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.

Methods
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 Habitat Cover:

Photo quadrats (40 control, 31 seep) were taken at 1 meter distance from the substrate.
Coral cover was analysed using CoralPointCount software, categorising each point into live or dead coral and three morphologies (branching, massive, rubble).

Fish Density:

Visual surveys conducted on SCUBA using belt transects (10x4 m for larger species, 10x2 m for smaller species).
Surveys recorded densities of five damselfish species.

Habitat Use:

In situ video recordings captured fish interactions with habitats.
Videos analysed to determine time spent in each habitat type, categorised by health and morphology.

Aquarium Habitat Choice

P. moluccensis individuals placed in aquaria with paired habitat types.
Time spent in each habitat was recorded to assess preference without external influences.

Flight Initiation Distance (FID):

FID measured as the distance at which fish fled from an approaching artificial startle cue.
FID data collected for five species across both sites, with additional detail for P. moluccensis regarding habitat type.

 Data Processing:

1.     Coral Cover Analysis:

Digital images processed with CoralPointCount to calculate percentage cover for each category.

Fish Surveys:

Densities calculated per 10 square meters, with data recorded in Excel.

Video Analysis:

Videos analysed using VLC media player.
Habitat use expressed as a percentage of total observations for each fish.

Aquarium Experiments:

Time in each habitat type recorded and expressed as a proportion of total observation time.

Flight Initiation Distance measurements:

Distances measured from video recordings using VLC, referencing a ruler for accuracy.