Pocillopora host-symbiont interactions along an extreme environmental gradient

Pocilloporid corals and their Symbiodiniaceae symbionts have co-evolved. Host-symbiont associations and symbiont species might be driven by adaptation to distinct ecological niches. in the coral host is exhibit co-phylogeny with niche diversification in the coral host, which serves as a primary driver of symbiont speciation. However, strong environmental constraints may favour the proliferation of alternative, but compatible Symbiodiniaceae species present within a coral colony. Here we used single nucleotide polymorphisms (SNPs) to examine host population structure, characterised Symbiodiniaceae associations in shaded and exposed areas of coral using Internal Transcribed Spacer (ITS2) metabarcoding, and identified photobiological phenotypes of Pocillopora acuta colonies from two acidic, deoxygenated and highly variable temperature mangrove environments versus two adjacent reef locations. We found two genetic clusters of P. acuta with evidence of potential hybrid individuals. Limited admixture suggests low levels of gene flow between the reef and mangrove sites. Each of the two lineages were predominantly associated with either reef or mangrove habitats, with distinct dominant symbionts (Cladocopium (reef) and Durusdinium (mangrove)), each with different photobiological strategies. Hybrid individuals exhibited greatest heterogeneity in ITS2 profiles compared to the two other populations. Our results provide evidence that the two lineages are part of a known species complex as suggested by population structure and morphological differences. The genetic distinctiveness of the sampled populations emphasises the unique diversity within the extreme environments. Consequently, conservation efforts should aim to minimise additional anthropogenic impacts at these sites.