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Widespread coral bleaching occurred in the central Red Sea in 2010 and 2015. During both events, a cross-shelf and depth gradient of bleaching severity was identified within the Thuwal reef system, central Red Sea, Saudi Arabia. While bleaching and survival of coral taxa were monitored, neither in situ reef temperatures nor coral-associated algal communities (family Symbiodiniaceae) were characterized. Here, we determined coral host-associated Symbiodiniaceae communities and monitored temperatures along the same cross-shelf and depth gradient on six reefs in 2017 to better understand the role of these factors in the observed bleaching patterns and to generate a baseline for further studies. We characterized > 600 coral–algal associations across winter and summer in six genera of scleractinian coral (Pocillopora, Stylophora, Seriatopora, Galaxea, Gardineroseris, and Porites) and one fire coral (Family Milleporidae) using ITS2 next-generation sequencing in conjunction with the SymPortal analytical framework. We show that previous bleaching patterns correlate poorly with the largely coral host-specific structure of the 2017 Symbiodiniaceae community and are in better agreement with absolute and intraday sea water temperature variations monitored on the reefs. We demonstrate a greater distinctiveness of Symbiodiniaceae communities at the more severely bleached inshore reefs compared to those reefs further offshore. However, the potential Symbiodiniaceae community changes at these reefs prior to our sampling prevent us from evaluating this distinctiveness as determinative of the differences in bleaching severities. Based on our analyses, we discuss how fine-scale delineation of algal genotypes, including host-specific putative genotypes of Durusdinium trenchii that represent alluring targets for further taxonomic identification, corroborate a niche-adapted rather than generalist character of many coral–Symbiodiniaceae associations. In conclusion, as studies such as this one continue to build the global catalogue of coral–Symbiodiniaceae associations, we may be afforded a better oversight of how specialized coral–algal associations really are and how restricted their modification may be, both of which are critical considerations in predicting the adaptive potential of corals and the reef ecosystems they build.

Fatal infestations of land-based Acropora cultures with so-called Acropora-eating flatworms (AEFWs) are a global phenomenon. We evaluate the hypothesis that AEFWs represent a risk to coral reefs by studying the biology and the invasive potential of an AEFW strain from the UK. Molecular analyses identified this strain as Amakusaplana acroporae, a new species described from two US aquaria and one natural location in Australia. Our molecular data together with life history strategies described here suggest that this species accounts for most reported cases of AEFW infestations. We show that local parasitic activity impairs the light-acclimation capacity of the whole host colony. A. acroporae acquires excellent camouflage by harbouring photosynthetically competent, host-derived zooxanthellae and pigments of the green-fluorescent protein family. It shows a preference for Acropora valida but accepts a broad host range. Parasite survival in isolation (5–7 d) potentially allows for an invasion when introduced as non-native species in coral reefs.