Can breeding with warm-reef corals help young corals tolerate heat?

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Climate change is accelerating coral reef degradation, prompting consideration of interventions to increase coral resilience and speed recovery. Researchers collected Acropora spathulata colonies that survived mass bleaching in 2016 and 2017 from a warmer, bleaching-impacted northern Great Barrier Reef (GBR) reef. They crossed these corals with colonies from a recently bleached but historically cooler central GBR reef, producing purebred and crossbred offspring groups: warm–warm, warm–cool, and cool–warm.

The researchers tested whether corals from the warmer reef produced more heat-tolerant hybrid and purebred offspring than crosses produced with corals from the cooler reef, and whether different symbiont taxa affected heat tolerance. Juveniles were infected with Symbiodinium tridacnidorum, Cladocopium goreaui, or Durusdinium trenchii, and their survival, bleaching, and growth were assessed at 27.5°C and 31°C.

The contributions of host genetic background and symbiont identity varied across fitness traits. Offspring with one or both parents from the northern population had 13 to 26-fold higher survival odds than all other treatments; familial cross identity significantly influenced survival probability at 31°C but not at 27.5°C (Kaplan–Meier p=0.001 versus 0.2). Among juveniles in symbiosis with D. trenchii, a warm sire and cool dam gave the best survival odds.

Bleaching was predominantly driven by the Symbiodiniaceae treatment: juveniles hosting D. trenchii bleached significantly less than the other treatments at 31°C. The greatest overall fold-benefits in growth and survival at 31°C occurred with at least one warm dam and symbiosis with D. trenchii. Juveniles associated with D. trenchii grew the most at 31°C, while those associated with C. goreaui grew fastest at 27.5°C.

The researchers concluded that selective breeding with corals from warmer GBR reefs, combined with algal symbiont manipulation, can help increase thermal tolerance on cooler but warming reefs. Such interventions have the potential to improve coral fitness in warming oceans.