# Concordant Patterns of Population Genetic Structure and Symbiont Communities in a Broadcasting Spawning Coral Along a Western Australian Fringing Reef

## TL;DR

This study examines how coral and their symbionts are genetically connected along a fringing reef in Western Australia, finding patterns shaped by geographic distance.

## Contribution

The study reveals shared biogeographic drivers structuring coral host and symbiont communities along a fringing reef system.

## Key findings

- Coral host genetic structure follows isolation by distance with a 50 km genetic neighborhood.
- Symbiont communities show similar spatial patterns but no strong link to host genetic diversity.
- Shared biogeographic factors influence both coral and symbiont connectivity across the reef.

## Abstract

The degree of connectivity across ecosystems is a key determinant of resilience, directly influencing recovery potential after disturbance and long‐term ecosystem stability. In reef‐building corals, there is added complexity to these processes because both the coral host and their symbiotic dinoflagellates determine resilience. Given these complexities, we investigated the connectivity of a broadcast spawning coral and its associated algal symbiont communities along the Ningaloo Reef Marine Park and Muiron Island Management Area. Using reduced representation sequencing and DNA metabarcoding in 158 colonies of Acropora cf. tenuis across 14 sampling sites, we detected significant spatial genetic structure in the coral host consistent with a pattern of isolation by distance (IBD). Spatial Autocorrelation analyses revealed that the genetic neighbourhood extends up to 50 km suggesting that this coral species has multiple demographically independent populations across Ningaloo Reef. Symbiont communities were dominated by Cladocopium and followed a similar IBD pattern of between‐site differences in community composition. We did not identify a significant correlation between host genetic diversity and symbiont community diversity at the colony level. However, spatial patterns of genetic differentiation between sample sites for the host and symbiont community composition were significantly associated suggesting that connectivity along a fringing reef system for both coral hosts and their symbionts is driven by similar biogeographic factors.

We explored fine‐scale patterns of connectivity and symbiont associations across the Ningaloo reefscape to inform on post‐disturbance recovery, larval dispersal capabilities, and recruitment dynamics. We detected low but significant population genetic structure among sample sites spread across Ningaloo Reef with the highest diversity in southern sites. There is substantial evidence of isolation by distance, with an increasing signal of genetic differentiation with increasing geographic distance between sites. Interestingly, symbiont ITS2 metabarcoding revealed a positive association between coral genetic differentiation and symbiont community dissimilarity across sites, indicating that both are structured by shared biogeographic drivers.