressed by roughly 80%. Most common coral colonies host at least two gobiids, which remain in that same colony for most of their adult life. In coral colonies occupied by gobies, the harmful effect of C. fastigiata on A. nasuta declined by 70–80% compared to those colonies without gobies. Dixson and Hay also determined that gobies only responded to chemical cues from Acropora nasuta, not any other—even closely related—coral species. When the coral cues gobies, the gobies begin removing alga within minutes of seaweed contact. As reefs continue to evolve due to climate change, it is important to understand interactions that enhance survival and re-growth of coral. Chemically mediated behaviors such as these can potentially be suppressed or reversed if the ocean environment changes. 

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