Coral-algal competition: macroalgae with different properties have different effects on corals

Competition between hard corals and macroalgae is a key ecological process on coral reefs, especially during reef degradation, which often involves a 'phase-shift' from coral- to alga-dominated reefs. However, there are relatively few published studies exploring the variability in this interaction. This paper expands the range of documented coral-algal interactions by comparing the mechanisms and outcomes of interactions involving 3 different algal species, as well as general, mixed algal turfs. Mixed filamentous turfs had relatively minor effects on corals. However, the turfing filamentous red alga Corallophila huysmansii provided a dramatic exception to this pattern, being able to settle on, overgrow and kill live coral tissue, perhaps due to allelochemical production by the alga, although this was not directly demonstrated. The larger filamentous alga Chlorodesmis fastigiata ('Turtle Weed'), which is conspicuous and abundant on Indo-Pacific reefs, caused polyp retraction but had little other noticeable effect on coral tissue. A corticated red alga Hypnea pannosa, frequently observed living within colonies of the branching coral Porites cylindrica, did not have a major impact on underlying coral tissue, even over a period of 1 yr, apparently because its relatively translucent and porous thallus structure does not strongly inhibit coral tissue functions. Together, the results demonstrate the considerable potential variability in both the process and outcome of coral-algal competition. This variability can be effectively interpreted in terms of the limited number of mechanisms by which algae can affect corals, with these mechanisms depending largely on the properties (physical, biological, chemical) of the algae. Given the central importance of coral-algal competition to the process of coral reef phase-shifts, understanding the variability and complexity in such competition will have important implications for the prediction and consequences of such phase-shifts.