The colour of massive corals is not linked to the thermal sensitivity of photoendosymbiosis on Norfolk Island’s subtropical coral reefs

Context Approximately half of scleractinian corals rely on endosymbiotic Symbiodiniaceae, making them vulnerable to bleaching during thermal stress. Accurately interpreting bleaching responses is essential for monitoring reef health, yet visual assessments of coral colour often overlook symbiotic status or physiological health. Aims This study aimed to determine whether coral colour is a reliable indicator of symbiosis or bleaching response in massive coral genera Platygyra and Paragoniastrea in the subtropical reef system of Norfolk Island. Methods We conducted year-long ecological surveys to quantify colour morph frequency and photophysiology in Platygyra and Paragoniastrea colonies. Coral colouration, photosynthetic yield, symbiont density, tissue condition, and skeletal colonisation were assessed under both ambient and elevated sea surface temperatures. Key results Approximately 20% of Platygyra and Paragoniastrea colonies exhibited white or pale colouration year-round, independent of elevated sea surface temperatures. White and pale colour morphs maintained high photosynthetic yield and symbiont densities, comparable to darker pigmented colonies, with no decline in photophysiology or increase in frequency during periods of thermal stress. Conclusions Colony colouration in Platygyra and Paragoniastrea is not a reliable indicator of symbiotic health or bleaching susceptibility in the subtropical reef system of Norfolk Island. Visual assessments alone may lead to inaccurate evaluations of coral health in these taxa. Implications Our findings highlight the need for species- and location-specific approaches to coral health assessments, particularly in subtropical reefs that may exhibit distinct functional traits. Our results also underscore the limitations of using visual monitoring methods alone, and the importance of including physiological metrics.