Out of shape: Ocean acidification simplifies coral reef architecture and reshuffles fish assemblages

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.