Settlement from the plankton to the benthos is a key component of recruitment and recovery of marine organisms. During settlement, larvae rely on facilitative physical and chemical cues to select optimal habitats that maximise post-settlement survival, as well as inhibitive cues to avoid suboptimal habitats. Yet many ecosystems are experiencing shifts in community structure resulting from local and global stressors and it is uncertain how such changes influence settlement cues and therefore the future recruitment of populations. Coral reefs typify these concerns, with many reefs losing coral and coralline algae and experiencing an increase of fleshy macroalgae, which likely alter the balance of cues on which coral larvae rely. Many macroalgal species release chemical cues (allelochemicals) that deter coral settlement. It remains unclear, however, how larvae detect and discriminate between positive cues, and negative cues released by algae, or the scale at which larvae exhibit sensitivity to allelochemicals. Here, we focused on the effects of a particularly pervasive macroalga, Lobophora, on the larvae of three reef-building corals from the genus Acropora. We used aquarium experiments and field observations to assess the response of coral larvae to cues from Lobophora at multiple spatial scales (from millimetres to a metre).
Results/Conclusions
Our results demonstrate that at an intermediate spatial scale (5-15 cm), larval settlement in all species was inversely related to algal biomass, while larval mortality increased in proportion to algal biomass. Water-soluble secondary metabolites were identified as a key mechanism of inhibition, whereas organic compounds had limited effects. At the smallest spatial scale (0-10 cm), settlement in two coral species was sensitive to the spatial arrangement of algae, with greater inhibition when patches were distributed uniformly rather than clumped (i.e., where available settlement distances from algae were minimised). Lobophora also impaired overall settlement at the largest spatial scale (50-100 cm), regardless of the location of the alga relative to the settlement tiles or the direction of the water flow. Analysis of Acropora settlement onto tiles in the field confirmed avoidance behaviour by the larvae, with corals settling further from Lobophora patches than by chance. Subsequently, larvae settled closer to other invertebrates, such as bryozoans, which are known competitors of corals during early post-settlement stages. Our results reveal that Lobophora can inhibit coral settlement at multiple spatial scales, from millimetres to metres, through the release of water-soluble compounds, and may be responsible for large-scale recruitment failure on coral reefs following disturbances.