This study whether corals from the warmer reef produced more thermally tolerant hybrid and purebred offspring compared with crosses produced with colonies sourced from the cooler reef and whether different symbiont taxa affect heat tolerance. Juveniles were infected with Symbiodinium tridacnidorum, Cladocopium goreaui and Durusdinium trenchii and survival, bleaching and growth were assessed at 27.5°C and 31°C. The experiment focused on familial crosses produced from (n=3) parents from a warm far northern reef (WW1, WW2, WW3), one cross with a warm dam and cool sire (WC) and one cross with a cool dam and warm sire (CW).
Larvae from each cross were allowed to settle, and grown on plugs for 11 days. Juveniles were exposed to one of three treatments of the following Symbiodiniaceae taxa cultured at the Australian Institute of Marine Science Algal Culture Facility: S. tridacnidorum (monoclonal SCF022.01), C. goreaui (monoclonal SCF055-01.10) and D. trenchii (heterogeneous SCF082) as described in Quigley et al. (2014)
All inoculated juveniles were subsequently kept at 27.5°C for 8 days and symbiosis establishment was visually confirmed over this period under a microscope.
Plugs were then randomly divided across treatment tanks, and half from each symbiosis-establishment treatment were placed into 31°C treatment tanks without ramping, totalling six tanks (three replicate tanks at 27.5°C and three replicate tanks at 31°C).
Juvenile survival, bleaching and growth were assessed through image analysis, starting on the first day of exposure to 31°C, with five time points measured and analysed at 1, 9, 35, 49 and 70 days of heat exposure.
Juveniles were scored as highly pigmented (3=D6), pale (2=D4), bleached (1=D1, translucent tissue), or dead (0, missing or bare skeleton with or without algal or cyanobacterial overgrowth)
Statistical analyses completed in R using generalisezd linear models and percent change in the bleaching score and juvenile area were calculated for each individual juvenile across host genetic background and symbiont type. See Quigley et al (2020) for full details.
