Our first aim was to carry out tests of heat-tolerance on a large number of corals from three species (Acropora digitifera, Goniastrea retiformis and Echinopora lamellosa). These experiments involve tests on small fragments of coral (“nubbins”) in flow-through sea-water aquarium tanks where we increase temperatures gradually over time to mimic a natural coral bleaching event. We assess the average health status (i.e. whether coral nubbins bleach and die) to compare performance of each coral colony over time. Our results show that even within a single population of corals, there is a great deal of variation in the way individuals respond to temperature stress. By doing repeated temperature stress experiments tests on fragments from several adult parent colonies we have also shown consistency in heat tolerance from colonies tested in consecutive years. By examining the difference between the most and the least tolerant corals in the population we were able to show that the most tolerant corals could withstand twice as much heat stress as the least tolerant corals. If we take action on climate change, this extra tolerance may “buy” an additional 17 years before they succumb to heat stress compared to the least tolerant.
The temperature stress tests have allowed us to select parents for breeding from two of our study species (Acropora digitifera and Goniastrea retiformis) with relatively high and low heat tolerance. We were successful in spawning and selectively breeding these corals in 2018 and 2019 and produce multiple unique crosses from parents with varying heat tolerance. To date we have produced hundreds of 3 and 4- year old individual F1 corals with over 600 outplanted to the reef to assess their survivorship and growth rates. We have found that survivorship varied considerably, but that rearing corals for longer periods in nurseries led to much higher survivorship rates. To overcome these observed early mortality bottlenecks, we devised and tested an innovative technique called the Coralassist Plug (CAPs) for transplanting juvenile corals that we have shown reduces nursery times and increase early survivorship of outplanted corals. CAPs consist of a ceramic substrate containing small grooves that provide protection for newly settled corals from fish grazing and can be attached quickly and easily to the reef large as part of large-scale restoration interventions. In trials, we found that these were 4-8 times faster to outplant and that survivorship was doubled compared to conventional techniques. The remainder of our selectively bred corals were kept in ocean nurseries and used for further experiments on heritability of heat stress tolerance. We have now conducted heat tolerance assays on embryos, larvae, juvenile and adult corals that were selectively bred and in each case they showed evidence of heritability, with crosses from parents with high relative heat tolerance having higher tolerance than those from parents with low relative heat tolerance. This has allowed us to estimate the extent of heritability for the first in and adult coral.
To assess whether more heat tolerant corals tend to produce fewer eggs or have slower growth we collected samples to assess reproductive outputs and have carried out 3D imaging of all individual tagged coral colonies at two or three time points. These images and samples were analysed to gain insights into potential resource trade-offs. Surprisingly, we found no evidence of a trade-off and in fact, more heat tolerant corals tended to grow faster than les heat tolerant corals. This provides some hope that corals selectively bred for heat tolerance will not show disadvantages in other traits, such as growth. We found that coral heat tolerance was not associated with either the Symbiont or microbial communities, suggesting any differences I heat tolerance are coming from the corals themselves.
A major goal of CORALASSIST is to understand the role that different proteins play in protecting certain corals from heat stress. We have established a library of a comprehensive set of proteins produced by one of our species (A. digitifera), a vital first step in validating our experimental approach. So far, these experiments include assessing the overall protein expression profiles for different coral life stages; comparing the baseline protein abundance differences between coral colonies (and their offspring) with different heat tolerance levels and; testing differences in protein abundance during heat stress between corals from different selective crosses. While we do not see differences in protein abundances between groups under baseline conditions, we do see differences between our heat tolerant groups when put under heat stress. These data are currently being analysed to look for specific proteins they may be involved in causing differences in heat tolerance.
