Copper and thermal perturbations on the early life processes of the hard coral <Emphasis Type="Italic">Platygyra acuta</Emphasis>

Anthropogenic pollutants and climate change are major threats to coral reefs today. Yet interactions between chemical and thermal perturbations have not been fully explored in reef studies. Here, we present the single and combined effects of copper (Cu) with thermal stress on five early life-history stages/processes (fertilization, larval mortality, swimming ability, metamorphosis and growth of juvenile recruits) of the massive coral Platygyra acuta in Hong Kong. In the first four experiments, coral gametes and larvae were exposed to different Cu doses (0–200 μg L^?1, apart from the fertilization assay in which 0–1000 μg L^?1 was used) and temperature treatments (ambient and ambient +2 or +3 °C as a thermal stress treatment) following a factorial experimental design. Exposure time was 5 h for the fertilization assay and 48 h for the other experiments. The last experiment on growth of coral recruits was conducted over 56 d with 0–80 μg L^?1 Cu used. Cu significantly reduced percent fertilization success, percentage of active swimming larvae and larval survivorship (EC₅₀s, the half maximal effective concentrations, for percent fertilization success and percentage of active swimming larvae were 92–145 and 45–47 μg L^?1 respectively. While LC₅₀, the lethal concentration that kills 50% of the population, was 101–110 μg L^?1), while growth of coral recruits was not affected at 80 μg L^?1 Cu for 56 d. No settling cues were used in the settlement experiment. In their absence, percent metamorphosis increased with Cu doses, in sharp contrast to earlier findings. Settlement and metamorphosis may thus be strategies for coral larvae to escape from Cu toxicity. Thermal treatment did not significantly affect any experimental end points. This is likely because the thermal regimes used in the experiments were within the range experienced by local corals. The high variability in Cu toxicities indicates differential susceptibilities of the various life-history stages/processes of P. acuta. The level of Cu tolerance was also markedly higher than that reported in the literature for other coral species. This provides evidence to suggest possible adaptation of this species to survive in a highly polluted marine environment like that in Hong Kong.