Corals are hypersensitive organisms that are unable to cope with the increasingly harsh conditions induced by climate change. The cnidarian-dinoflagellate symbiosis provides the structural and trophic basis of coral reef ecosystems. Coral mucus, integral to this endosymbiosis, has many critical functions: it hosts a distinct suite of microorganisms and has key roles including lubricating surfaces and aiding heterotrophic feeding. Despite this importance, our knowledge of the intra- and extra-cellular secretory processes in corals remain understudied. Secreted proteins (‘secretome’) constitute an important class of molecules, which play an active role in processes necessary for cell survival and immune functioning. In my research I have used proteomics to characterise the secretome of Acropora millepora and its response to thermal stress. Preliminary results indicate a clear separation between the coral host tissue and mucus, potentially providing a non-invasive biomarker for stress. The results indicate distinct differences in protein composition, functionality and abundance between control and thermally stressed secretory proteins, indicative of endoplasmic reticulum stress caused by the conserved unfolded protein response. This research provides a platform for future studies of secreted proteins in this symbiosis and addresses this major knowledge gap, as we seek new tools to combat the ‘coral reef crisis'.