We found evidence of strong interactions between Cu and temperature as determinants of metamorphosis success for two common coral species. The combined effects of Cu and increased SST were additive for A. tenuis larvae above 29uC and became synergistic for both species at sea surface temperatures above 31uC, with the combined effect of Cu contamination and excessive SST being stronger than the sum of the independent effects of each stressor. These findings provide empirical evidence to support the implementation of environmental policies that aim to increase the resilience of corals to elevated SSTs by improving water quality. Although synergistic effects of these two stressors were only apparent at temperatures above those which would be presently experienced by the majority of settling larvae on the GBR, additive effects of Cu and temperature were apparent at ecologically relevant temperatures. Molecular techniques indicate that some of the sub-cellular responses of corals to Cu may be similar to those identified for thermal stress. Heat-shock proteins, normally associated with thermal stress, were up-regulated in adult Montastraea franksi branches exposed to Cu during an 8 h period. While shared stress response pathways may be partially responsible for the interactive effects of increased SST and Cu on metamorphosis, the effects of temperature on larval metabolism or biochemistry may also influence the toxicity of Cu. Uptake of Cu by the larvae can increase with temperature due to: increased active transport, increased membrane SB-271046A permeability, increased metabolic rate, and/or reduced Cu elimination. Furthermore, each of these mechanisms can be influenced by the species of Cu, which is likely to change as the SST increases. In seawater, Cu exists in equilibrium between a range of species, from the most toxic and bioavailable form of free copper to less bioavailable carbonate complexes. The present study demonstrates that coral larval metamorphosis, a critical step in the process of coral recruitment, is more CP-424,174 sensitive to Cu as SSTs increase further above current summer temperatures of $31uC. Copper affected metamorphosis at concentrations close to or below the US EPA guideline figure of 3.1 mg L21, at temperatures $32uC.