| Summary: | Climate change stressors are progressively simplifying biogenic habitats in the terrestrial and marine realms, and consequently altering the structure of associated species communities. Here, we used a volcanic CO2 seep in Papua New Guinea to test in situ if altered reef architecture due to ocean acidification reshuffles associated fish assemblages. We observed replacement of branching corals by massive corals at the seep, with simplified coral architectural complexity driving abundance declines between 60-86% for an assemblage of damselfishes associated with branching corals. An experimental test of habitat preference for a focal species indicated that acidification does not directly affect habitat selection behaviour, with changes in habitat structural complexity consequently appearing to be the stronger driver of assemblage reshuffling. Habitat health affected anti-predator behaviour, with P. moluccensis becoming less bold on dead branching corals relative to live branching corals, irrespective of ocean acidification. We conclude that coral reef fish assemblages are likely to be more sensitive to changes in habitat structure induced by increasing pCO2 than any direct effects on behaviour, indicating that changes in coral architecture and live cover may act as important mediators of reef fish community structures in a future ocean. Funding provided by: Agence Nationale de la Recherche ROR ID: https://ror.org/00rbzpz17 Award Number: ANR-15-CE02-0006-01 Funding provided by: ARC Centre of Excellence for Coral Reef Studies ROR ID: https://ror.org/028cdc266 Award Number: Funding provided by: Okinawa Institute of Science and Technology Graduate University ROR ID: https://ror.org/02qg15b79 Award Number: Funding provided by: Australian Research Council ROR ID: https://ror.org/05mmh0f86 Award Number: FT120100183 Data Collection: Study sites: data were collected from two coral reefs near Upa-Upasina, Papua New Guinea: one acidified by natural CO2 seeps and a nearby control reef with ambient pCO2 levels. Benthic ...
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