# Skeleton-Forming Responses of Reef-Building Corals under Ocean Acidification

## TL;DR

This study explores how ocean acidification affects the skeleton formation of four coral species, revealing diverse strategies and vulnerabilities.

## Contribution

The study identifies unique skeleton-forming responses and vulnerabilities in corals under acidified conditions.

## Key findings

- Acropora muricata forms a 'cavity-like' structure to protect its polyp–canal system at the expense of skeletal density.
- Pocillopora damicornis and Montipora species show 'osteoporosis'-like skeletal patterns with disordered structures and low bone mass.
- Ocean acidification primarily damages pre-existing coral skeletal structures.

## Abstract

Ocean acidification is becoming more prevalent and may contribute to coral reef degradation, yet our understanding of its role in global reef decline remains limited. Therefore, there is an urgent need to study the impact of reduced pH levels on the growth patterns of major reef-building corals. Here, we studied the skeleton-forming strategies of 4 widely distributed coral species in a simulated acidified habitat with a pH of 7.6 to 7.8. We reconstructed and visualized the skeleton-forming process, quantified elemental calcium loss, and determined gene expression changes. The results suggest that different reef-building corals have diverse growing strategies in lower pH conditions. A unique “cavity-like” forming process starts from the inside of the skeletons of Acropora muricata, which sacrifices skeletal density to protect its polyp–canal system. The forming patterns in Pocillopora damicornis, Montipora capricornis, and Montipora foliosa were characterized by “osteoporosis”, exhibiting disordered skeletal structures, insufficient synthesis of adhesion proteins, and low bone mass, correspondingly. In addition, we found that damage from acidification particularly affects pre-existing skeletal structures in the colony. These results enhance our understanding of skeleton-forming strategies in major coral species under lower pH conditions, providing a foundation for coral reef protection and restoration amidst increasing ocean acidification.