Stable symbiont communities persist in parents, gametes, and larvae of Montipora capitata across historical bleaching phenotypes

Parental effects on early life history stages of corals are poorly understood, but with severe environmental disturbances, these impacts may be increasingly important in understanding future coral survival trajectories on reefs. This study investigated whether parental bleaching of Montipora capitata in 2015 influenced symbiont community composition and offspring size three years after recovery. In July 2018, gametes were collected from Reef 13 in Kāne‘ohe Bay, O‘ahu, Hawai‘i and selectively crossed to produce three different parental phenotype histories: 1) both parents previously bleached (“bleached” phenotype), 2) both parents previously non-bleached (“non-bleached” phenotype), and 3) crosses from a combination of both parental histories (“crossed” phenotype). Parental bleaching history affected the symbiont community composition in three different life history stages – parents, gametes, and larvae, with the bleached phenotype dominated by Cladocopium and non-bleached phenotype dominated by Durusdinium. Symbiont densities were also different between bleaching phenotypes in parents and gamete bundles but not in larvae, with non-bleached phenotypes having slightly higher symbiont densities than their bleached counterparts. Larvae from each phenotype were then exposed to either ambient or high-temperature conditions for 72 h and larvae from bleached phenotype parents were smallest regardless of temperature treatment. With these findings, larval recruitment to the reef from previously bleached parents is suspected to decline as ocean warming becomes more frequent and severe, potentially leading to generational symbiont community shifts. The direct heritability of thermal tolerance from parent to offspring in M. capitata provides opportunities for restoration by selectively breeding for traits that may increase community resilience to thermal stress.