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, i..., 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 ex..., , # Out of shape: ocean acidification simplifies coral reef architecture and reshuffles fish assemblages

[https://doi.org/10.5061/dryad.np5hqc02d](https://doi.org/10.5061/dryad.np5hqc02d)

This dataset originates from a study examining the effects of ocean acidification on coral reef structural complexity and associated impacts on damselfish assemblages at Upa-Upasina, Milne Bay Province, Papua New Guinea. The study focused on comparative analyses between a naturally acidified shallow coral reef and a control site with ambient pCO2 conditions. Data collection occurred in June 2018.

## Description of the data and file structure

The dataset includes the following six spreadsheets:

1. **Benthic cover** - Contains data on the percentage cover of five broad habitat categories (live branching, dead branching, live massive, dead massive, rubble) across two treatment locations (control, seep). Data were collected from photo quadrats (40 control, 31 seep) taken at 1 meter distance from the su...