Adaptation to local thermal regimes by crustose coralline algae does not affect rates of recruitment in coral larvae

Crustose coralline algae (CCA) are well known for their ability to induce settlement in coral larvae. While their wide distribution spans reefs that differ substantially in temperature regimes, the extent of local adaptation to these regimes and the impact they have on CCA inductive ability are unknown. CCA Porolithon onkodes from Heron (southern) and Lizard (northern) islands on Australia’s Great Barrier Reef (separated by 1181 km) were experimentally exposed to acute or prolonged thermal stress events and their thermal tolerance and recruitment capacity determined. A sudden onset bleaching model was developed to determine the health status of CCA based on the rate of change in the CCA live surface area (LSA). The interaction between location and temperature was significant (F_(2,119) = 6.74, p = 0.0017), indicating that thermally driven local adaptation had occurred. The southern population remained healthy after prolonged exposure to 28 °C and exhibited growth compared to the northern population (p = 0.022), with its optimum temperature determined to be slightly below 28 °C. As expected, at the higher temperatures (30 and 32 °C) the Lizard Island population performed better that those from Heron Island, with an optimum temperature of 30 °C. Lizard Island CCA displayed the lowest bleaching rates at 30 °C, while levels consistently increased with temperature in their southern counterparts. The ability of those CCA deemed thermally tolerant (based on LSA) to induce Acropora millepora larval settlement was then assessed. While spatial differences influenced the health and bleaching levels of P. onkodes during prolonged and acute thermal exposure, thermally tolerant fragments, regardless of location, induced similar rates of coral larval settlement. This confirmed that recent thermal history does not influence the ability of CCA to induce settlement of A. millepora larvae.